Absolute Rate Constants for Collisional Vibrational Relaxation in Dense Vibrational Regions of S₁ p-Difluorobenzene

Abstract

To gain insight into the possible magnitude of vibrational activation/deactivation rate constants for large molecules with the high vibrational excitation of thermal unimolecular reactions, absolute vibrational relaxation rate constants have been measured in S1 p-difluorobenzene energy regions where the vibrational levels begin to form a quasi-continuum. The measured rate constants define vibrational energy transfer from three initially pumped S1 levels into the surrounding vibrational field. The observed rate constants are about 60% of the Lennard-Jones value for the collision partner Ar and about 40% for He. The initial levels lie in the εvib range 2887 to 3310 cm-1, where the level densities are approximately 800−2000 per cm-1. New He rate constants are also measured for many lower initial levels. When combined with earlier measurements, the He and Ar rate constants span the range 0 ≤ εvib ≤ 3310 cm-1. Both sets of rate constants show a trend to larger values with increasing εvib and have prominent variations in response to the zero order quantum identity of the initial level. When the high level density starts to create overlapping states, the Ar rate constants for the three highest levels appear to have leveled off, and the initial quantum state variations are damped out. The He rate constants, on the other hand, sustain the trend to larger values even at the highest energies where the data are ambiguous on whether initial quantum state sensitivity persists.

Non-Stern−Volmer Quenching of S1 pDFB Fluorescence by O2 and the Charge Transfer Complex

“…We are using O 2 fluorescence quenching as part of a study of collisional vibrational energy transfer from regions of the vibrational manifold with high state densities. 7 In support of the study, we report here a reinvestigation of O 2 fluorescence quenching kinetics designed to gain an understanding of why deviation from the standard Stern-Volmer model occurs. Quenching of S 1 pDFB fluorescence has been characterized over a pressure range spanning more than 4 orders of magnitude, starting with O 2 pressures below one Torr where, on average, less than one hard-sphere collision occurs during the fluorescence lifetime.…”

“…We are using O 2 fluorescence quenching as part of a study of collisional vibrational energy transfer from regions of the vibrational manifold with high state densities . In support of the study, we report here a reinvestigation of O 2 fluorescence quenching kinetics designed to gain an understanding of why deviation from the standard Stern−Volmer model occurs.…”

Collisions of electronically excited molecules: differential cross-sections for rotationally inelastic scattering of NO(A2Σ+) with Ar and He

“…Most studies of collisional dynamics of electronically excited species have focused on measuring quenching rates of excited molecules in thermal samples. Spectroscopic resolution of fluorescence from the excited state has allowed detailed study of rotational and vibrational energy transfer in small and large molecular systems [4][5][6][7][8][9]. Such fluorescence-based measurements of NO(A 2 AE þ ) collisional quenching have also served as useful quantitative diagnostics of chemistry at high pressures [10][11][12].…”

Semiempirical Model of Vibrational Relaxation for Estimating Absolute Rate Coefficients

“…Our focus here is on vibrational relaxation rates, and several experiments have demonstrated that the hard sphere potential is not always appropriate. 24,[28][29][30][31] The Lennard-Jones (6, 12) potential has been demonstrated to contain the essential features required to model various trends in observed vibrational relaxation rates. [32][33][34][35][36][37][38][39] The Lennard-Jones cross section σ LJ is related to the hard sphere cross section via the omega integral, Ω (2,2)* :…”

“…The calculated collision rate is sensitive to the choice of intermolecular potential and hence σ . Our focus here is on vibrational relaxation rates, and several experiments have demonstrated that the hard sphere potential is not always appropriate. ,− The Lennard-Jones (6, 12) potential has been demonstrated to contain the essential features required to model various trends in observed vibrational relaxation rates. − The Lennard-Jones cross section σ LJ is related to the hard sphere cross section via the omega integral, Ω (2,2)* : where Ω (2,2)* contains the temperature dependence of σ LJ . The choice of Ω (2,2)* over other Ω ( l , s )* integrals is to some extent arbitrary, but there is a precedent for its use. − In any case, the difference between the various forms of the Ω ( l , s )* integrals is slight…”