Search

Spectrally resolved three-pulse stimulated vibrational echo experiments are used as the basis for structure assignments of the A1 and A3 spectroscopic substates in the IR spectrum of the carbon monoxide (CO) stretch of carbonmonoxymyoglobin (MbCO). The measured dephasing dynamics of these substates is compared to the dephasing dynamics of MbCO predicted from molecular dynam ics (MD) simulations.

The vibrational relaxation in methanol-d, ethanol-d, and 1-propanol-d dissolved in CCl4 calculated with ultra-fast infrared pump-probe experiment is presented. A model in which hydrogen bond dissociation, subsequent to vibrational relaxation, raises the concentration of non-hydrogen bond donating hydroxyl clusters.

Ultrafast infrared frequency selected vibrational echo (FSVE) experiments are used to study temperature-dependent dynamic interactions in liquids, glasses, and proteins. The calculated decay shows substantial agreement with the experimentally measured decays. The FSVE experiment probes protein dynamics and provides an observable that can be used to test structural assignments for the Mb-Co conformational substates.

Multidimensional visible spectroscopy using pulse shaping to produce pulses with stable controllable phases and delays has emerged as an elegant tool to acquire electronic spectra faster and with greatly reduced instrumental and data processing errors. Recent migration of this approach using acousto-optic modulator (AOM) pulse shaping to the mid-infrared region has proved useful for acquiring two dimensional infrared (2D IR) vibrational echo spectra. The measurement of spectral diffusion in 2D IR experiments hinges on obtaining accurate 2D line shapes. To date, pulse shaping 2D IR has not been used to study the time-dependent spectral diffusion of a vibrational chromophore. Here we compare the spectral diffusion data obtained from a standard non-collinear 2D IR spectrometer using delay lines to the data obtained from an AOM pulse shaper based 2D IR spectrometer. The pulse shaping experiments are performed in stationary, partially rotating, and fully rotating reference frames and are the first in the infrared to produce 2D spectra collected in a fully rotating frame using a phase controlled pulse sequence. Rotating frame experiments provide a dramatic reduction in the number of time points that must be measured to obtain a 2D IR spectrum, with the fully rotating frame giving the greatest reduction. Experiments were conducted on the transition metal carbonyl complex tricarbonylchloro(1,10-phenanthroline)rhenium(I) in chloroform. The time dependent data obtained from the different techniques and with different reference frames are shown to be in agreement. [ABSTRACT FROM AUTHOR]

The center line slope (CLS) method is often used to extract the frequency-frequency correlation function (FFCF) from 2D IR spectra to delineate dynamics and to identify homogeneous and inhomogeneous contributions to the absorption line shape of a system. While the CLS method is extremely efficient, quite accurate, and immune to many experimental artifacts, it has only been developed and properly applied to systems that have a single vibrational band, or to systems of two species that have spectrally resolved absorption bands. In many cases, the constituent spectra of multiple component systems overlap and cannot be distinguished from each other. This situation creates ambiguity when analyzing 2D IR spectra because dynamics for different species cannot be separated. Here a mathematical formulation is presented that extends the CLS method for a system consisting of two components (chemically distinct uncoupled oscillators). In a single component system, the CLS corresponds to the time-dependent portion of the normalized FFCF. This is not the case for a two component system, as a much more complicated expression arises. The CLS method yields a series of peak locations originating from slices taken through the 2D spectra. The slope through these peak locations yields the CLS value for the 2D spectra at a given Tw. We derive analytically that for two component systems, the peak location of the system can be decomposed into a weighted combination of the peak locations of the constituent spectra. The weighting depends upon the fractional contribution of each species at each wavelength and also on the vibrational lifetimes of both components. It is found that an unknown FFCF for one species can be determined as long as the peak locations (referred to as center line data) of one of the components are known, as well as the vibrational lifetimes, absorption spectra, and other spectral information for both components. This situation can arise when a second species is introduced into a well characterized single species system. An example is a system in which water exists in bulk form and also as water interacting with an interface. An algorithm is presented for back-calculating the unknown FFCF of the second component. The accuracy of the algorithm is tested with a variety of model cases in which all components are initially known. The algorithm successfully reproduces the FFCF for the second component within a reasonable degree of error. [ABSTRACT FROM AUTHOR]

