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Interference is widely regarded as a foundational attribute of quantum mechanics. However, for a given experimental arrangement, interference can either contribute or not contribute to the outcome depending upon the basis in which it is measured. This observation is both foundational and particularly relevant to coherent control of molecular processes, an approach based upon quantum interference. Here, we address this issue and its relevance to controlling molecular processes via the "coherent control scattering (CCS) matrix," a formalism that allows for an analysis of modifications in an interference structure resulting from a change of basis. This analysis reveals that the change in the interference structure can be attributed to the non-commutativity of the transformation matrix with the CCS matrix and the non-orthogonality of the transformation. Additionally, minimal interference is shown to be associated with the CCS eigenbasis and that the Fourier transform of the eigenvectors of the CCS matrix provides the maximal interference and hence the best coherent control. The change of controllability through a change of basis is illustrated with an example of 85Rb+ 85Rb scattering. In addition, the developed formalism is applied to explain recent experimental results on He + D2 inelastic scattering demonstrating the presence or absence of interference depending on the basis. [ABSTRACT FROM AUTHOR]

A frequency-dependent dielectric constant characterizes the dielectric response of a medium and also represents the time scale of system's collective dynamics. Although it is valuable not only academically but also practically for developing advanced devices, getting the value of a solution at the interface with a solid or electrode surface is challenging both experimentally and computationally. Here, we propose a computational method that imitates the dielectric spectroscopy and AC impedance measurement. It combines a time-dependent voltage applied molecular dynamics simulation with an equivalent circuit representation of a system composed of a solution confined between two identical electrodes. It gives the frequency-dependent dielectric constants of the bulk solution and the interface simultaneously. Unlike the conventional method, it does not require computation of a dipole autocorrelation function and its Fourier transformation. Application of the method on a system of water confined between polarizable Pt electrodes gives the static dielectric constant and the relaxation time of the bulk water in good agreement with previous simulation results and experimental values. In addition, it gives a much smaller static dielectric constant at the interface, consistent with previous observations. The outline of the dielectric dispersion curve of the interface seems similar to that of the bulk, but the relaxation time is several times faster. [ABSTRACT FROM AUTHOR]

A new free-energy functional is proposed for inhomogeneous associating fluids. The general formulation of Wertheim's thermodynamic perturbation theory is considered as the starting point of the derivation. We apply the hypotheses of the statistical associating fluid theory in the classical density functional theory (DFT) framework to obtain a tractable expression of the free-energy functional for inhomogeneous associating fluids. Specific weighted functions are introduced in our framework to describe association interactions for a fluid under confinement. These weighted functions have a mathematical structure similar to the weighted densities of the fundamental-measure theory (i.e., they can be expressed as convolution products) such that they can be efficiently evaluated with Fourier transforms in a 3D space. The resulting free-energy functional can be employed to determine the microscopic structure of inhomogeneous associating fluids of arbitrary 3D geometry. The new model is first compared with Monte Carlo simulations and previous versions of DFT for a planar hard wall system in order to check its consistency in a 1D case. As an example of application in a 3D configuration, we then investigate the extreme confinement of an associating hard-sphere fluid inside an anisotropic open cavity with a shape that mimics a simplified model of zeolite. Both the density distribution and the corresponding molecular bonding profile are given, revealing complementary information to understand the structure of the associating fluid inside the cavity network. The impact of the degree of association on the preferential positions of the molecules inside the cavity is investigated as well as the competition between association and steric effect on adsorption. [ABSTRACT FROM AUTHOR]

Ultrafast spectroscopy is a powerful technique that utilizes short pulses on the femtosecond time scale to generate and probe coherent responses in molecular systems. While the specific ultrafast methodologies vary, the most common data analysis tools rely on discrete Fourier transformation for recovering coherences that report on electronic or vibrational states and multi-exponential fitting for probing population dynamics, such as excited-state relaxation. These analysis tools are widely used due to their perceived reliability in estimating frequencies and decay rates. Here, we demonstrate that such "black box" methods for parameter estimation often lead to inaccurate results even in the absence of noise. To address this issue, we propose an alternative approach based on Bayes probability theory that simultaneously accounts for both population and coherence contributions to the signal. This Bayesian inference method offers accurate parameter estimations across a broad range of experimental conditions, including scenarios with high noise and data truncation. In contrast to traditional methods, Bayesian inference incorporates prior information about the measured signal and noise, leading to improved accuracy. Moreover, it provides estimator error bounds, enabling a systematic statistical framework for interpreting confidence in the results. By employing Bayesian inference, all parameters of a realistic model system may be accurately recovered, even in extremely challenging scenarios where Fourier and multi-exponential fitting methods fail. This approach offers a more reliable and comprehensive analysis tool for time-resolved coherent spectroscopy, enhancing our understanding of molecular systems and enabling a better interpretation of experimental data. [ABSTRACT FROM AUTHOR]

The first finite basis set implementation of the real-time time-dependent self-consistent field method in a dynamic (time-dependent) magnetic field using London atomic orbitals (LAOs) is presented. The accuracy of the finite basis approach using LAOs is benchmarked against numerical results from the literature for the hydrogen atom and H2 in the presence of rapidly oscillating magnetic fields. This comparison is used to inform the choice of appropriate basis sets for studies under such conditions. Remarkably, relatively modest compact LAO basis sets are sufficient to obtain accurate results. Analysis of electron dynamics in the hydrogen atom shows that LAO calculations correctly capture the time evolution of orbital occupations. The Fourier transformation of the autocorrelation function yields a power spectrum exhibiting harmonics associated with coherent emission, which closely matches the literature and further confirms the accuracy of this approach. The dynamical response of the electron density in H2 for a magnetic field parallel to the internuclear axis shows similar behavior to benchmark studies. The flexibility of this implementation is then demonstrated by considering how the dynamical response changes as a function of the orientation of the molecule relative to the applied field. At non-parallel orientations, the symmetry of the system is lowered and numerical benchmark data, which exploit cylindrical symmetry, are no-longer readily available. The present study demonstrates the utility of LAO-based calculations for extreme dynamic magnetic fields, providing a stress-test on the choice of basis. Future applications of this approach for less extreme dynamic magnetic fields are briefly discussed. [ABSTRACT FROM AUTHOR]

Non-impact effects in the absorption spectra of HCl in various collision-partners are investigated both experimentally and theoretically. Fourier transform spectra of HCl broadened by CO2, air, and He have been recorded in the 2-0 band region at room temperature and for a wide pressure range, from 1 to up to 11.5 bars. Comparisons between measurements and calculations using Voigt profiles show strong super-Lorentzian absorptions in the troughs between successive lines in the P and R branches for HCl in CO2. A weaker effect is observed for HCl in air, while for HCl in He, Lorentzian wings are in very good agreement with measurements. In addition, the line intensities retrieved by fitting the Voigt profile on the measured spectra decrease with the density of the perturber. This perturber-density dependence decreases with the rotational quantum number. For HCl in CO2, the decrease in the retrieved line intensity can reach 2.5% per amagat for the first rotational quantum numbers. This number is about 0.8% per amagat for HCl in air, while for HCl in He, no density dependence of the retrieved line intensity is observed. Requantized classical molecular dynamics simulations have been performed for HCl–CO2 and HCl–He in order to simulate the absorption spectra for various perturber-density conditions. The density dependence of the intensities retrieved from the simulated spectra and the predicted super-Lorentzian behavior in the troughs between lines are in good agreement with experimental determinations for both HCl–CO2 and HCl–He. Our analysis shows that these effects are due to incomplete or ongoing collisions, which govern the dipole auto-correlation function at very short times. The effects of these ongoing collisions strongly depend on the details of the intermolecular potential: they are negligible for HCl–He but significant for HCl–CO2 for which a line-shape model beyond the impact approximation will be needed to correctly model the absorption spectra from the center to the far wings. [ABSTRACT FROM AUTHOR]

