Vyacheslav S. Vikhrenko

Molecular dynamics simulation of vibrational relaxation of highly excited molecules in fluids. II. Nonequilibrium simulation of azulene in CO2 and Xe

Results of nonequilibrium molecular dynamics simulations of vibrational energy relaxation of azulene in carbon dioxide and xenon at low and high pressure are presented and analyzed. Simulated relaxation times are in good agreement with experimental data for all systems considered. The contribution of vibration–rotation coupling to vibrational energy relaxation is shown to be negligible. A normal mode analysis of solute-to-solvent energy flux reveals an important role of high-frequency modes in the process of vibrational energy relaxation. Under all thermodynamic conditions considered they take part in solvent-assisted intramolecular energy redistribution and, moreover, at high pressure they considerably contribute to azulene-to-carbon dioxide energy flux. Solvent-assisted (or collision-induced) intermode energy exchange seems to be the main channel, ensuring fast intramolecular energy redistribution. For isolated azulene intramolecular energy redistribution is characterized by time scales from several to ...

Nonequilibrium molecular dynamics simulations of vibrational energy relaxation of HOD in D2O

Vibrational energy relaxation of HOD in deuterated water is investigated performing classical nonequilibrium molecular dynamics simulations. A flexible SPC/E model is employed to describe the intermolecular interactions and the intramolecular potential of the D(2)O solvent. A more accurate intramolecular potential is used for HOD. Our results for the OH stretch, OD stretch, and HOD bend vibrational relaxation times are 2.7, 0.9, and 0.57 ps, respectively. Exciting the OH stretching mode the main relaxation pathway involves a transition to the bending vibration. These results are in agreement with recent semiclassical Landau-Teller calculations. Contrary to this previous work, however, we observe a strong coupling of bending and OH stretching mode to the HOD rotation. As a result almost half of the total vibrational energy is transferred through the HOD rotation to the bath. At the same time the most efficient acceptor mode is the D(2)O rotation indicating the importance of resonant libration-to-libration energy transfer. We also find significant vibrational excitation of the D(2)O bending mode of the D(2)O solvent by V-V energy transfer from the HOD bending mode.

Molecular dynamics simulation of heat conduction through a molecular chain.

This work deals with a molecular dynamics simulation analysis of the intramolecular vibrational energy transfer in a system of two chromophores, azulene and anthracene, bridged by an aliphatic chain and is motivated by corresponding laser experiments. After selective excitation of the azulene chromophore, the subsequent intramolecular vibrational energy redistribution is monitored by analyzing the transient temperatures of the two chromophores and the chain between them. The main focus concerns the heat conduction process in the chain. Therefore, the chain length was varied from 0 to 19 CH(2) units. In addition, methoxymethyl, 1,2-dimethoxyethyl, and a thiomethoxymethyl chains were studied. The investigation of the intramolecular vibrational energy process was decomposed into a temporal analysis and a spatial analysis. For short alkyl chains, the time constant of energy relaxation increases proportionally to the chain length. However, for longer chains, the time constant characterizing the energy decay of the azulene chromophore saturates and becomes independent of the chain length. This behavior is consistent with experimental findings. The spatial analysis shows more or less exponential decay of the temperature along the chain near the excited chromophore. In additional simulations, the two chromophores were thermostatted at different temperatures to establish a constant heat flux from the azulene to the anthracene side. The steady-state temperature profiles for longer alkyl chains show strong gradients near the two chromophores and constant but weak gradients in the central part of the chain. Both simulation methods indicate that strong Kapitza effects at the boundaries between each chromophore and the molecular chain dominate the intramolecular energy flux.

Molecular dynamics simulation of vibrational energy relaxation of highly excited molecules in fluids. I. General considerations

Methods of implementation of classical molecular dynamics simulations of moderate size molecule vibrational energy relaxation and analysis of their results are proposed. Two different approaches are considered. The first is concerned with modeling a real nonequilibrium cooling process for the excited molecule in a solvent initially at equilibrium. In addition to the solute total, kinetic, and potential energy evolution, that define the character of the process and the rate constant or relaxation time, a great deal of important information is provided by a normal mode specific analysis of the process. Expressions for the decay of the normal mode energies, the work done by particular modes, and the vibration–rotation interaction are presented. The second approach is based on a simulation of a solute–solvent system under equilibrium conditions. In the framework of linear nonequilibrium statistical thermodynamics and normal mode representation of the solute several expressions for the rate constant are derived. In initial form, they are represented by integrals of the time correlation functions of the capacities of the solute–solvent interaction atomic or normal mode forces and include the solute heat capacity. After some approximations, which are adequate for specific cases, these expressions are transformed to combinations of those for individual oscillators with force–force time correlation functions. As an attempt to consider a strongly nonequilibrium situation we consider a two-temperature model and discuss the reason why the rate constant can be independent on the solute energy or temperature. Expressions for investigation of the energy redistribution in the solvent are derived in two forms. One of them is given in the usual form of a heat transfer equation with the source term describing the energy flux from the excited solute. The other form describes the energy redistribution in the solvent in terms of capacity time correlation functions and can be more convenient if memory effects and spatial dispersion play an important role in energy redistribution in the solvent.

Mode specificity of vibrational energy relaxation of azulene in CO2 at low and high density

Abstract Results of the normal mode analysis of classical molecular dynamics simulations of azulene vibrational energy relaxation in carbon dioxide are presented for the first time. There is strong evidence for different energy transfer mechanisms dominating at low and high densities. At low pressure only the low-frequency vibrational modes of the solute participate in the energy transfer to the solvent whereas at high pressure almost all the vibrational modes contribute to the solute-to-solvent energy flow. The relevance of these results for experimental studies is discussed.