Water dynamics as reflected by the spectral diffusion of the water hydroxyl stretch were measured in w0 = 2 (1.7 nm diameter) Aerosol-OT (AOT)/water reverse micelles in carbon tetrachloride and in isooctane solvents using ultrafast 2D IR vibrational echo spectroscopy. Orientational relaxation and population relaxation are observed for w0 = 2, 4, and 7.5 in both solvents using IR pump-probe measurements. It is found that the pump-probe observables are sensitive to w0, but not to the solvent. However, initial analysis of the vibrational echo data from the water nanopool in the reverse micelles in the isooctane solvent seems to yield different dynamics than the CCl4 system in spite of the fact that the spectra, vibrational lifetimes, and orientational relaxation are the same in the two systems. It is found that there are beat patterns in the interferograms with isooctane as the solvent. The beats are observed from a signal generated by the AOT/isooctane system even when there is no water in the system. A beat subtraction data processing procedure does a reasonable job of removing the distortions in the isooctane data, showing that the reverse micelle dynamics are the same within experimental error regardless of whether isooctane or carbon tetrachloride is used as the organic phase. Two time scales are observed in the vibrational echo data, 1 and 10 ps. The slower component contains a significant amount of the total inhomogeneous broadening. Physical arguments indicate that there is a much slower component of spectral diffusion that is too slow to observe within the experimental window, which is limited by the OD stretch vibrational lifetime. [ABSTRACT FROM AUTHOR]

The orientational correlation functions measured in the time-resolved second-harmonic generation (TRSHG) and time-resolved sum-frequency generation (TRSFG) experiments are derived. In the laboratory coordinate system, the lm(Ωlab(t))Y2m(Ωlab(0))> (l=1,3 and m=0,2) correlation functions, where the Ylm are spherical harmonics, describe the orientational relaxation observables of molecules at interfaces. A wobbling-in-a-cone model is used to evaluate the correlation functions. The theory demonstrates that the orientational relaxation diffusion constant is not directly obtained from an experimental decay time in contrast to the situation for a bulk liquid. Model calculations of the correlation functions are presented to demonstrate how the diffusion constant and cone half-angle affect the time-dependence of the signals in TRSHG and TRSFG experiments. Calculations for the TRSHG experiments on Coumarin C314 molecules at air-water and air-water-surfactant interfaces are presented and used to examine the implications of published experimental results for these systems. [ABSTRACT FROM AUTHOR]

Ultrafast two-dimensional infrared (2D-IR) vibrational echo spectroscopy can probe the fast structural evolution of molecular systems under thermal equilibrium conditions. Structural dynamics are tracked by observing the time evolution of the 2D-IR spectrum, which is caused by frequency fluctuations of vibrational mode(s) excited during the experiment. However, there are a variety of effects that can produce line shape distortions and prevent the correct determination of the frequency-frequency correlation function (FFCF), which describes the frequency fluctuations and connects the experimental observables to a molecular level depiction of dynamics. In addition, it can be useful to analyze different parts of the 2D spectrum to determine if dynamics are different for subensembles of molecules that have different initial absorption frequencies in the inhomogeneously broadened absorption line. Here, an important extension to a theoretical method for extraction of the FFCF from 2D-IR spectra is described. The experimental observable is the center line slope (CLSωm) of the 2D-IR spectrum. The CLSωm is obtained by taking slices through the 2D spectrum parallel to the detection frequency axis (ωm). Each slice is a spectrum. The slope of the line connecting the frequencies of the maxima of the sliced spectra is the CLSωm. The change in slope of the CLSωm as a function of time is directly related to the FFCF and can be used to obtain the complete FFCF. CLSωm is immune to line shape distortions caused by destructive interference between bands arising from vibrational echo emission, from the 0-1 vibrational transition (positive), and from the 1-2 vibrational transition (negative) in the 2D-IR spectrum. The immunity to the destructive interference enables the CLSωm method to compare different parts of the bands as well as comparing the 0-1 and 1-2 bands. Also, line shape distortions caused by solvent background absorption and finite pulse durations do not affect the determination of the FFCF with the CLSωm method. The CLSωm can also provide information on the cross correlation between frequency fluctuations of the 0-1 and 1-2 vibrational transitions. [ABSTRACT FROM AUTHOR]

A series of pyrene photoacids is used to investigate excited-state proton transfer with time-dependent pump-probe spectroscopy. The deprotonation dynamics of a cationic photoacid, 8-aminopyrene-1,3,6-trisulfonic acid trisodium salt (APTS), shows single exponential dynamics(∼30 ps) in water. This is in contrast to what is observed for the neutral photoacids 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) and 8-hydroxy-N,N,N′,N′,N″,N″-hexamethylpyrene-1,3,6-trisulfonamide, which display biexponential dynamics. For the cationic photoacid, the vast majority of the intramolecular charge redistribution does not occur in the protonated state. Instead, the charge redistribution, which is responsible for the photoacidity and the observed spectroscopic changes, occurs primarily following the excited-state proton transfer. The lack of charge redistribution prior to proton transfer causes APTS to display single exponential kinetics. In contrast, the dynamics for the neutral photoacids are multiexponential because major charge redistribution precedes proton transfer followed by additional charge redistribution that accompanies proton transfer. Previous studies of HPTS in water are discussed in terms of the results presented here. [ABSTRACT FROM AUTHOR]