The double quantum coherence (DQC) signal in nonlinear spectroscopy gives information about the many-body correlation effects not easily available by other methods. The signal is short-lived, consequently, a significant part of it is generated during the pulse overlap. Since the signal is at two times the laser frequency, one may intuitively expect that the pulse overlap-related artifacts are filtered out by the Fourier transform. Here, we show that this is not the case. We perform explicit calculations of phase-modulated two-pulse experiments of a two-level system where the DQC is impossible. Still, we obtain a significant signal at the modulation frequency, which corresponds to the DQC, while the Fourier transform over the pulse delay shows a double frequency. We repeat the calculations with a three-level system where the true DQC signal occurs. We conclude that with realistic dephasing times, the pulse-overlap artifact can be significantly stronger than the DQC signal. Our results call for great care when analyzing such experiments. As a rule of thumb, we recommend that only delays larger than 1.5 times the pulse length should be used. [ABSTRACT FROM AUTHOR]

In this work, we have studied the nuclear and electron dynamics in the glycine cation starting from localized hole states using the quantum Ehrenfest method. The nuclear dynamics is controlled both by the initial gradient and by the instantaneous gradient that results from the oscillatory electron dynamics (charge migration). We have used the Fourier transform (FT) of the spin densities to identify the "normal modes" of the electron dynamics. We observe an isomorphic relationship between the electron dynamics normal modes and the nuclear dynamics, seen in the vibrational normal modes. The FT spectra obtained this way show bands that are characteristic of the energy differences between the adiabatic hole states. These bands contain individual peaks that are in one-to-one correspondence with atom pair (+·) ↔ (·+) resonances, which, in turn, stimulate nuclear motion involving the atom pair. With such understanding, we anticipate "designer" coherent superpositions that can drive nuclear motion in a particular direction. [ABSTRACT FROM AUTHOR]

We propose a methodology to tackle the laser control of a non-stationary dark ro-vibrational state of acetylene (C2H2), given realistic experimental limitations in the 7.7 μm (1300 cm−1) region. Simulations are performed using the Lindblad master equation, where the so-called Lindblad parameters are used to describe the effect of the environment in the dilute gas phase. A phenomenological representation of the parameters is used, and they are extracted from high-resolution spectroscopy line broadening data. An effective Hamiltonian is used for the description of the system down to the rotational level close to experimental accuracy. The quality of both the Hamiltonian and Lindblad parameters is assessed by a comparison of a calculated infrared spectrum with the available experimental data. A single shaped laser pulse is used to perform the control, where elements of optics and pulse shaping using masks are introduced with emphasis on experimental limitations. The optimization procedure, based on gradients, explicitly takes into account the experimental constraints. Control performances are reported for shaping masks of increasing complexity. Although modest performances are obtained, mainly due to the strong pulse shaping constraints, we gain insights into the control mechanism. This work is the first step toward the conception of a realistic experiment that will allow for population characterization and manipulation of a non-stationary vibrational "dark" state. Effects of the collisions on the laser control in the dilute gas phase, leading to decoherence in the molecular system, are clearly shown. [ABSTRACT FROM AUTHOR]

Chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy is a powerful near-universal detection method finding application in many areas. We have previously coupled it with supersonic flows (CPUF) to obtain product branching in reaction and photodissociation. Because chirped-pulse microwave detection requires monitoring the free induction decay on the timescale of microseconds, it cannot be employed with good sensitivity at the high densities achieved in some uniform supersonic flows. For application to low-temperature kinetics studies, a truly uniform flow is required to obtain reliable rate measurements and enjoy all the advantages that CP-FTMW has to offer. To this end, we present a new setup that combines sampling of uniform supersonic flows using an airfoil-shaped sampling device with chirped-pulse mmW detection. Density and temperature variations in the airfoil-sampled uniform flow were revealed using time-dependent rotational spectroscopy of pyridine and vinyl cyanide photoproducts, highlighting the use of UV photodissociation as a sensitive diagnostic tool for uniform flows. The performance of the new airfoil-equipped CPUF rotational spectrometer was validated using kinetics measurements of the CN + C2H6 reaction at 50 K with detection of the HCN product. Issues relating to product detection by rotational spectroscopy and airfoil sampling are discussed. We show that airfoil sampling enables direct measurements of low temperature reaction kinetics on a microsecond timescale, while rotational spectroscopic detection enables highly specific simultaneous detection of reactants and products. [ABSTRACT FROM AUTHOR]

We consider the longitudinal quadrupole relaxation rate enhancement (QRE) of a 1H nucleus due to the time fluctuations of the local dipolar magnetic field created by a close quadrupole 14N nucleus, the electric-field gradient (EFG) Hamiltonian of which changes with time because of vibrations/distortions of its chemical environment. The QRE is analytically expressed as a linear combination of the cosine Fourier transforms of the three quantum time auto-correlation functions GAA(t) of the 14N spin components along the principal axes A = X, Y, and Z of the mean (time-averaged) EFG Hamiltonian. Denoting the three transition frequencies between the energy levels of this mean Hamiltonian by νA, the functions GAA(t) oscillate at frequencies νA + sA/(2π) with mono-exponential decays of relaxation times τA, where the frequency dynamic shifts sA and the relaxation times τA are closed expressions of the magnitude of the fluctuations of the instantaneous EFG Hamiltonian about its mean and of the characteristic fluctuation time. Thus, the theoretical QRE is the sum of three Lorentzian peaks centered at νA + sA/(2π) with full widths at half maxima 1/(πτA). The predicted peak widths are nearly equal. The predicted dynamic shifts of the peaks are much smaller than their widths and amazingly keep proportional to the transition frequencies νA for reasonably fast EFG fluctuations. The theory is further improved by correcting the transition frequencies by the 14N Zeeman effects of second order. It is successfully applied to reinterpret the QRE pattern measured by Broche, Ashcroft, and Lurie [Magn. Reson. Med. 68, 358 (2012)] in normal cartilage. [ABSTRACT FROM AUTHOR]

Over the past two decades, coherent multidimensional spectroscopies have been implemented across the terahertz, infrared, visible, and ultraviolet regions of the electromagnetic spectrum. A combination of coherent excitation of several resonances with few-cycle pulses, and spectral decongestion along multiple spectral dimensions, has enabled new insights into wide ranging molecular scale phenomena, such as energy and charge delocalization in natural and artificial light-harvesting systems, hydrogen bonding dynamics in monolayers, and strong light–matter couplings in Fabry–Pérot cavities. However, measurements on ensembles have implied signal averaging over relevant details, such as morphological and energetic inhomogeneity, which are not rephased by the Fourier transform. Recent extension of these spectroscopies to provide diffraction-limited spatial resolution, while maintaining temporal and spectral information, has been exciting and has paved a way to address several challenging questions by going beyond ensemble averaging. The aim of this Perspective is to discuss the technological developments that have eventually enabled spatially resolved multidimensional electronic spectroscopies and highlight some of the very recent findings already made possible by introducing spatial resolution in a powerful spectroscopic tool. [ABSTRACT FROM AUTHOR]

