Collisional Relaxation of the Three Vibrationally Excited Difluorobenzene Isomers by Collisions with CO<sub>2</sub>:  Effect of Donor Vibrational Mode

199

Molecular dynamics simulations were used to study relaxation of a vibrationally excited C6F6* molecule in a N2 bath. Ab initio calculations were performed to develop N2-N2 and N2-C6F6 intermolecular potentials for the simulations. Energy transfer from "hot" C6F6 is studied versus the bath density (pressure) and number of bath molecules. For the large bath limit, there is no heating of the bath. As C6F6* is relaxed, the average energy of C6F6* is determined versus time, i.e., ⟨E(t)⟩, and for each bath density ⟨E(t)⟩ is energy dependent and cannot be fit by a single exponential. In the long-time limit C6F6 is fully equilibrated with the bath. For a large bath and low pressures, the simulations are in the fixed temperature, independent collision regime and the simulation results may be compared with gas phase experiments of collisional energy transfer. The derivative d[⟨E(t)⟩]/dt divided by the collision frequency ω of the N2 bath gives the average energy transferred from C6F6* per collision ⟨ΔE(c)⟩, which is in excellent agreement with experiment. For the ~100-300 ps simulations reported here, energy transfer from C6F6* is to N2 rotation and translation in accord with the equipartition model, with no energy transfer to N2 vibration. The energy transfer dynamics from C6F6* is not statistically sensitive to fine details of the N2-C6F6 intermolecular potential. Tests, with simulation ensembles of different sizes, show that a relatively modest ensemble of only 24 trajectories gives statistically meaningful results.

Section: Introductionmentioning

confidence: 99%

A unified model for simulating liquid and gas phase, intermolecular energy transfer: N2 + C6F6 collisions

Paul

Kohale

Pratihar

et al. 2014

199

“…There have been numerous studies on CET between highly vibrationally excited polyatomic molecules and various bath gases. Previous studies include highly vibrationally excited benzene, [1][2][3] deuterated and halogenated benzenes, 1,[3][4][5][6][7] alkyl benzenes, 8 azulene, 9 naphthalene, 10 cycloheptatrienes, 11 and pyrazine 12 in a range of atomic and molecular baths including noble gases, 1 CO 2 , [3][4][5]12,13 N 2 , 6,7 and H 2 O. 14 The photophysics of highly vibrationally excited benzene has been the subject of numerous studies.…”

Section: Introductionmentioning

confidence: 99%

“…7,31 Often a functional form is chosen for P(E, E ′ ) to describe the energy transfer probability for collisions that transfer energy from the vibrationally excited molecule to the bath. Then, detailed balance, Equation (4), is utilized to determine P(E ′ , E), which describes the energy transfer probability for collisions that transfer energy from the bath to the vibrationally excited molecule,…”

Section: Introductionmentioning

confidence: 99%

Resolving the energy and temperature dependence of C6H6∗ collisional relaxation via time-dependent bath temperature measurements

West

Winner

Bowersox

et al. 2016

The relaxation of highly vibrationally excited benzene, generated by 193 nm laser excitation, was studied using the transient rotational-translational temperature rise of the N2 bath, which was measured by proxy using two-line laser induced fluorescence of seeded NO. The resulting experimentally measured time-dependent N2 temperature rises were modeled with MultiWell based simulations of Collisional Energy Transfer (CET) from benzene vibration to N2 rotation-translation. We find that the average energy transferred in benzene deactivating collisions depends linearly on the internal energy of the excited benzene molecules and depends approximately linearly on the N2 bath temperature between 300 K and 600 K. The results are consistent with experimental studies and classical trajectory calculations of CET in similar systems.

“…These include infrared (IR) fluorescence detection, 7 high-resolution IR laser transient absorption spectroscopy, 8 time-resolved diode laser spectroscopy, 9 time-resolved ultraviolet absorption spectroscopy, 10 time-sliced velocity map ion imaging in molecular beams, 11 and laser-Schlieren 12 measurements. Most of the experimental measurements using these techniques have been carried out at low pressures for which a collision of the excited molecule with the buffer gas is rare and well isolated from any subsequent collisions of the excited molecule.…”

Section: Introductionmentioning

confidence: 99%

Pressure effects on the relaxation of an excited nitromethane molecule in an argon bath

Rivera‐Rivera

Wagner

Sewell

et al. 2015

Classical molecular dynamics simulations were performed to study the relaxation of nitromethane in an Ar bath (of 1000 atoms) at 300 K and pressures 10, 50, 75, 100, 125, 150, 300, and 400 atm. The molecule was instantaneously excited by statistically distributing 50 kcal/mol among the internal degrees of freedom. At each pressure, 1000 trajectories were integrated for 1000 ps, except for 10 atm, for which the integration time was 5000 ps. The computed ensemble-averaged rotational energy decay is ∼100 times faster than the vibrational energy decay. Both rotational and vibrational decay curves can be satisfactorily fit with the Lendvay-Schatz function, which involves two parameters: one for the initial rate and one for the curvature of the decay curve. The decay curves for all pressures exhibit positive curvature implying the rate slows as the molecule loses energy. The initial rotational relaxation rate is directly proportional to density over the interval of simulated densities, but the initial vibrational relaxation rate decreases with increasing density relative to the extrapolation of the limiting low-pressure proportionality to density. The initial vibrational relaxation rate and curvature are fit as functions of density. For the initial vibrational relaxation rate, the functional form of the fit arises from a combinatorial model for the frequency of nitromethane "simultaneously" colliding with multiple Ar atoms. Roll-off of the initial rate from its low-density extrapolation occurs because the cross section for collision events with L Ar atoms increases with L more slowly than L times the cross section for collision events with one Ar atom. The resulting density-dependent functions of the initial rate and curvature represent, reasonably well, all the vibrational decay curves except at the lowest density for which the functions overestimate the rate of decay. The decay over all gas phase densities is predicted by extrapolating the fits to condensed-phase densities.