The photoacid 8-hydroxy-N,N,N′,N′,N′,N′-hexamethylpyrene-1,3,6-trisulfonamide (HPTA) and related compounds are used to investigate the steps involved in excited-state deprotonation in polar solvents using pump-probe spectroscopy and time correlated single photon counting fluorescence spectroscopy. The dynamics show a clear two-step process leading to excited-state proton transfer. The first step after electronic excitation is charge redistribution occurring on a tens of picoseconds time scale followed by proton transfer on a nanosecond time scale. The three states observed in the experiments (initial excited state, charge redistributed state, and proton transfer state) are recognized by distinct features in the time dependence of the pump-probe spectrum and fluorescence spectra. In the charge redistributed state, charge density has transferred from the hydroxyl oxygen to the pyrene ring, but the OH sigma bond is still intact. The experiments indicate that the charge redistribution step is controlled by a specific hydrogen bond donation from HPTA to the accepting base molecule. The second step is the full deprotonation of the photoacid. The full deprotonation is clearly marked by the growth of stimulated emission spectral band in the pump-probe spectrum that is identical to the fluorescence spectrum of the anion. [ABSTRACT FROM AUTHOR]

Ultrafast two-dimensional infrared (2D-IR) vibrational echo spectroscopy can probe structural dynamics under thermal equilibrium conditions on time scales ranging from femtoseconds to ∼100 ps and longer. One of the important uses of 2D-IR spectroscopy is to monitor the dynamical evolution of a molecular system by reporting the time dependent frequency fluctuations of an ensemble of vibrational probes. The vibrational frequency-frequency correlation function (FFCF) is the connection between the experimental observables and the microscopic molecular dynamics and is thus the central object of interest in studying dynamics with 2D-IR vibrational echo spectroscopy. A new observable is presented that greatly simplifies the extraction of the FFCF from experimental data. The observable is the inverse of the center line slope (CLS) of the 2D spectrum. The CLS is the inverse of the slope of the line that connects the maxima of the peaks of a series of cuts through the 2D spectrum that are parallel to the frequency axis associated with the first electric field-matter interaction. The CLS varies from a maximum of 1 to 0 as spectral diffusion proceeds. It is shown analytically to second order in time that the CLS is the Tw (time between pulses 2 and 3) dependent part of the FFCF. The procedure to extract the FFCF from the CLS is described, and it is shown that the Tw independent homogeneous contribution to the FFCF can also be recovered to yield the full FFCF. The method is demonstrated by extracting FFCFs from families of calculated 2D-IR spectra and the linear absorption spectra produced from known FFCFs. Sources and magnitudes of errors in the procedure are quantified, and it is shown that in most circumstances, they are negligible. It is also demonstrated that the CLS is essentially unaffected by Fourier filtering methods (apodization), which can significantly increase the efficiency of data acquisition and spectral resolution, when the apodization is applied along the axis used for obtaining the CLS and is symmetrical about τ=0. The CLS is also unchanged by finite pulse durations that broaden 2D spectra. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulations and quantum mechanical electronic structure calculations are used to investigate the nature and dynamics of the phenol-benzene complex in the mixed solvent, benzene/CCl4. Under thermal equilibrium conditions, the complexes are continuously dissociating and forming. The MD simulations are used to calculate the experimental observables related to the phenol hydroxyl stretching mode, i.e., the two dimensional infrared vibrational echo spectrum as a function of time, which directly displays the formation and dissociation of the complex through the growth of off-diagonal peaks, and the linear absorption spectrum, which displays two hydroxyl stretch peaks, one for the complex and one for the free phenol. The results of the simulations are compared to previously reported experimental data and are found to be in quite reasonable agreement. The electronic structure calculations show that the complex is T shaped. The classical potential used for the phenol-benzene interaction in the MD simulations is in good accord with the highest level of the electronic structure calculations. A variety of other features is extracted from the simulations including the relationship between the structure and the projection of the electric field on the hydroxyl group. The fluctuating electric field is used to determine the hydroxyl stretch frequency-frequency correlation function (FFCF). The simulations are also used to examine the number distribution of benzene and CCl4 molecules in the first solvent shell around the phenol. It is found that the distribution is not that of the solvent mole fraction of benzene. There are substantial probabilities of finding a phenol in either a pure benzene environment or a pure CCl4 environment. A conjecture is made that relates the FFCF to the local number of benzene molecules in phenol’s first solvent shell. [ABSTRACT FROM AUTHOR]

Optically heterodyne-detected optical Kerr effect (OHD-OKE) experiments are conducted to study the orientational dynamics of a discotic liquid crystal 2,3,6,7,10,11-hexakis(pentyloxy)triphenylene (HPT) in the isotropic phase near the columnar-isotropic (C-I) phase transition. The OHD-OKE signal of HPT is characterized by an intermediate power law t-0.76±0.02 at short times (a few picoseconds), a von Schweidler power law t-0.26±0.01 at intermediate times (hundreds of picoseconds), and an exponential decay at long times (tens of nanoseconds). The exponential decay has Arrhenius temperature dependence. The functional form of the total time dependent decay is identical to the one observed previously for a large number of molecular supercooled liquids. The mode coupling theory schematic model based on the Sjögren [Phys. Rev. A 33, 1254 (1986)] model is able to reproduce the HPT data over a wide range of times from <1 ps to tens of nanoseconds. The studies indicate that the HPT C-I phase transition is a strong first order transition, and the dynamics in the isotropic phase display a complex time dependent profile that is common to other molecular liquids that lack mesoscopic structure. [ABSTRACT FROM AUTHOR]