We develop a fast method for computing the electrostatic energy and forces for a collection of charges in doubly periodic slabs with jumps in the dielectric permittivity at the slab boundaries. Our method achieves spectral accuracy by using Ewald splitting to replace the original Poisson equation for nearly singular sources with a smooth far-field Poisson equation, combined with a localized near-field correction. Unlike existing spectral Ewald methods, which make use of the Fourier transform in the aperiodic direction, we recast the problem as a two-point boundary value problem in the aperiodic direction for each transverse Fourier mode for which exact analytic boundary conditions are available. We solve each of these boundary value problems using a fast, well-conditioned Chebyshev method. In the presence of dielectric jumps, combining Ewald splitting with the classical method of images results in smoothed charge distributions, which overlap the dielectric boundaries themselves. We show how to preserve the spectral accuracy in this case through the use of a harmonic correction, which involves solving a simple Laplace equation with smooth boundary data. We implement our method on graphical processing units and combine our doubly periodic Poisson solver with Brownian dynamics to study the equilibrium structure of double layers in binary electrolytes confined by dielectric boundaries. Consistent with prior studies, we find strong charge depletion near the interfaces due to repulsive interactions with image charges, which points to the need for incorporating polarization effects in understanding confined electrolytes, both theoretically and computationally. [ABSTRACT FROM AUTHOR]

The vibrational spectra of gas phase tert-butyl hydroperoxide have been recorded in the OH-stretching fundamental and overtone regions (ΔvOH = 1–5) at room temperature using conventional Fourier transform infrared (ΔvOH = 1–3) and cavity ring-down (ΔvOH = 4–5) spectroscopy. In hydroperoxides, the OH-stretching and COOH torsion vibrations are strongly coupled. The double-well nature of the COOH torsion potential leads to tunneling splitting of the energy levels and, combined with the low frequency of the torsional vibration, results in spectra in the OH-stretching regions with multiple vibrational transitions. In each of the OH-stretching regions, both an OH-stretching and a stretch–torsion combination feature are observed, and we show direct evidence for the tunneling splitting in the OH-stretching fundamental region. We have developed two complementary vibrational models to describe the spectra of the OH-stretching regions, a reaction path model and a reduced dimensional local mode model, both of which describe the features of the vibrational spectra well. We also explore the torsional dependence of the OH-stretching transition dipole moment and show that a Franck–Condon treatment fails to capture the intensity in the region of the stretch–torsion combination features. The accuracy of the Franck–Condon treatment of these features improves with increasing ΔvOH. [ABSTRACT FROM AUTHOR]

Amide I difference spectroscopy is widely used to investigate protein function and structure changes. In this article, we show that the common approach of assigning features in amide I difference signals to distinct secondary structure elements in many cases may not be justified. Evidence comes from Fourier transform infrared (FTIR) and 2D-IR spectroelectrochemistry of the protein cytochrome c in the amide I range, in combination with computational spectroscopy based on molecular dynamics (MD) simulations. This combination reveals that each secondary structure unit, such as an alpha-helix or a beta-sheet, exhibits broad overlapping contributions, usually spanning a large part of the amide I region, which in the case of difference absorption experiments (such as in FTIR spectroelectrochemistry) may lead to intensity-compensating and even sign-changing contributions. We use cytochrome c as the test case, as this small electron-transferring redox-active protein contains different kinds of secondary structure units. Upon switching its redox-state, the protein exhibits a different charge distribution while largely retaining its structural scaffold. Our theoretical analysis suggests that the change in charge distribution contributes to the spectral changes and that structural changes are small. However, in order to confidently interpret FTIR amide I difference signals in cytochrome c and proteins in general, MD simulations in combination with additional experimental approaches such as isotope labeling, the insertion of infrared labels to selectively probe local structural elements will be required. In case these data are not available, a critical assessment of previous interpretations of protein amide I 1D- and 2D-IR difference spectroscopy data is warranted. [ABSTRACT FROM AUTHOR]

A molecular dynamics (MD) simulation was performed to study the propagation of soundwaves in a fluid. Soundwaves are generated by a sinusoidally oscillating wall and annihilated by a locally applied Langevin thermostat near the opposite wall. The waveform changes from sinusoidal to sawtooth with increasing wave amplitude. For low-frequency sounds, the simulation results show a very good agreement with Burgers's equation without any fitting parameters. In contrast, for high-frequency sounds, significant deviations are obtained because of acoustic streaming. The speed of sound can be directly determined from the Fourier transform of a waveform with high accuracy. Although obtaining the attenuation rate directly from the simulation results is difficult because of the nonlinear effects of the wave amplitude, it can be estimated via Burgers's equation. The results demonstrate that MD simulations are a useful tool for the quantitative analysis of soundwaves. [ABSTRACT FROM AUTHOR]

A semiempirical potential energy curve for the a3Σ+-state of the KRb molecule with only five parameters is reported. The potential is continuous over the entire range of internuclear distances and has the correct long-range attractive dispersion potential from established theory. The potential provides an equally good fit of the laser induced fluorescence Fourier transform spectroscopic data of Pashov et al. [Phys. Rev. A 76, 022511 (2007)] as their multi-parameter potential. The new potential is supported by the good agreement of the well parameters De, Re and the harmonic vibrational constant ωe with combining-rule estimates. The scattering lengths for all six isotopologues are also in excellent agreement with experiment with a 0.2% adjustment within theoretical uncertainty of the leading dispersion coefficient C6. An analysis of the harmonic vibrational constant ωe and the constant ωexe of the potential of Pashov et al. reveals a significant difference to the present potential which turned out to be due to an oscillatory deviation in their potential in the vicinity of the potential minimum. The new potential is, thus, the best available because its simplicity is ideal for further applications. [ABSTRACT FROM AUTHOR]

Pure rotational spectra of Sc13C2 ( X ̃ 2A1) and Sc12C13C ( X ̃ 2A′) have been measured using Fourier transform microwave/millimeter-wave methods. These molecules were synthesized in a DC discharge from the reaction of scandium vapor, produced via laser ablation, with 13CH4 or 13CH4/12CH4, diluted in argon. The NKa,Kc = 10,1 → 00,0, 20,2 → 10,1, 30,3 → 20,2, and 40,4 → 30,3 transitions in the frequency range of 14 GHz–61 GHz were observed for both species, each exhibiting hyperfine splittings due to the nuclear spins of 13C (I = 1/2) and/or Sc (I = 7/2). These data have been analyzed with an asymmetric top Hamiltonian, and rotational, spin-rotation, and hyperfine parameters have been determined for Sc13C2 and Sc12C13C. In addition, a quartic force field was calculated for ScC2 and its isotopologues using a highly accurate coupled cluster-based composite method, incorporating complete basis set extrapolation, scalar relativistic corrections, outer core and inner core electron correlation, and higher-order valence correlation effects. The agreement between experimental and computed rotational constants, including the effective constant (B + C), is ∼0.5% for all three isotopologues. This remarkable agreement suggests promise in predicting rotational spectra of new transition metal-carbon bearing molecules. In combination with previous work on Sc12C2, an accurate structure for ScC2 has been established using combined experimental (B, C) and theoretical (A) rotational constants. The radical is cyclic (or T-shaped) with r(Sc–C) = 2.048(2) Å, r(C–C) = 1.272(2) Å, and ∠(C–Sc–C) = 36.2(1)°. The experimental and theoretical results also suggest that ScC2 contains a C2− moiety and is largely ionic. [ABSTRACT FROM AUTHOR]

