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A detailed three-dimensional time-dependent quantum dynamical study of the He+H2+(v=0–3,j=0)→HeH++H reaction is reported for different vibrational v states of H2+ in its ground rotational (j=0) state over a range of translational Etrans energies on an accurate ab initio potential energy surface published by Palmieri et al. Plots of reaction probability as a function of total energy E reveal a large number of oscillations indicating the presence of a number of reactive scattering resonances. When averaged over total angular momentum J, some of the oscillations survive, indicating that they may be amenable to experimental observation. A comparison of our present results with our earlier results on the McLaughlin–Thompson–Joseph–Sathyamurthy surface and the experimental results from different research groups reveal a good deal of agreement as well as some discrepancies between theory and experiment at the level of state-selected gas phase dynamics. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A theoretical and experimental study of the gas phase and liquid acetic acid based on resonant inelastic x-ray scattering (RIXS) spectroscopy is presented. We combine and compare different levels of theory for an isolated molecule for a comprehensive analysis, including electronic and vibrational degrees of freedom. The excitation energy scan over the oxygen K-edge absorption reveals nuclear dynamic effects in the core-excited and final electronic states. The theoretical simulations for the monomer and two different forms of the dimer are compared against high-resolution experimental data for pure liquid acetic acid. We show that the theoretical model based on a dimer describes the hydrogen bond formation in the liquid phase well and that this bond formation sufficiently alters the RIXS spectra, allowing us to trace these effects directly from the experiment. Multimode vibrational dynamics is accounted for in our simulations by using a hybrid time-dependent stationary approach for the quantum nuclear wave packet simulations, showing the important role it plays in RIXS. [ABSTRACT FROM AUTHOR]

The influence of ring-puckering on the light-induced ring-opening dynamics of heterocyclic compounds was studied on the sample 5-membered ring molecules γ-valerolactone and 5H-furan-2-one using time-resolved photoelectron spectroscopy and ab initio molecular dynamics simulations. In γ-valerolactone, ring-puckering is not a viable relaxation channel and the only available reaction pathway is ring-opening, which occurs within one vibrational period along the C—O bond. In 5H-furan-2-one, the C=C double bond in the ring allows for ring-puckering which slows down the ring-opening process by about 150 fs while only marginally reducing its quantum yield. This demonstrates that ring-puckering is an ultrafast process, which is directly accessible upon excitation and which spreads the excited state wave packet quickly enough to influence even the outcome of an otherwise expectedly direct ring-opening reaction. [ABSTRACT FROM AUTHOR]

The Caldeira-Leggett model provides a compact characterization of a thermal environment in terms of a spectral density function, which has led to a variety of numerically exact quantum methods for reduced density matrix propagation. Since spectral densities are often computed from classical molecular dynamics simulations, we investigate in this paper whether quantum effects should be accounted for in the calculations. Therefore, we reformulate the recently developed Fourier method for spectral density calculations from semiclassical simulations which approximately allow for quantum effects. We propose two possible protocols based on either correlation functions or expectation values. These protocols are tested on a generic Calderra-Leggett model for the linearized semiclassical initial-value representation (LSC-IVR), the thawed Gaussian wave packet dynamics (TGWD), and hybrid schemes combining the two with the more accurate Herman-Kluk formula. Surprisingly, spectral densities from the LSC-IVR method, which treats the dynamics completely classically, are found to be extremely accurate, even in the quantum regime, where this method does not give a correct description of the correlation functions and expectation values. In contrast, the TGWD method turns out as too inaccurate for spectral density calculations, and the hybrid schemes perform well only if the system is close to the classical regime. This implies that, if the bath has a Caldeira-Leggett form, spectral densities are insensitive to quantum effects and any effort to approximately account for them rather leads to errors. Hence, in this case, spectral densities can be computed from classical simulations and used in a reduced quantum simulation as well. [ABSTRACT FROM AUTHOR]