The spectroscopic locations of the 1La and 1Lb electronic states of pyranine (1-hydroxy-3,6,8-pyrenetrisulfonic acid, commonly referred to as HPTS), as well as several related compounds, are found using magnetic circular dichroism spectroscopy as well as absorption and fluorescence spectroscopies. These electronic states have been discussed in connection with the photoacid properties of HPTS. Polarization selective fluorescence spectroscopy is used to identify the transition dipole directions of the electronic states of the compounds studied. The issue of the origin for the changes in vibronic structure of HPTS in different solvents is addressed. It is demonstrated that a Brownian oscillator model, in which the strength of the coupling of the electronic states to the solvent changes with solvent, is sufficient to reproduce the trends in the shapes of the vibronic structure. [ABSTRACT FROM AUTHOR]

A pump-probe spectroscopic study of photoinduced forward electron transfer and geminate recombination between donors and acceptors located in the head group regions of micelles is presented. The hole donor is octadecyl-rhodamine B (ODRB) and the hole acceptor is N,N-dimethyl-aniline (DMA). The experiments are conducted as a function of the DMA concentration in the dodecyltrimethylammonium bromide and tetradecyltrimethylammonium bromide micelles. In spite of the fact that the absorptions of both the ODRB radical and ground state bleach spectrally overlap with the ODRB excited state absorption, a procedure that makes it possible to determine the geminate recombination dynamics is presented. These experiments are the first to measure the dynamics of geminate recombination in micelles, and the experiments have two orders of magnitude better time resolution than previous studies of forward transfer. The experimental data are compared to statistical mechanics theoretical calculations of both the forward transfer and the geminate recombination. The theory includes important aspects of the topology of the micelle and the diffusion of the donor-acceptors in the micelle head group region. A semiquantitative but nonquantitative agreement between theory and experiments is achieved. [ABSTRACT FROM AUTHOR]

The coupled processes of intermolecular photoinduced forward electron transfer and geminate recombination between the (hole) donor (Rhodamine 3B) and (hole) acceptors (N,N-dimethylaniline) are studied in three molecular liquids: acetonitrile, butyronitrile, and benzonitrile. Two color pump-probe experiments on time scales from ∼100 fs to hundreds of picoseconds give information about the depletion of the donor excited state due to forward electron transfer and the survival kinetics of the radicals produced by forward electron transfer. The data are analyzed with a model presented previously that includes distance dependent forward and back electron transfer rates, donor and acceptor diffusion, solvent structure, and the hydrodynamic effect in a mean-field theory of through solvent electron transfer. The forward electron transfer is in the normal regime, and the Marcus equation for the distance dependence of the transfer rate is used. The forward electron transfer data for several concentrations in the three solvents are fitted to the theory with a single adjustable parameter, the electronic coupling matrix element Jf at contact. Within experimental error all concentrations in all three solvents are fitted with the same value of Jf. The geminate recombination (back transfer) is in the inverted region, and semiclassical treatment developed by Jortner [J. Chem. Phys. 64, 4860 (1976)] is used to describe the distance dependence of the back electron transfer. The data are fitted with the single adjustable parameter Jb. It is found that the value of Jb decreases as the solvent viscosity increases. Possible explanations are discussed. [ABSTRACT FROM AUTHOR]

The dynamics of two nematic liquid crystals, 4-(trans-4′-n-octylcyclohexyl)isothiocyanatobenzene and 4-(4-pentyl-cyclohexyl)-benzonitrile, are investigated as a function of temperature both in the homeotropically aligned nematic phase and in the isotropic phase using optical heterodyne-detected optical Kerr effect experiments, which measures the time derivative of the polarizability–polarizability-correlation function (orientational relaxation). Data are presented over a time range of 500 fs-70 μs for the nematic phase and 500 fs to a few hundred nanoseconds for the isotropic phase. The nematic dynamics are compared with a previously studied liquid crystal in the nematic phase. All three liquid crystals have very similar dynamics in the nematic phase that are very different from the isotropic phase. On the slowest time scale (20 ns–70 μs), a temperature-independent power law, the final power law, t-f with f∼0.5, is observed. On short time scales (∼3 ps to ∼1 ns), a temperature-dependent intermediate power law is observed with an exponent that displays a linear dependence on the nematic order parameter. Between the intermediate power law and the final power law, there is a crossover region that has an inflection point. For times that are short compared to the intermediate power law (<=2 ps), the data decay much faster, and can be described as a third power law, although this functional form is not definitive. The isotopic phase data have the same features as found in previous studies of nematogens in the isotropic phase, i.e., the temperature-independent intermediate power law and von Schweidler power law at short to intermediate times, and a highly temperature-dependent long time exponential decay that is well described by the Landau-de Gennes theory. The results show that liquid-crystal dynamics in the nematic phase exhibit universal behavior. [ABSTRACT FROM AUTHOR]