The Fourier transform microwave spectrum of 4-methylacetophenone recorded from 8 GHz to 18 GHz under jet-cooled conditions has revealed large tunneling splittings arising from a low barrier to internal rotation of the ring methyl group and small splittings from a high torsional barrier of the acetyl methyl group. The large splittings are especially challenging to model, while the small splittings are difficult to analyze due to the resolution limit of 120 kHz. The combination of two methyl groups undergoing internal rotations caused each rotational transition to split into five torsional species, which were resolved and fitted using a modified version of the XIAM code and the newly developed ntop code to a root-mean-square deviation close to measurement accuracy, providing an estimate of the V3 potential barriers of about 22 cm−1 and 584–588 cm−1 for the ring and the acetyl methyl groups, respectively. The assignment was aided by separately fitting the five torsional species using odd-power order operators. Only one conformer in which all heavy atoms are located on a symmetry plane could be identified in the spectrum, in agreement with results from conformation analysis using quantum chemical calculations. [ABSTRACT FROM AUTHOR]

This paper presents core excitation spectra from coupled-cluster (CC) theory obtained from both a time-independent and a new time-dependent formalism. The conventional time-independent CC formulation for excited states is the equation-of-motion (EOM-CC) method whose eigenvalues and eigenvectors describe the core excited states. An alternative computational procedure is offered by a time-dependent CC description. In that case, the dipole transition operator is expressed in the CC effective Hamiltonian form and propagated with respect to time. The absorption spectrum is obtained from the CC dipole autocorrelation function via a Fourier transformation. Comparisons are made among the time-dependent results obtained from second-order perturbation theory, to coupled cluster doubles and their linearized forms (CCD and LCCD), to CC singles and doubles (CCSD) and the linearized form (LCCSD). In the time-independent case, considerations of triples (EOM-CCSDT) and quadruples (EOM-CCSDTQ) are used to approach sub-electron volt accuracy. A particular target is the allyl radical, as an example of an open-shell molecule. As the results have to ultimately be the same, the two procedures offer a complementary approach toward analyzing experimental results. [ABSTRACT FROM AUTHOR]

In the weak-field limit in which microwave spectroscopy is typically carried out, an application of a single-frequency pulse that is resonant with a molecular transition will create a coherence between the pair of states involved in the rotational transition, producing a free-induction decay (FID) that, after Fourier transform, produces a molecular signal at that same resonance frequency. With the advent of chirped-pulse Fourier transform microwave methods, the high-powered amplifiers needed to produce broadband microwave spectra also open up other experiments that probe the molecular response in the high-field regime. This paper describes a series of experiments involving resonant frequency pulses interrogating jet-cooled molecules under conditions of sufficient power to Rabi oscillate the two-state system through many Rabi cycles. The Fourier-transformed FID shows coherent signal not only at the applied resonant frequency but also at a series of transitions initially connected to the original one by sharing an upper or lower level with it. As the duration of the single-frequency excitation is increased from 250 to 1500 ns, the number of observed off-resonant, but dipole-allowed, molecular coherences grow. The phenomenon is quite general, having been demonstrated in Z-phenylvinylnitrile, E-phenylvinylnitrile (E-PVN), benzonitrile, guaiacol, and 4-pentynenitrile. In E-PVN, the highest power/longest pulse duration, coherent signal is also present at energetically nearby but not directly connected transitions. Even in molecular samples containing more than one independent species, only transitions due to the single species responsible for the original resonant transition are present. We develop a time-dependent model of the molecular/photon system and use it in conjunction with the experiment to test possible sources of the phenomenon. [ABSTRACT FROM AUTHOR]

The vibrational modes of chemisorbed CO on a Si(001) surface are investigated by means of transmission Fourier-transform infrared absorption spectroscopy. We observed the three components corresponding to the stretching vibration of the terminal-site CO adsorbed on the down-dimer sites of a Si(001) surface. The symmetric stretching vibration and asymmetric stretching vibration are observed separately. This assignment is consistent with the polarization dependence of the incident light and with the vibrational modes obtained by performing theoretical calculations. We found that both the intradimer row and interdimer row coupling should be considered to explain the appearance of the three components. [ABSTRACT FROM AUTHOR]

The rotational spectra of two isotopologues of methylphenylsilane were measured and assigned by using the supersonic-jet Fourier transform microwave spectroscopic technique in the 2.0–20.5 GHz range. The feature of rotational spectra of methylphenylsilane indicates that the doublets of rotational transitions in methylphenylsilane are contributable to the methyl internal rotation with a V3 barrier of 559 (25) cm−1. No splitting has been observed due to the methyl internal rotation in its carbon analog, ethylbenzene, which indicated that the barrier to such motion should be higher than 700 cm−1. Silicon incorporation of the ethyl group in ethylbenzene leads to a much lower barrier to the methyl internal rotation. [ABSTRACT FROM AUTHOR]

Two different numerically exact methods for open quantum system dynamics, the hierarchy of pure states (HOPS) method, and the multi-Davydov-Ansatz are discussed. We focus on the suitability of the underlying representations of bath correlations. While in the HOPS case the correct description of the bath correlation function (BCF) in the time domain is decisive, it turns out that a windowed Fourier transform of the BCF is an appropriate indicator of the quality of the discretization in the multi-Davydov-Ansatz. For the spin-boson model with sub-Ohmic spectral density considered here, a discretization of the spectral density based on an exponential distribution, used previously, turns out to be most favorable. [ABSTRACT FROM AUTHOR]

Using Fourier Transform Infrared (FTIR) spectroscopy, we investigate the temperature dependence of the bending vibrations of water confined in the pores of a silica hydrogel in the temperature interval of 270–180 K. We also investigate the presence of thermal hysteresis by cooling and reheating temperature scans. The results clearly show the presence, at about 230 K, of a crossover in the temperature dependence of the IR spectra; moreover, the presence of hysteresis is clearly demonstrated. By comparing FTIR data with neutron diffraction data and previous calorimetric data on the same samples, we conclude that the crossover and the hysteretical behavior do not involve a water glass transition or crystallization but are related to a first-order-like liquid-liquid transition. [ABSTRACT FROM AUTHOR]

We propose two schemes for interpolation of the one-particle Green's function (GF) calculated within a coupled-cluster singles and doubles (CCSD) method for a periodic system. These schemes use Wannier orbitals for circumventing huge cost for a large number of sampled k points. One of the schemes is the direct interpolation, which obtains the GF straightforwardly by using Fourier transformation. The other is the self-energy-mediated interpolation, which obtains the GF via the Dyson equation. We apply the schemes to a LiH chain and trans-polyacetylene and examine their validity in detail. It is demonstrated that the direct-interpolated GFs suffer from numerical artifacts stemming from slow convergence of CCSD GFs in real space, while the self-energy-mediated interpolation provides more physically appropriate GFs due to the localized nature of CCSD self-energies. Our schemes are also applicable to other correlated methods capable of providing GFs. [ABSTRACT FROM AUTHOR]

