Collisional depolarization of OH(A) with Ar: Experiment and theory

Abstract

Zeeman quantum beat spectroscopy has been used to measure the 300 K rate constants for the angular momentum depolarization of OH(A (2)Sigma(+)) in the presence of Ar. We show that the beat amplitude at short times, in the absence of collisions, is well described by previously developed line strength theory for (1+1) laser induced fluorescence. The subsequent pressure dependent decay of the beat amplitude is used to extract depolarization rate constants and estimates of collisional depolarization cross sections. Depolarization accompanies both inelastic collisions, giving rise to rotational energy transfer, and elastic collisions, which change m(j) but conserve j. Previous experimental studies, as well as classical theory, suggest that elastic scattering contributes around 20% to the observed total depolarization rate at low j. Simulation of the experimental beat amplitudes, using theoretical calculations presented in the preceding paper, reveals that depolarization of OH(A) by Ar has a rate constant comparable to, if not larger than, that for energy transfer. This is consistent with a significant tilting or realignment of j(') away from j on collision. The experimental data are used to provide a detailed test of quantum mechanical and quasiclassical trajectory scattering calculations performed on a recently developed ab initio potential energy surface of Kłos et al. [J. Chem. Phys. 129, 054301 (2008)]. The calculations and simulations account well for the observed cross sections at high N, but underestimate the experimental results by between 10% and 20% at low N, possibly due to remaining inaccuracies in the potential energy surface or perhaps to limitations in the dynamical approximations made, particularly the freezing of the OH(A) bond.

Elastic depolarization and polarization transfer in CN(A2Π,v= 4)+Ar collisions

“…17,[19][20][21][22][23]33 Comparison with full quantum scattering calculations on accurate ab-initio PESs has helped to unravel the influence of different parts of the PES on RET and elastic depolarization. Brouard and co-workers have used an independent technique, Zeeman quantum beat spectroscopy, to study elastic and inelastic depolarization in the OH(A)-Ar system, [23][24][25]34 and most recently NO(A 2 Σ + )-Ar/He. 35 For OH(A), excellent processes.…”

“…Experiments have used various techniques under both controlled collision conditions [15][16][17][18][19][20][21][22][23][24][25][26] and in atmospheric pressure flames. [27][28][29][30][31] We have exploited Polarization Spectroscopy, a 3 rd -order non-linear spectroscopic method, 16,32 to measure rate constants for elastic depolarization of both orientation and alignment of OH(X 2 Π or A 2 Σ + ) in collisions with rare gases and simple molecular colliders.…”

Depolarizing Collisions With Hydrogen: Neutral and Singly Ionized Alkaline Earths

“…The total depolarizing rate of the αJ level is defined as the term within the brackets in Equation (9), according to Kerkeni (2002), Brouard et al (2009), and Dagdigian & Alexander (2009) …”

Rotationally inelastic scattering of methyl radicals with Ar and N2

“…Previous studies of inelastic scattering with Ar concentrated mostly on NO, [8][9][10][31][32][33][34][35][36][37][38][39] OH, 18,21,22,24 and HCl, 40 although scattering of a polyatomic molecule with Ar has been explored in the case of NH 3 . 41,42 Molecular nitrogen was previously used as a collider in studies of the inelastic scattering of HCl.…”

“…6 This body of work extended the scope of inelastic scattering of labile free radicals, which had previously concentrated on collisions of diatomic radicals with atomic species. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] A recent article by Dagdigian reviewed collisional energy transfer calculations for small hydrocarbon intermediates 26 and highlighted computational studies of integral cross sections for collisions of methylene (CH 2 ) 27,28 and methyl radicals. 5,28,29 The methyl radical is also the first polyatomic free radical to be slowed in a Zeeman decelerator using a pulsed magnetic field, 30 opening up new possibilities to study inelastic and reactive scattering at very low collision energies.…”