Optical heterodyne-detected optical Kerr effect (OHD-OKE) experimental data are pre-sented on nematogens 4-(trans-4′-n-octylcyclohexyl)isothiocyanatobenzene (8-CHBT), and 4-(4′-pentyl-cyclohexyl)-benzonitrile (5-PCH) in the isotropic phase. The 8-CHBT and 5-PCH data and previously published data on 4′-pentyl-4-biphenylcarbonitrile (5-CB) are analyzed using a modification of a schematic mode coupling theory (MCT) that has been successful in describing the dynamics of supercooled liquids. At long time, the OHD-OKE data (orientational relaxation) are well described with the standard Landau-de Gennes (LdG) theory. The data decay as a single exponential. The decay time diverges as the isotropic to nematic phase transition is approached from above. Previously there has been no theory that can describe the complex dynamics that occur at times short compared to the LdG exponential decay. Earlier, it has been noted that the short-time nematogen dynamics, which consist of several power laws, have a functional form identical to that observed for the short time behavior of the orientational relaxation of supercooled liquids. The temperature-dependent orientational dynamics of supercooled liquids have recently been successfully described using a schematic mode coupling theory. The schematic MCT theory that fits the supercooled liquid data does not reproduce the nematogen data within experimental error. The similarities of the nematogen data to the supercooled liquid data are the motivation for applying a modification of the successful MCT theory to nematogen dynamics in the isotropic phase. The results presented below show that the new schematic MCT theory does an excellent job of reproducing the nematogen isotropic phase OHD-OKE data on all time scales and at all temperatures. [ABSTRACT FROM AUTHOR]

The coupling between the OD stretch v=2 level and benzene-ring modes in 2-methoxyphenol-OD (hydroxyl H replaced by D) is observed with ultrafast two-dimensional (2D) IR vibrational echo spectroscopy. Because of this coupling, the 1-2 transition peak in the 2D spectrum is split into a doublet with peaks of approximately equal amplitudes. Several molecules and solvents were used to study this phenomenon. Near-IR (NIR) spectroscopy measurements and density-functional theory calculations (B3LYP/6-31+G(d,p) level) were also applied. Experimental results and calculations show that the OD stretch 1-2 transition is coupled to a combination band related to the benzene-ring motions. A simple quantum-mechanical model indicates that the combination band has a frequency of 5172 and 5176.5 cm-1 in CCl4 and hexane, respectively. The transition between this combination band and the ground state is too weak to detect by NIR. The transition between this band and the OD stretch first excited state is also so weak that most of the intensity of the doublet comes from the oscillator strength produced by coupling to the OD stretch. The model gives the coupling strengths as 6.5 and 7 cm-1 in CCl4 and hexane, respectively. [ABSTRACT FROM AUTHOR]

Optical heterodyne-detected optical Kerr effect (OHD-OKE) experiments are presented for the supercooled liquid acetylsalicylic acid (aspirin - ASP). The ASP data and previously published OHD-OKE data on supercooled dibutylphthalate (DBP) display highly damped oscillations with a periods of ∼2 ps as the temperature is reduced to and below the mode coupling theory (MCT) temperature TC. The oscillations become more pronounced below TC. The oscillations can be interpreted as the time domain signature of the boson peak. Recently a schematic MCT model, the Sjögren model, was used to describe the OHD-OKE data for a number of supercooled liquids by Götze and Sperl [W. Götze and M. Sperl, Phys. Rev. E 92, 105701 (2004)] , but the short-time and low-temperature behaviors were not addressed. Franosch et al. [T. Franosch, W. Gotze, M. R. Mayr, and A. P. Singh, Phys. Rev. E 55, 3183 (1997)] found that the Sjögren model could describe the boson peak observed by depolarized light-scattering (DLS) experiments on glycerol. The OHD-OKE experiment measures a susceptibility that is a time domain equivalent of the spectrum measured in DLS. Here we present a detailed analysis of the ASP and DBP data over a broad range of times and temperatures using the Sjögren model. The MCT schematic model is able to describe the data very well at all temperatures and relevant time scales. The trajectory of MCT parameters that fit the high-temperature data (no short-time oscillations) when continued below TC results in calculations that reproduce the oscillations seen in the data. The results indicate that increasing translational-rotational coupling is responsible for the appearance of the boson peak as the temperature approaches and drops below TC. [ABSTRACT FROM AUTHOR]