Molecular dynamics simulation of vibrational energy relaxation of highly excited molecules in fluids. III. Equilibrium simulations of vibrational energy relaxation of azulene in carbon dioxide

The expressions for vibrational energy relaxation (VER) rates of polyatomic molecules in terms of equilibrium capacity time correlation functions (TCFs) derived in the first paper of this series [J. Chem. Phys. 110, 5273 (1999)] are used for the investigation of VER of azulene in carbon dioxide at low (3.2 MPa) and high (270 MPa) pressure. It is shown that for both cases the VER times evaluated on the basis of the same potential model via solute–solvent interaction capacity TCFs by means of equilibrium molecular dynamics (EMD) simulations satisfactorily agree with the nonequilibrium (NEMD) molecular dynamics [J. Chem. Phys. 110, 5286 (1999)] and experimental [J. Chem. Phys. 105, 3121 (1996)] results as well. Thus it follows that these methods can complement each other in characterizing VER from different points of view. Although more computational power and refined methods of dealing with simulated data are required for EMD simulations, they allow the use of powerful tools of equilibrium statistical mecha...

Microscopic description of vibrational energy relaxation in supercritical fluids: on the dominance of binary solute-solvent contributions.

The representation of the frequency dependent friction coefficient ia a four-particle two-time correlation function is used to analyze the applicability of collisional and hydrodynamical models of vibrational energy relaxation (VER). The solute–solvent binary dynamics is separated from collective equilibrium correlations by means of Greens functions. The collective contributions manifest themselves mainly ia the solute–solvent radial distribution function (RDF), which reflects peculiarities of the particular solvent thermodynamical (e.g., n supercritical) state. The binary dynamics is also closely related to many-body equilibrium correlations, as initial conditions sample microscopic system states in the vicinity of the solute which are the most important for n VER. VER rates along a close to critical isotherm are calculated on the basis of the breathing sphere model n and the Douglas approximation for force–time correlation functions, while Monte Carlo simulations are used n for calculating RDFs. The results are compared with molecular dynamics simulations at low, intermediate and n high densities. It is shown that at near-critical conditions as well as far from the critical point the key contribution to VER comes from the short and intermediate time behavior of the force–time correlation function. In configuration space only short range binary solute–solvent correlations are important. Analytical n estimations, Monte Carlo and molecular dynamics simulations clearly show that the dynamics of VER can only be understood on the basis of a detailed description of local solute–solvent interactions and correlations.

Vibrational cooling of a highly excited anharmonic oscillator: Evidence for strong vibration–rotation coupling during relaxation

The n model of a diatomic molecule in a monoatomic solvent bath is considered to directly elucidate the role n of vibration–rotation coupling for vibrational energy relaxation in liquids. Capacities of solute–solvent interactions n as well as of Coriolis and centrifugal forces are used as a means for displaying different energy exchange n channels and manifesting the strength of vibration–rotation coupling. The ensemble averaged energy exchange n between different degrees of freedom during specific solute vibrational phases is investigated. As an aside, n we show that in the reduced density regime 0.5 to 0.75 the scaling of the frequency dependent frictional response n with solvent density is practically the same for all effective n vibrational n frequencies explored n by the anharmonic oscillator during energy relaxation.

Mode-specific energy absorption by solvent molecules during CO2 vibrational cooling

Non-equilibrium molecular dynamics (NEMD) simulations of energy transfer from vibrationally excited CO(2) to CCl(4) and CH(2)Cl(2) solvent molecules are performed to identify the efficiency of different energy pathways into the solvent bath. Studying in detail the work performed by the vibrationally excited solute on the different solvent degrees of freedom, it is shown that vibration-to-vibration (V-V) processes are strongly dominant and controlled by those accepting modes which are close in frequency to the CO(2) bend and symmetric stretch vibration.

Molecular dynamics modeling of cooling of vibrationally highly excited carbon dioxide produced in the photodissociation of organic peroxides in solution

Non-equilibrium (NEMD) and equilibrium (EMD) molecular dynamics simulations are performed to investigate the vibrational cooling and asymmetric stretch spectral evolution of highly excited carbon dioxide produced in the photodissociation of organic peroxides in the solvents dichloromethane, carbon tetrachloride and xenon. Due to strong Fermi resonance the symmetric stretching and bending modes of carbon dioxide in CH2Cl2 and CCl4 jointly relax on a ten and hundred picosecond timescale, respectively, which is in accordance with experiment. However, the high frequency CO2 asymmetric stretch vibration relaxes on a considerably longer time scale because of weak interaction with the other modes. The relaxation rate coefficients of (and works done by) different modes obtained from NEMD and the Landau-Teller rate coefficients calculated through equilibrium force time correlation functions are in reasonable agreement. The analysis of these results leads to the conclusion that, in contrast to xenon where the relaxation takes about 20 ns, the shorter time scales in CH2Cl2 and CCl4 are caused by efficient near resonant vibration to vibration energy transfer from carbon dioxide to solvent molecules. The results of the non-equilibrium simulations are used to monitor the quasi-stationary asymmetric stretch infrared spectra of carbon dioxide during the cooling process. Comparison of the corresponding experimental results suggests that carbon dioxide initially is produced with a broad distribution of energy disposed in its bend and symmetric stretch modes while the asymmetric stretch mode remains unexcited.