This study presents quantum dynamical simulations, using the Gaussian-based multiconfigurational time-dependent Hartree (G-MCTDH) method, of time-resolved coherent Raman four-wave-mixing spectroscopic experiments for the iodine molecule embedded in a cryogenic crystal krypton matrix [D. Picconi et al., J. Chem. Phys. 150, 064111 (2019)]. These experiments monitor the time-evolving vibrational coherence between two wave packets created in a quantum superposition (i.e., a "Schrödinger cat state") by a pair of pump pulses which induce electronic B Π u 3 0 + ⟵ X Σ g + 1 transitions. A theoretical description of the spectroscopic measurement is developed, which elucidates the connection between the nonlinear signals and the wave packet coherence. The analysis provides an effective means to simulate the spectra for several different optical conditions with a minimum number of quantum dynamical propagations. The G-MCTDH method is used to calculate and interpret the time-resolved coherent Raman spectra of two selected initial superpositions for a I2Kr18 cluster embedded in a frozen Kr cage. The time- and frequency-dependent signals carry information about the molecular mechanisms of dissipation and decoherence, which involve vibrational energy transfer to the stretching mode of the four "belt" Kr atoms. The details of these processes and the number of active solvent modes depend in a non-trivial way on the specific initial superposition. [ABSTRACT FROM AUTHOR]

We have used a generalised coherent state resolution of the identity to map the quantum canonical statistical average for a general system onto a phase-space average over the centre and width parameters of a thawed Gaussian wave packet. We also propose an artificial phase-space density that has the same behaviour as the canonical phase-space density in the low-temperature limit, and have constructed a novel Nosé-Hoover looped chain thermostat that generates this density in conjunction with variational thawed Gaussian wave-packet dynamics. This forms a new platform for evaluating statistical properties of quantum condensed-phase systems that has an explicit connection to the time-dependent Schrödinger equation, whilst retaining many of the appealing features of pathintegral molecular dynamics. [ABSTRACT FROM AUTHOR]

We performed simulations of the prototypical femtosecond 'double-slit' experiment with strong pulsed laser fields for a chromophore in solution. The chromophore is modeled as a system with two electronic levels and a single Franck-Condon active underdamped vibrational mode. All other (intra- and inter-molecular) vibrational modes are accounted for as a thermal bath. The system-bath coupling is treated in a computationally accurate manner using the hierarchy equations of motion approach. The double-slit signal is evaluated numerically exactly without invoking perturbation theory in the matter-field interaction. We show that the strong-pulse double-slit signal consists of a superposition of N-wave-mixing (N = 2, 4, 6...) responses and can be split into population and coherence contributions. The former reveals the dynamics of vibrational wave packets in the ground state and the excited electronic state of the chromophore, while the latter contains information on the dephasing of electronic coherences of the chromophore density matrix. We studied the influence of heat baths with different coupling strengths and memories on the double-slit signal. Our results show that the double-slit experiment performed with strong (nonperturbative) pulses yields substantially more information on the photoinduced dynamics of the chromophore than the weak-pulse experiment, in particular, if the bath-induced dephasings are fast. [ABSTRACT FROM AUTHOR]

A practical quantum theory for unifying electronic and nuclear dynamics, which were separated by the Born–Oppenheimer approximation, is proposed. The theory consists of two processes. Nonadiabatic (quantum) electron wavepacket dynamics on branching (non-Born–Oppenheimer) nuclear paths are first constructed. Since these paths are not the classical trajectories, most of the existing semiclassical theories to generate quantum wavepacket do not work. Therefore, we apply our own developed semiclassical wavepacket theory to these generated non-Born–Oppenheimer paths. This wavepacket is generated based on what we call the action decomposed function, which does not require the information of the so-called stability matrix. Thus, the motion of nuclei is also quantized, and consequently the total wave function is represented as a series of entanglement between the electronic and nuclear wavepackets. In the last half of the article, we show the practice to demonstrate how these independent theories can be unified to give electron-nuclear wavepackets in a two-state model. The wavepackets up to the phases and resultant transition probabilities are compared to the full quantum-mechanical counterparts. It turns out that the lowest level approximation to the wavepacket approach already shows a good agreement with the full quantum quantities. Thus, the present theoretical framework gives a basic method with which to study non-Born–Oppenheimer electronic and nuclear wavepacket states relevant to ultrafast chemical events. [ABSTRACT FROM AUTHOR]