The time-resolved orientational anisotropies of the OD hydroxyl stretch of dilute HOD in H2O confined on a nanometer length scale in sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles are studied using ultrafast infrared polarization and spectrally resolved pump-probe spectroscopy, and the results are compared to the same experiments on bulk water. The orientational anisotropy data for three water nanopool sizes (4.0, 2.4, and 1.7 nm) can be fitted well with biexponential decays. The biexponential decays are analyzed using a wobbling-in-a-cone model that involves fast orientational diffusion within a cone followed by slower, full orientational relaxation. The data provide the cone angles, the diffusion constants for motion within the cones, and the final diffusion constants as a function of the nanopool size. The two processes can be interpreted as a local angular fluctuation of the OD and a global hydrogen bond network rearrangement process. The trend in the relative amplitudes of the long and short exponential decays suggest an increasing rigidity as the nanopool size decreases. The trend in the long decay constants indicates a longer hydrogen bond network rearrangement time with decreasing reverse micelle size. The anisotropy measurements for the reverse micelles studied extrapolate to ∼0.33 rather than the ideal value of 0.4, suggesting the presence of an initial inertial component in the anisotropy decay that is too fast to resolve. The very fast decay component is consistent with initial inertial orientational motion that is seen in published molecular-dynamics simulations of water in AOT reverse micelles. The angle over which the inertial orientational motion occurs is determined. The results are in semiquantitative agreement with the molecular-dynamics simulations. [ABSTRACT FROM AUTHOR]

Vibrational echo correlation spectroscopy experiments on the OD stretch of dilute HOD in H2O are used to probe the structural dynamics of water. A method is demonstrated for combining correlation spectra taken with different infrared pulse bandwidths (pulse durations), making it possible to use data collected from many experiments in which the laser pulse properties are not identical. Accurate measurements of the OD stretch anharmonicity (162 cm-1) are presented and used in the data analysis. In addition, the recent accurate determination of the OD vibrational lifetime (1.45 ps) and the time scale for the production of vibrational relaxation induced broken hydrogen bond “photoproducts” (∼2 ps) aid in the data analysis. The data are analyzed using time dependent diagrammatic perturbation theory to obtain the frequency time correlation function (FTCF). The results are an improved FTCF compared to that obtained previously with vibrational echo correlation spectroscopy. The experimental data and the experimentally determined FTCF are compared to calculations that employ a polarizable water model (SPC-FQ) to calculate the FTCF. The SPC-FQ derived FTCF is much closer to the experimental results than previously tested nonpolarizable water models which are also presented for comparison. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We apply the combined electronic structure/molecular dynamics approach of Corcelli, Lawrence, and Skinner [J. Chem. Phys. 120, 8107 (2004)] to the fluctuating charge (SPC-FQ) model of liquid water developed by Rick, Stuart, and Berne [J. Chem. Phys. 101, 6141 (1994)]. For HOD in H2O the time scale for the long-time decay of the OD stretch frequency time-correlation function, which corresponds to the time scale for hydrogen-bond rearrangement in the liquid, is about 1.5 ps. This result is significantly longer than the 0.9 ps decay previously calculated for the nonpolarizable SPC/E water model. Our results for the SPC-FQ model are in better agreement with recent vibrational echo experiments.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The fifth order contributions to the signals of ultrafast infrared spectrally resolved stimulated vibrational echoes at high intensities have been investigated in carbonmonoxy heme proteins. High intensities are often required to obtain good data. Intensity dependent measurements are presented on hemoglobin-CO (Hb–CO) and a mutant of myoglobin, H64V–CO. The spectrally resolved vibrational echoes demonstrate that fifth order effects arise at both the 1-0 and the 2-1 emission frequencies of the stretching mode of the CO chromophore bound at the active site of heme proteins. Unlike one-dimensional experiments, in which the signal is integrated over all emission frequencies, spectrally resolving the signal shows that the fifth order contributions have a much more pronounced influence on the 2-1 transition than on the 1-0 transition. By spectrally isolating the 1-0 transition, the influence of fifth order contributions to vibrational echo data can be substantially reduced. Analysis of fifth order Feynman diagrams that contribute in the vibrational echo phase-matched direction demonstrates the reason for the greater influence of fifth order processes on the 1-2 transition, and that the fifth order contributions are heterodyne amplified by the third order signal. Finally, it is shown that the anharmonic oscillations in vibrational echo data of Hb–CO that previous work had attributed strictly to fifth order effects arise even without fifth order contributions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Photoinduced electron transfer and geminate recombination are studied for the systems rhodamine 3B (R3B+) and rhodamine 6G (R6G+), which are cations, in neat neutral N,N-dimethylaniline (DMA). Following photoexcitation of R3B+ or R6G+ (abbreviated as R+), an electron is transferred from DMA to give the neutral radical R and the cation DMA+. Because the DMA hole acceptor is the neat solvent, the forward transfer rate is very large, ∼5×1012 s-1. The forward transfer is followed by geminate recombination, which displays a long-lived component suggesting several percent of the radicals escape geminate recombination. Spectrally resolved pump-probe experiments are used in which the probe is a ''white'' light continuum, and the full time-dependent spectrum is recorded with a spectrometer/charge-coupled device. Observations of stimulated emission (excited state decay--forward electron transfer), the R neutral radical spectrum, and the DMA+ radical cation spectrum as well as the ground-state bleach recovery (geminate recombination) make it possible to unambiguously follow the electron transfer kinetics. Theoretical modeling shows that the long-lived component can be explained without invoking hole hopping or spin-forbidden transitions. [ABSTRACT FROM AUTHOR]