We present a general approach for the calculation and assignment of X-ray absorption spectra based on electronic wavepacket propagations performed using explicitly time-dependent electronic structure calculations. Such calculations have the appeal of yielding the entire absorption spectrum for the cost of a single set of electronic wavepacket propagations, obviating the need to explicitly calculate large numbers of core-excited states. The spectrum can either be calculated from the Fourier transform of the time-dependent dipole moment or from the Fourier transform of the wavepacket autocorrelation function. We propose that calculating the absorption spectrum using the latter approach will generally be the preferred option. This method has two important advantages. First, the autocorrelation functions can be obtained for twice the propagation time, resulting in a halving of the computational effort required to calculate the spectrum relative to the time-dependent dipole moment approach. Second, using the tools of filter diagonalisation, the autocorrelation functions may be used to determine the time-independent final core-excited states underlying the peaks of interest in the spectrum. The proposed scheme is validated by calculating and characterizing the X-ray absorption spectra of benzene and trifluoroacetonitrile at the time-dependent second-order algebraic diagrammatic construction level of theory. [ABSTRACT FROM AUTHOR]

The rotational spectra of five isotopic species of the Ar–SiO complex have been observed at high-spectral resolution between 8 and 18 GHz using chirped Fourier transform microwave spectroscopy and a discharge nozzle source; follow-up cavity measurements have extended these measurements to as high as 35 GHz. The spectrum of the normal species is dominated by an intense progression of a-type rotational transitions arising from increasing quanta in the Si–O stretch, in which lines up to v = 12 (∼14 500 cm−1) were identified. A structural determination by isotopic substitution and a hyperfine analysis of the Ar–Si17O spectrum both suggest that the complex is a highly fluxional prolate symmetric rotor with a vibrationally averaged structure between T-shaped and collinear in which the oxygen atom lies closer to argon than the silicon atom, much like Ar–CO. To complement the experimental studies, a full dimensional potential and a series of effective vibrationally averaged, two-dimensional potential energy surfaces of Ar + SiO have been computed at the CCSD(T)-F12b/CBS level of theory. The equilibrium structure of Ar–SiO is predicted to be T-shaped with a well depth of 152 cm−1, but the linear geometry is also a minimum, and the potential energy surface has a long, flat channel between 140 and 180°. Because the barrier between the two wells is calculated to be small (of order 5 cm−1) and well below the zero-point energy, the vibrationally averaged wavefunction is delocalized over nearly 100° of angular freedom. For this reason, Ar–SiO should exhibit large amplitude zero-point motion, in which the vibrationally excited states can be viewed as resonances with long lifetimes. Calculations of the rovibrational level pattern agree to within 2% with the transition frequencies of normal and isotopic ground state Ar–SiO, and the putative Ka = ±1 levels for Ar–28SiO, suggesting that the present theoretical treatment well reproduces the salient properties of the intramolecular potential. [ABSTRACT FROM AUTHOR]

A typical broadband rotational spectrum may contain several thousand observable transitions, spanning many species. While these spectra often encode troves of chemical information, identifying and assigning the individual spectra can be challenging. Traditional approaches typically involve visually identifying a pattern. A more modern approach is to apply an automated fitting routine. In this approach, combinations of 3 transitions are searched by trial and error, to fit the A, B, and C rotational constants in aWatson-type Hamiltonian. In this work, we develop an alternative approach--to utilize machine learning to train a computer to recognize the patterns inherent in rotational spectra. Broadband high-resolution rotational spectra are perhaps uniquely suited for pattern recognition, assignment, and species identification using machine learning. Repeating patterns of transition frequencies and intensities are now routinely recorded in broadband chirped-pulse Fourier transform microwave experiments in which both the number of resolution elements and the dynamic range surpass 104. At the same time, these high-resolution spectra are extremely sensitive to molecular geometry with each polar species having a unique rotational spectrum. Here we train the feed forward neural network on thousands of rotational spectra that we calculate, using the rules of quantum mechanics, from randomly generated sets of rotational constants and other Hamiltonian parameters. Reasonable physical constraints are applied to these parameter sets, yet they need not belong to existing species. A trained neural network presented with a spectrum identifies its type (e.g., linear molecule, symmetric top, or asymmetric top) and infers the corresponding Hamiltonian parameters (rotational constants, distortion, and hyperfine constants). The classification and prediction times, about 160 μs and 50 μs, respectively, seem independent of the spectral complexity or the number of molecular parameters. We describe how the network works, provide benchmarking results, and discuss future directions. [ABSTRACT FROM AUTHOR]

The inter- and intra-molecular interactions as they evolve in the course of glassy solidification are studied by broadband dielectric-and Fourier-transform infrared-spectroscopy for oligomeric derivatives of poly(ethylene glycol) derivatives, namely, poly(ethylene glycol) phenyl ether acrylate and poly(ethylene glycol) dibenzoate in the bulk and under confinement in nanoporous silica having mean pore diameters 4, 6, and 8 nm, with native and silanized inner surfaces. Analyzing the spectral positions and the oscillator strengths of specific IR absorption bands and their temperature dependencies enables one to trace the changes in the intra-molecular potentials and to compare it with the dielectrically determined primarily inter-molecular dynamics. Special emphasis is given to the calorimetric glass transition temperature Tg and Tαβ ≈ 1.25Tg, where characteristic changes in conformation appear, and the secondary β-relaxation merges with the dynamic glass transition (α-relaxation). Furthermore, the impact of main chain conformations, inter- and intra-molecular hydrogen bonding, and nanometric confinement on the dynamic glass transition is unraveled. [ABSTRACT FROM AUTHOR]

The rotational spectra of two isotopologues have been recorded and interpreted for the van derWaals complex chlorotrifluoroethylene-argon, by using pulsed jet Fourier transform microwave technique. Argon locates out of the ethylene plane, in the FCCF cavity of chlorotrifluoroethylene. The preference of the CFCl side over CF2 can be attributed to the presence of "π-hole" over the double bond in chlorotrifluoroethylene. Non-covalent interaction analysis indicated that the van derWaals interaction occurs between argon and two sites of chlorotrifluoroethylene (the π-system and the F atom of the CFCl top). The steric effect between Ar and the C--Cl bond repulses the Ar atom toward the C--F side. [ABSTRACT FROM AUTHOR]

Gd3+-based spin labels are useful as an alternative to nitroxides for intramolecular distance measurements at high fields in biological systems. However, double electron-electron resonance (DEER) measurements using model Gd3+ complexes featured a low modulation depth and an unexpected broadening of the distance distribution for short Gd3+-Gd3+ distances, when analysed using the software designed for S = 1/2 pairs. It appears that these effects result from the different spectroscopic characteristics of Gd3+--the high spin, the zero field splitting (ZFS), and the flip-flop terms in the dipolar Hamiltonian that are often ignored for spin-1/2 systems. An understanding of the factors affecting the modulation frequency and amplitude is essential for the correct analysis of Gd3+-Gd3+ DEER data and for the educated choice of experimental settings, such as Gd3+ spin label type and the pulse parameters. This work uses time-domain simulations of Gd3+-Gd3+ DEER by explicit density matrix propagation to elucidate the factors shaping Gd3+ DEER traces. The simulations show that mixing between the |+½, -½> and |-½> +½i states of the two spins, caused by the flip-flop term in the dipolar Hamiltonian, leads to dampening of the dipolar modulation. This effect may be mitigated by a large ZFS or by pulse frequency settings allowing for a decreased contribution of the central transition and the one adjacent to it. The simulations reproduce both the experimental line shapes of the Fourier-transforms of the DEER time domain traces and the trends in the behaviour of the modulation depth, thus enabling a more systematic design and analysis of Gd3+ DEER experiments. [ABSTRACT FROM AUTHOR]