We propose a simple and tractable method to treat quantum electron wavepacket dynamics that nonadiabatically couples with “classical” nuclear motions in mixed quantum-classical representation. The electron wavepacket is propagated inducing electronic-state mixing along our proposed paths. It has been shown in our previous studies that classical force working on nuclei in a nonadiabatic region is represented in a matrix form (called the force matrix), and the solutions of the Hamilton canonical equations of motion for nuclei based on this force matrix give rise to a cascade of infinitely many branching paths when solved simultaneously with electronic-state mixing. As a tractable approximation to these rigorous solutions, we here devise a method to provide much simpler nonadiabatic paths: (i) extract one or a few number of representative paths by taking an average over the paths in phase space (not averaging over the forces) that should be otherwise undergo the fine branching. (ii) After the nonadiabatic coupling becomes sufficiently small, let these paths naturally branch by running them with their own individual eigenforces (the eigenvalues of the force matrix). Since the eigenforces coincide with the forces of adiabatic potential energy surfaces in the limit of zero nonadiabatic coupling, these branching paths eventually run on one of possible adiabatic potential energy surfaces, converging to a classical path (Born–Oppenheimer path). The paths thus created are theoretically satisfactory in that they realize the coherent mixing of electronic states in the manner of quantum entanglement and yet eventually become consistent with the Born–Oppenheimer classical trajectories. We test the present method numerically with the use of two- and three-state systems that are extracted from ab initio calculations for the excited states of LiH molecule. [ABSTRACT FROM AUTHOR]

We illustrate how the preparation and probing of rotational Raman wave packets in O2 detected by time-dependent degenerate four-wave mixing (TD-DFWM) can be manipulated by an additional time-delayed control pulse. By controlling the time delay of this field, we are able to induce varying amounts of additional Rabi cycling among multiple rotational states within the system. The additional Rabi cycling is manifested as a change in the signal detection from homodyne detected to heterodyne detected, depending on the degree of rotational alignment induced. At the highest laser intensities, Rabi cycling among multiple rotational states cannot account for the almost complete transformation to a heterodyne-detected signal, suggesting a second mechanism involving ionization. The analysis we present for these effects, involving the formation of static alignment by Rabi cycling at moderate laser intensities and possibly ion gratings at the highest intensities, appears to be consistent with the experimental findings and may offer viable explanations for the switching from homodyne to heterodyne detection observed in similar DFWM experiments at high laser field intensities (>1013 W/cm2). [ABSTRACT FROM AUTHOR]

We develop a wave packet interferometry description of multidimensional ultrafast electronic spectroscopy for energy-transfer systems. After deriving a general perturbation-theory-based expression for the interference signal quadrilinear in the electric field amplitude of four phase-locked pulses, we analyze its form in terms of the underlying energy-transfer wave packet dynamics in a simplified oriented model complex. We show that a combination of optical-phase cycling and polarization techniques will enable the experimental isolation of complex-valued overlaps between a "target" vibrational wave packet of first order in the energy-transfer coupling J, characterizing the one-pass probability amplitude for electronic energy transfer, and a collection of variable "reference" wave packets prepared independently of the energy-transfer process. With the help of quasiclassical phase-space arguments and analytic expressions for local signal variations, the location and form of peaks in the two-dimensional interferogram are interpreted in terms of the wave packet surface-crossing dynamics accompanying and giving rise to electronic energy transfer. [ABSTRACT FROM AUTHOR]

The fourth-rank tensorial nonlinear response function which controls the response of a chromophore in the condensed phase to three arbitrarily polarized external fields is calculated. Internal (vibrational and rotational) degrees of freedom are incorporated using wave packets in Liouville space (the doorway/window picture). The rotational contribution of the chromophore is expressed in terms of a conditional probability for the rotational diffusion model. We apply this formalism to transient dichroism and birefringence spectroscopies. [ABSTRACT FROM AUTHOR]

The A˜ continuum photoexcitation of ICN in an Ar matrix is studied using an implementation of the molecular dynamics with quantum transitions method of Tully. Five excited electronic potential energy surfaces of the ICN molecule, [sup 3]Π[sub 0+], [sup 1]Π[sub 1](A[sup ′],A[sup ″]), [sup 3]Π[sub 1](A[sup ′],A[sup ″]), as well as its ground state, are included in these calculations. The couplings between electronic states at large I-CN internuclear distances are modeled using a diatomic in molecules treatment of the mixing of the different spin-orbit states of iodine induced by the Ar atoms. The electronic motion, as well as the I-CN distance and the corresponding bending angle, are treated quantum mechanically using wave-packet techniques. The rotation and translation of the ICN molecule in the Ar cage are treated classically, as well as the motion of the Ar atoms. In contrast with previous calculations, in which all nuclear degrees of freedom were treated classically, we found a 2% of CN cage exit during the first 0.5 ps of the dynamics. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]