A temperature scaling analysis using the same mode coupling theory (MCT) scaling relationships employed for supercooled liquids is applied to optical heterodyne detected optical Kerr effect data for four liquid crystals. The data cover a range of times from ∼1 ps to 100 ns and a range of temperatures from ∼50 K above the isotropic to nematic phase transition temperature T[sub NI] down to ∼T[sub NI]. The slowest exponential component of the data obeys the Landau–de Gennes (LdG) theory for the isotropic phase of liquid crystals. However, it is also found that the liquid crystal data obey MCT scaling relationships, but, instead of a single scaling temperature T[sub C] as found for supercooled liquids, in the liquid crystals there are two scaling temperatures T[sub CL] (L for low temperature) and T[sub CH] (H for high temperature). T[sub CH] is very close to T[sup *], which results from LdG scaling, just below the isotropic to nematic phase transition temperature, T[sub NI], but is 30–50 K higher than T[sub CL]. The liquid crystal time dependent data have the identical functional form as supercooled liquid data, that is, a fast power law decay with temperature independent exponent, followed by a slower power law decay with temperature independent exponent, and on the longest time scales, an exponential decay with highly temperature dependent decay constant. For each liquid crystal, the amplitudes of the two power laws scale with expressions that involve T[sub CL], but the exponential decay time constant (long time dynamics) scales with an expression that involves T[sub CH]. The existence of two scaling temperatures can be interpreted as a signature of two “glass transitions” in liquid crystals. In ideal MCT developed for spheres, T[sub C] is the “ideal glass transition temperature,” although it is found experimentally to be ∼20%–30% above the experimental glass transition temperature, T[sub g]. The transition in nematic liquid crystals at T[sub CL] corresponds to the conventional ideal MCT glass transition, while the transition at T[sub CH] can occur for nonspherical molecules, and may correspond to the freezing in of local nematic order. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Frequency resolved pump-probe experiments have been conducted on the deuterated hydroxyl stretch of methanol-d in a solution containing 0.8% methanol-d/23% methanol-h in carbon tetrachloride. Methanol-d molecules that both donate and receive hydrogen bonds have an inhomogeneously broadened hydroxyl stretch absorption line centered at 2487 cm[SUP-1]. With a laser tuned to 2513 cm[SUP-1], the high-frequency side of the absorption spectrum is excited. The equilibration of the excited state peak and the ground-state hole results in the time-dependent shift in the frequency of the signal, which is used to monitor the dynamics of spectral diffusion. Model calculations were conducted to address the influence of spectral diffusion in the ground and excited states on the experimental observables when the vibrational lifetime is comparable to the spectral diffusion time. The model calculations illustrate the influence on the signal of absorbers in the ground state that have relaxed from the excited state. This aspect of the problem has not been addressed in previous descriptions of frequency resolved pump-probe spectroscopy. The calculations were used to fit the time-dependent peak maximum, resulting in a bi-exponential frequency-frequency correlation function, with a fast time constant of roughly 0.1 ps and a slower time constant of 1.6 ± 0.3 ps. The observed dynamics have been compared with the predictions of dielectric continuum theory. The inability of a simple dielectric continuum theory to predict the observed spectral diffusion dynamics suggests that these dynamics do not result from the long-wavelength, collective orientational relaxation of the solvent. Instead the dynamics are attributed to fluctuations in the local hydrogen bond network, which is consistent with recent molecular-dynamics simulations of vibrational transient hole burning in water. [ABSTRACT FROM AUTHOR]

A comparison is made of the dynamics observed over wide ranges of time and temperature between five supercooled liquids and four isotropic phase liquid crystals that have been previously studied separately. Optical-heterodyne-detected optical Kerr effect (OHD-OKE) measurements were employed to obtain the orientational relaxation dynamics over time scales from sub-ps to tens of ns. For the supercooled liquids, the temperatures range from above the melting point down to ∼ T[SUBc], the mode coupling theory critical temperature. For the liquid crystals, the temperatures range from well above the isotropic-to-nematic phase transition temperature T[SUBNI] down to ∼ T[SUBNI]. For time scales longer than those dominated by intramolecular vibrational dynamics (&lsim;1 ps), the fundamental details of the dynamics are identical. All nine liquids exhibit decays of the OHD-OKE signal that begin (> 1 ps) with a temperature-independent power law t[SUP-z], where z is somewhat less than or equal to 1. The power law decay is followed in both the supercooled liquids and liquid crystals by a crossover region, modeled as a second power law. The longest time scale decay for all nine liquids is exponential. In supercooled liquids, the exponential decay is the a relaxation (complete structural relaxation). In liquid crystals, the exponential decay is the Landau-de Gennes decay (relaxation of pseudonematic domains). As T[SUBc] (supercooled liquids) and T[SUBNI] (liquid crystals) are approached from above, the time range over which the ''intermediate'' power law can be observed increases, until near T[SUBc] and T[SUBNI], the power law can be observed from >1 ps to many ns. The data for all nine liquids are described accurately by the same functional form and exhibit a scaling relation in common. The nature of the dynamics in the liquid crystals is understood in terms of pseudonematic domains that have a correlation length j, which increases as T[SUBNI] is approached. It is... [ABSTRACT FROM AUTHOR]