Valuable dynamical and structural information about neat liquid DMSO at ambient conditions can be obtained through a study of low frequency vibrations in the far infrared (FIR), that is, terahertz regime. For DMSO, collective excitations as well as single molecule stretches and bends have been measured by different kinds of experiments such as OHD-RIKES and terahertz spectroscopy. In the present work, we investigate the intermolecular vibrational spectrum of DMSO through three different computational techniques namely (i) the far-infrared spectrum obtained through the Fourier transform of total dipole moment auto-time correlation function, (ii) from the Fourier transform of the translational and angular velocity time autocorrelation functions, and (iii) a quenched normal mode analysis of the parent liquid at 300 K. The three spectra, although exhibit differences among each other, reveal similar features which are in good, semi-quantitative, agreement with experimental results. The study of participation ratio of the density of states obtained from the normal mode analysis shows that the broad spectrum around 100 cm-1 involves collective oscillations of 300-400 molecules. Dipolar solvation dynamics exhibit ultrafast energy relaxation with an initial time constant around 157 fs which can be attributed to the coupling to the collective excitations.We compare the properties of DMSO with those of water vis-a-vis the existence of the low frequency collective modes. Last, we find that the collective excitation spectrum exhibits strong temperature dependence. [ABSTRACT FROM AUTHOR]

Terahertz pulses effects are investigated in an asymmetric top C2v molecule using numerical simulations. The average value of the direction cosine ΦZx is computed solving the time dependent Schrödinger equation for several types of pulses. The H2S molecule taken as a test case is first subject to two short terahertz pulses with a duration smaller than 5 ps, an identical maximum value of the electric field of 2 MV/cm, but a different shape. The thermal average ⟨⟨ΦZx⟩⟩ is calculated for several temperatures, and non-periodic time variations are found even for the lowest temperature. For a given temperature, the maximum orientation achieved is shown to be dependent on the overlap between the absorption spectrum of the molecule and the Fourier transform of the pulse. The maximum orientation is also shown to be closely related to the molecular energy increase. In a second step, the optimal control theory is used to build a 14 ps long few-cycle pulse with the same maximum value of the electric field allowing us to reach a large maximum value of ⟨ΦZx⟩ equal to 0.93. A fairly good understanding of the wavefunction describing the molecule after the pulse was achieved. [ABSTRACT FROM AUTHOR]

Using standard hardware available in chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy, an experimental method is introduced to selectively extract from the microwave spectrum of an otherwise complicated multicomponent mixture a set of transitions due to a single component, thereby speeding spectral assignment. The method operates the broadband chirped-pulse used to excite the sample in the strong-field limit through a combination of high power and control of the sweep rate. A procedure is introduced that leads to selection of three transition frequencies that can be incorporated as a set of resonant sequential single-frequency microwave pulses that follow broadband chirped-pulse excitation, resulting in a reduction in the coherent signal from a set of transitions ascribable to the component of interest. The difference in the CP-FTMW spectrum with and without this set of multi-resonant single-frequency pulses produces a set of transitions that can confidently be assigned to a single component of the mixture, aiding the analysis of its spectrum. The scheme is applied to (i) selectively extract the spectrum of one of five singly 13C-subsituted isotopologues of benzonitrile in natural abundance, (ii) obtain the microwave spectra of the two structural isomers (E)- and (Z)-phenylvinylnitrile, and (iii) obtain conformer-specific microwave spectra of methylbutyrate. [ABSTRACT FROM AUTHOR]

The paper deals with a model-free approach to the analysis of quasielastic neutron scattering intensities from anomalously diffusing quantum particles. All quantities are inferred from the asymptotic form of their time-dependent mean square displacements which grow ?ta, with 0 = a < 2. Confined diffusion (a = 0) is here explicitly included. We discuss in particular the intermediate scattering function for long times and the Fourier spectrum of the velocity autocorrelation function for small frequencies. Quantum effects enter in both cases through the general symmetry properties of quantum time correlation functions. It is shown that the fractional diffusion constant can be expressed by a Green-Kubo type relation involving the real part of the velocity autocorrelation function. The theory is exact in the diffusive regime and at moderate momentum transfers. [ABSTRACT FROM AUTHOR]

Small-angle X-ray scattering measurements can provide valuable information about the solvent environment around biomolecules, but it can be difficult to extract solvent-specific information from observed intensity profiles. Intensities are proportional to the square of scattering amplitudes, which are complex quantities. Amplitudes in the forward direction are real, and the contribution from a solute of known structure (and from the waters it excludes) can be estimated from theory; hence, the amplitude arising from the solvent environment can be computed by difference. We have found that this "square root subtraction scheme" can be extended to non-zero q values, out to 0.1 Å-1 for the systems considered here, since the phases arising from the solute and from the water environment are nearly identical in this angle range. This allows us to extract aspects of the water and ion distributions (beyond their total numbers), by combining experimental data for the complete system with calculations for the solutes. We use this approach to test molecular dynamics and integral-equation (3D-RISM (three-dimensional reference interaction site model)) models for solvent structure around myoglobin, lysozyme, and a 25 base-pair duplex DNA. Comparisons can be made both in Fourier space and in terms of the distribution of interatomic distances in real space. Generally, computed solvent distributions arising from the MD simulations fit experimental data better than those from 3D-RISM, even though the total small-angle X-ray scattering patterns are very similar; this illustrates the potential power of this sort of analysis to guide the development of computational models. [ABSTRACT FROM AUTHOR]

Thirteen bands for the B¹Δg-A¹Πu system and eleven bands for the B'¹Σg+-A¹Πu system of C2 were identified in the Fourier transform infrared emission spectra of hydrocarbon discharges. The B'¹Σg+ v = 4 and the B¹Δg v = 6, 7, and 8 vibrational levels involved in nine bands were studied for the first time. A direct global analysis with Dunham parameters was carried out satisfactorily for the B¹Δg-A¹Πu system except for a small perturbation in the B¹Δg v = 6 level. The calculated rovibrational term energies up to B¹Δg v = 12 showed that the level crossing between the B¹Δg and d³Πg states is responsible for many of the prominent perturbations in the Swan system observed previously. Nineteen forbidden transitions of the B¹Δg-a3?u transition were identified and the off-diagonal spin-orbit interaction constant AdB between d³Πg and B¹Δg was derived as 8.3(1) cm-1. For the B'¹Σg+-A¹Πu system, only individual band analyses for each vibrational level in the B'¹Σg+ state could be done satisfactorily and Dunham parameters obtained from these effective parameters showed that the anharmonic vibrational constant wexe is anomalously small (nearly zero). Inspection of the RKR (Rydberg-Klein-Rees) potential curves for the B'¹Σg+ and X¹Σg+ states revealed that an avoided crossing or nearly avoided crossing may occur around 30 000 cm-1, which is responsible for the anomalous molecular constants in these two states. [ABSTRACT FROM AUTHOR]

