Density dependence of the dephasing and energy relaxation times by computer simulations

Short time scale dynamics and a second correlation between liquid and gas phase chemical rates: diffusion processes in noble gas fluids

A theoretical formula for single-atom diffusion rates that predicts an isothermal correlation relation between the liquid (l) and gas (g) phase diffusion coefficients, D(T, ρl) and D(T, ρg) is developed. This formula is based on a molecular level expression for the atom’s diffusion coefficient, D(T, ρ), and on numerical results for 1715 thermodynamic states of 25 rare gas fluids. These numerical results show that at fixed temperature, T, the decay time, τDIF, which governs the shortest time decay of an appropriate force autocorrelation function, F(t) F0, is density (ρ)-independent. This independence holds since τDIF arises from the ρ-independent shortest time inertial motions of the solvent. The ρ independence implies the following l−g diffusion coefficient correlation equation: D−1(T, ρl) = (ρl/ρg) D−1(T, ρg) [ρl−1F0,l2/ρg−1F0,g2]. This relation is identical in form to the familiar (isolated binary-collision-like) empirical correlation formula for vibrational energy relaxation rate constants. This is because both correlation relations arise from inertial dynamics. Inertial dynamics always determines short-time fluid motions, so it is likely that similar correlation relations occur for all liquid phase chemical processes. These correlation relations will be most valuable for phenomena dominated by short time scale dynamics.

Study on the vibrational energy relaxation of p-nitroaniline, N,N-dimethyl-p-nitroaniline, and azulene by the transient grating method

The vibrational energy dissipation processes of the electronic ground states of p-nitroaniline and N,N-dimethyl-p-nitroaniline have been studied by transient grating spectroscopy with subpicosecond laser pulses. The rise time of the acoustic signal produced by the energy dissipation process of the hot ground state molecule was monitored. The acoustic signal was analyzed by an equation including the acoustic damping. The solvent temperature rise times in various solvents have been determined. The acoustic signals of azulene in previous papers [Y. Kimura et al., J. Chem. Phys. 123, 054512 (2005); 123, 054513 (2005)] were also reanalyzed using this equation. The temperature rise times in all cases are longer than the vibrational energy relaxation times of the solutes determined by the transient absorption measurements. The difference is discussed in terms of the energy transfer pathways from the solute to the solvent. We concluded that both the hydrogen bonding between the solute and the solvent and the lower frequency modes of the solutes play important roles in determining the energy transfer pathway from the solute to the solvent.

Vibrational energy relaxation of azulene studied by the transient grating method. I. Supercritical fluids

The vibrational energy dissipation process of the ground-state azulene in supercritical xenon, carbon dioxide, and ethane has been studied by the transient grating spectroscopy. In this method, azulene in these fluids was photoexcited by two counterpropagating subpicosecond laser pulses at 570 nm, which created a sinusoidal pattern of vibrationally hot ground-state azulene inside the fluids. The photoacoustic signal produced by the temperature rise of the solvent due to the vibrational energy relaxation of azulene was monitored by the diffraction of a probe pulse. The temperature-rise time constants of the solvents were determined at 383 and 298 K from 0.7 to 2.4 in rho(r), where rho(r) is the reduced density by the critical density of the fluids, by the fitting of the acoustic signal based on a theoretical model equation. In xenon, the temperature-rise time constant was almost similar to the vibrational energy-relaxation time constant of the photoexcited solute determined by the transient absorption measurement [D. Schwarzer, J. Troe, M. Votsmeier, and M. Zerezke, J. Chem. Phys. 105, 3121 (1996)] at the same reduced density irrespective of the solvent temperature. On the other hand, the temperature-rise time constants in ethane were larger than the vibrational energy-relaxation time constants by a factor of about 2. In carbon dioxide, the difference was small. From these results, the larger time constants of the solvent temperature rise than those of the vibrational energy relaxation in ethane and carbon dioxide were interpreted in terms of the vibrational-vibrational (V-V) energy transfer between azulene and solvent molecules and the vibrational-translational (V-T) energy transfer between solvent molecules. The contribution of the V-V energy transfer process against the V-T energy transfer process has been discussed.