Dynamics of supercooled ortho-terphenyl, salol, benzophenone, 2-biphenylmethanol, and dibutylphthalate have been studied using optical heterodyne detected optical Kerr effect experiments over a broad range of time, <1 ps to tens of ns. On time scales longer than those influenced by intramolecular vibrational dynamics, "intermediate" power law decays with temperature independent exponents close to -1 have been observed from ∼2 ps to 1-10 ns in all five samples. The intermediate power law decays occur over a wide range of temperatures from well above to somewhat below T[sub c], the mode-coupling theory (MCT) critical temperature. The intermediate power law corresponds to approximately a logarithmic decay of the polarizability-polarizability (orientational) correlation function. The amplitude of the intermediate power law increases with increasing temperature as [(T-T[sub c])/T[sub c]]½. The intermediate power law decay is followed by a second longer time scale power law, and the final portion of the decay is exponential. As a framework for discussion, the results are compared to the quantitative predictions of the MCT. The observations are in contrast to the standard MCT for the longer time portions of the decays. The observed intermediate power law decays may be consistent with MCT if the dynamics occur in the part of the MCT parameter space near a high order singularity (end point case). [ABSTRACT FROM AUTHOR]

Time and polarization resolved ultrafast infrared vibrational spectroscopy of the hydroxyl stretch of methanol dissolved in carbon tetrachloride has been utilized to investigate orientational relaxation and vibrational excitation transfer. The anisotropy decay of the deuterated hydroxyl stretch of methanol-d was measured in two solutions: Isotopically mixed 0.8 mol % methanol-d 23 mol % methanol-h in CCl[SUB4] and isotopically pure methanol-dat 26 mol % in CCl[SUB4]. The anisotropy decay in the isotopically mixed methanol solution is a biexponential characterized by 1.7±0.7 ps and 17±3 ps time constants, with 40±10% of the decay occurring with the slower time constant. The biexponential anisotropy decay has been analyzed with a restricted orientational diffusion model that involves fast orientational diffusion within a cone of semi-angle Θ[SUBc] followed by slower, full orientational relaxation. The fast orientational relaxation occurs within a cone semi-angle of Θ[SUBc]=45°±5°, with a diffusion coefficient of D[SUP-1,SUBc]=13±5 ps. The slower anisotropy decay results from the full orientational diffusion and occurs with a diffusion coefficient of D[SUP-1,SUBTheta;]=100±20 ps. The anisotropy decay for isotopically pure methanol-d in CCl[SUB4] is much faster because of vibrational excitation transfer in addition to the orientational relaxation. The excitation transfer has been successfully analyzed as transition dipole-transition dipole mediated transfer using a theory developed for randomly distributed chromophores. [ABSTRACT FROM AUTHOR]

Thermally induced gauche-trans isomerization and direct solute-solvent interactions of the solute, ethyl isocyanate (EIC), in the solvent, 2-methylpentane (2MP), are investigated using ultrafast infrared vibrational echo experiments and linear vibrational absorption spectroscopy of the isocyanate (N = C = O) antisymmetric stretching mode (2278 cm[sup -1]). Both the EIC vibrational echo measured pure vibrational dephasing and the absorption spectra show complex behavior as a function of temperature from room temperature to 8 K. The EIC data are compared to absorption experiments on the same mode of isocyanic acid (HNCO), which cannot undergo isomerization. To describe the observations, a model is presented that involves both intramolecular dynamics and intermolecular dynamical interactions. At room temperature, gauche-trans isomerization is very fast, and the isomerization dynamics contribution to the vibrational echo decay and the absorption line shape is small because it is motionally narrowed. The dominant contribution to both the vibrational echo decay and the absorption spectrum is from direct dynamical interactions of the solute with the solvent. As the temperature is lowered, the direct contribution to vibrational dephasing decreases rapidly, but the contribution from isomerization increases because the extent of motional narrowing diminishes. The combined effect is a very gradual decrease of the rate of pure dephasing as the temperature is initially lowered from room temperature. At very low temperature, below the 2MP glass transition, isomerization cannot occur. The absorption spectrum displays two peaks, interpreted as the distinct gauche and trans absorption bands. Even at 8 K, the pure dephasing is surprisingly fast. The direct solvent-induced dephasing is negligible. The dephasing is caused by motions of the ethyl group without isomerization occuring. At intermediate temperatures (150 K > T > 100 K), isomerization takes place, but its contribution... [ABSTRACT FROM AUTHOR]