Two-dimensional vibrational-electronic (2D VE) spectroscopy is a femtosecond Fourier transform (FT) third-order nonlinear technique that creates a link between existing 2D FT spectroscopies in the vibrational and electronic regions of the spectrum. 2D VE spectroscopy enables a direct measurement of infrared (IR) and electronic dipole moment cross terms by utilizing mid-IR pump and optical probe fields that are resonant with vibrational and electronic transitions, respectively, in a sample of interest. We detail this newly developed 2D VE spectroscopy experiment and outline the information contained in a 2D VE spectrum. We then use this technique and its single-pump counterpart (1D VE) to probe the vibrational-electronic couplings between high frequency cyanide stretching vibrations (νCN) and either a ligand-to-metal charge transfer transition ([FeIII(CN)6]3− dissolved in formamide) or a metal-to-metal charge transfer (MMCT) transition ([(CN)5FeIICNRuIII(NH3)5]− dissolved in formamide). The 2D VE spectra of both molecules reveal peaks resulting from coupled high- and low-frequency vibrational modes to the charge transfer transition. The time-evolving amplitudes and positions of the peaks in the 2D VE spectra report on coherent and incoherent vibrational energy transfer dynamics among the coupled vibrational modes and the charge transfer transition. The selectivity of 2D VE spectroscopy to vibronic processes is evidenced from the selective coupling of specific νCN modes to the MMCT transition in the mixed valence complex. The lineshapes in 2D VE spectra report on the correlation of the frequency fluctuations between the coupled vibrational and electronic frequencies in the mixed valence complex which has a time scale of 1 ps. The details and results of this study confirm the versatility of 2D VE spectroscopy and its applicability to probe how vibrations modulate charge and energy transfer in a wide range of complex molecular, material, and biological systems. [ABSTRACT FROM AUTHOR]

We provide a theoretical analysis for the intermediate scattering function typically measured in depolarized dynamic light scattering experiments. We calculate the field autocorrelation function g1VH (Q, t) in dependence on the wave vector Q and the time t explicitly in a vertical-horizontal scattering geometry for differently shaped solids of revolution. The shape of prolate cylinders, spherocylinders, spindles, and double cones with variable aspect ratio is expanded in rotational invariants flm(r). By Fourier transform of these expansion coefficients, a formal multipole expansion of the scattering function is obtained, which is used to calculate the weighting coefficients appearing in the depolarized scattering function. In addition to translational and rotational diffusion, especially the translational-rotational coupling of shape-anisotropic objects is considered. From the short-time behavior of the intermediate scattering function, the first cumulants G(Q) are calculated. In a depolarized scattering experiment, they deviate from the simple proportionality to Q². The coefficients flm(Q) strongly depend on the geometry and aspect ratio of the particles. The time dependence, in addition, is governed by the translational and rotational diffusion tensors, which are calculated by means of bead models for differently shaped particles in dependence on their aspect ratio. Therefore, our analysis shows how details of the particle shape--beyond their aspect ratio-- can be determined by a precise scattering experiment. This is of high relevance in understanding smart materials which involve suspensions of anisotropic colloidal particles. [ABSTRACT FROM AUTHOR]

We have used continuous-wave cavity ring-down and femto-Fourier transform-cavity-enhanced absorption spectrometers to record the spectrum of the OH-stretching + CH-stretching (1 + 2) combination band in trans-formic acid, with origin close to 6507 cm-1. They, respectively, allowed resolving and simplifying the rotational structure of the band near its origin under jet-cooled conditions (Trot = 10 K) and highlighting the overview of the band under room temperature conditions. The stronger B-type and weaker A-type subbands close to the band origin could be assigned, as well as the main B-type Q branches. The high-resolution analysis was hindered by numerous, severe perturbations. Rotational constants are reported with, however, limited physical meaning. The 1 + 2 transition moment is estimated from relative intensities to be 24 away from the principal b-axis of inertia. [ABSTRACT FROM AUTHOR]

Nonlinear two-dimensional Fourier transform (2DFT) and linear absorption spectroscopy are used to study the electronic structure and optical properties of excitons in the layered semiconductor GaSe. At the 1s exciton resonance, two peaks are identified in the absorption spectra, which are assigned to splitting of the exciton ground state into the triplet and singlet states. 2DFT spectra acquired for co-linear polarization of the excitation pulses feature an additional peak originating from coherent energy transfer between the singlet and triplet. At cross-linear polarization of the excitation pulses, the 2DFT spectra expose a new peak likely originating from bound biexcitons. The polarization dependent 2DFT spectra are well reproduced by simulations using the optical Bloch equations for a four level system, where many-body effects are included phenomenologically. Although biexciton effects are thought to be strong in this material, only moderate contributions from bound biexciton creation can be observed. The biexciton binding energy of ~2 meV was estimated from the separation of the peaks in the 2DFT spectra. Temperature dependent absorption and 2DFT measurements, combined with "ab initio" theoretical calculations of the phonon spectra, indicate strong interaction with the A'1 phonon mode. Excitation density dependent 2DFT measurements reveal excitation induced dephasing and provide a lower limit for the homogeneous linewidth of the excitons in the present GaSe crystal. [ABSTRACT FROM AUTHOR]

The pure rotational spectra of deuterated propiolic acids (HCCCOOD and DCCCOOH), 1- fluorobenzene (4-d1), and 1,2-difluorobenzene (4-d1) in their ground states have been measured using two Fourier transform microwave (FTMW) spectrometers at the University of Arizona. For 1-fluorobenzene (4-d1), nine hyperfine lines of three different ?J = 0 and 1 transitions were measured to check the synthesis method and resolution. For 1,2-difluorobenzene (4-d1), we obtained 44 hyperfine transitions from 1 to 12 GHz, including 14 different ?J = 0, 1 transitions. Deuterium quadrupole coupling constants along the three principal inertia axes were well determined. For deuterated propiolic acids, 37 hyperfine lines of Pro-OD and 59 hyperfine lines of Pro-CD, covering 11 and 12 different ?J = -1, 0, 1 transitions, respectively, were obtained from 5 to 16 GHz. Deuterium quadrupole coupling constants along the three inertia axes were well resolved for Pro-OD. For Pro-CD, only eQqaa was determined due to the near coincidence of the CD bond and the least principal inertia axis. Some measurements were made using a newer FTMW spectrometer employing multiple free induction decays as well as background subtraction. For 1-fluorobenzene (4-d1) and 1,2-difluorobenzene (4-d1), a very large-cavity (1.2 m mirror dia.) spectrometer yielded very high resolution (2 kHz) spectra. [ABSTRACT FROM AUTHOR]

The microwave spectrum of the mono-fluoro-benzoic acids, 2-fluoro-, 3-fluoro-, and 4-fluoro-benzoic acid have been measured in the frequency range of 4-14 GHz using a pulsed beam Fourier transform microwave spectrometer. Measured rotational transition lines were assigned and fit using a rigid rotor Hamiltonian. Assignments were made for 3 conformers of 2-fluorobenzoic acid, 2 conformers of 3-fluorobenzoic acid, and 1 conformer of 4-fluorobenzoic acid. Additionally, the gas phase homodimer of 3-fluorobenzoic acid was detected, and the spectra showed evidence of proton tunneling. Experimental rotational constants are A(0+) = 1151.8(5), B(0+) = 100.3(5), C(0+) = 87.64(3) MHz and A(0-) = 1152.2(5), B(0-) = 100.7(5), C(0-) = 88.85(3) MHz for the two ground vibrational states split by the proton tunneling motion. The tunneling splitting (E) is approximately 560 MHz. This homodimer appears to be the largest carboxylic acid dimer observed with F-T microwave spectroscopy. [ABSTRACT FROM AUTHOR]

We report the development of a new instrument that combines chirped-pulse microwave spectroscopy with a pulsed uniform supersonic flow. This combination promises a nearly universal detection method that can deliver isomer and conformer specific, quantitative detection and spectroscopic characterization of unstable reaction products and intermediates, product vibrational distributions, and molecular excited states. This first paper in a series of two presents a new pulsed-flow de-sign, at the heart of which is a fast, high-throughput pulsed valve driven by a piezoelectric stack actuator. Uniform flows at temperatures as low as 20 K were readily achieved with only modest pumping requirements, as demonstrated by impact pressure measurements and pure rotational spec-troscopy. The proposed technique will be suitable for application in diverse fields including fun-damental studies in spectroscopy, kinetics, and reaction dynamics. [ABSTRACT FROM AUTHOR]

Employing the multilayer multiconfiguration time-dependent Hartree (ML-MCTDH) method in conjunction with the multistate multimode vibronic coupling Hamiltonian (MMVCH) model, we perform a full dimensional (9D) quantum dynamical study on the simplest Criegee intermediate, formaldehyde oxide, in five lower-lying singlet electronic states. The ultraviolet (UV) spectrum is then simulated by a Fourier transform of the auto-correlation function. The MMVCH model is built based on extensive MRCI(8e,8o)/aug-cc-pVTZ calculations. To ensure a fast convergence of the final calculations, a large number of ML-MCTDH test calculations is performed to find an appropriate multilayer separations (ML-trees) of the ML-MCTDH nuclear wave functions, and the dynamical calculations are carefully checked to ensure that the calculations are well converged. To compare the computational efficiency, standard MCTDH simulations using the same Hamiltonian are also performed. A comparison of the MCTDH and ML-MCTDH calculations shows that even for the present not-too-large system (9D here) the ML-MCTDH calculations can save a considerable amount of computational resources while producing identical spectra as the MCTDH calculations. Furthermore, the present theoretical B¹A' ← X¹A' UV spectral band and the corresponding experimental measurements [J. M. Beames, F. Liu, L. Lu, and M. I. Lester, J. Am. Chem. Soc.134, 20045-20048 (2012); L. Sheps, J. Phys. Chem. Lett.4, 4201-4205 (2013); W.-L. Ting, Y.-H. Chen, W. Chao, M. C. Smith, and J. J.-M. Lin, Phys. Chem. Chem. Phys.16, 10438-10443 (2014)] are discussed. To the best of our knowledge, this is the first theoretical UV spectrum simulated for this molecule including nuclear motion beyond an adiabatic harmonic approximation. [ABSTRACT FROM AUTHOR]

The absolute photoabsorption cross section of propyne was recorded between 62 000 and 88 000 cm−1 by using the vacuum-ultraviolet, Fourier-transform spectrometer at the Synchrotron Soleil. This cross section spans the region including the lowest Rydberg bands and extends above the Franck-Condon envelope for ionization to the ground electronic state of the propyne cation, X ̃+. Room-temperature spectra were recorded in a flowing cell at 0.9 cm−1 resolution, and jet-cooled spectra were recorded at 1.8 cm−1 resolution and a rotational temperature of ∼100 K. The reduced widths of the rotational band envelopes in the latter spectra reveal new structure and simplify a number of assignments. Although nf Rydberg series have not been assigned previously in the photoabsorption spectrum of propyne, arguments are presented for their potential importance, and the assignment of one nf series is proposed. As expected from previous photoelectron spectra, Rydberg series are also observed above the adiabatic ionization threshold that converge to the v3+ = 1 and 2 levels of the C≡C stretching vibration. [ABSTRACT FROM AUTHOR]

Long-lived coherences (LLC's) are detectable magnetisation modes with favourable relaxation times that translate as sharp resonances upon Fourier transform. The frequency domain of LLC's was previously limited to the range of J-couplings within pairs of homonuclear spins. LLC evolution at high magnetic fields needs to be sustained by radio-frequency irradiation. We show that LLC-based spectral dispersion can be extended beyond the J-couplings domain using adapted carrier offsets and introduce a new reduced-power sustaining method to preserve LLC's within the required range of offsets. Spectral resolution is enhanced as the natively narrow lines of LLC's are further dispersed, making them potential probes for the study of biomolecules featuring strong resonance overlap and for media where NMR spectroscopy is commonly hindered by line broadening. [ABSTRACT FROM AUTHOR]

The kinetics of homogenization of binary AsxSe100 − x melts in the As concentration range 0% < x < 50% are followed in Fourier Transform (FT)-Raman profiling experiments, and show that 2 g sized melts in the middle concentration range 20% < x < 30% take nearly two weeks to homogenize when starting materials are reacted at 700 ◦C. In glasses of proven homogeneity, we find molar volumes to vary non-monotonically with composition, and the fragility index M displays a broad global minimum in the 20% < x < 30% range of x whereinM< 20. We show that properly homogenized samples have a lower measured fragility when compared to larger under-reacted melts. The enthalpy of relaxation at Tg, ΔHnr(x) shows a minimum in the 27% < x < 37% range. The super-strong nature of melt compositions in the 20% < x < 30% range suppresses melt diffusion at high temperatures leading to the slow kinetics of melt homogenization. [ABSTRACT FROM AUTHOR]

The rotational spectra of six isotopologues of a complex of ethyne and silver acetylide, C2H2· · ·AgCCH, are measured by both chirped-pulse and Fabry-Perot cavity versions of Fouriertransform microwave spectroscopy. The complex is generated through laser ablation of a silver target in the presence of a gas sample containing 1% C2H2, 1% SF6, and 98% Ar undergoing supersonic expansion. Rotational, A0, B0, C0, and centrifugal distortion ΔJ and ΔJK constants are determined for all isotopologues of C2H2· · ·AgCCH studied. The geometry is planar, C2v and T-shaped in which the C2H2 sub-unit comprises the bar of the “T” and binds to the metal atom through its π electrons. In the r0 geometry, the distance of the Ag atom from the centre of the triple bond in C2H2 is 2.2104(10) Å. The r(HC≡CH) parameter representing the bond distance separating the two carbon atoms and the angle, ∠(CCH), each defined within the C2H2 sub-unit, are determined to be 1.2200(24) Å and 186.0(5)◦, respectively. This distortion of the linear geometry of C2H2 involves the hydrogen atoms moving away from the silver atom within the complex. The results thus reveal that the geometry of C2H2 changes measurably on coordination to AgCCH. A value of 59(4) Nm−1 is determined for the intermolecular force constant, kσ , confirming that the complex is significantly more strongly bound than hydrogen and halogen-bonded analogues. Ab initio calculations of the re geometry at the CCSD(T)(F12∗)/ACVTZ level of theory are consistent with the experimental results. The spectra of the 107Ag13C13CH and 109Ag13C13CH isotopologues of free silver acetylide are also measured for the first time allowing the geometry of the AgCCH monomer to be examined in greater detail than previously. [ABSTRACT FROM AUTHOR]