Molecular beam electric resonance analysis of inelastic collisions: Vibrational relaxation of LiF scattered by polyatomic molecules

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Abstract
A thermal beam of LiF molecules at ∼1100°K is crossed by a target gas beam at temperatures ranging from 300–800°K and the LiF scattered at a laboratory angle of 20° is analyzed by electric resonance spectroscopy. The relative populations are measured for the lowest four vibrational states of LiF in the J = 1, MJ = 0 rotational state. With Ar, Xe, N2, O2, NO, CO, HF, HCl, or CO2 as the target beam, the vibrational populations of scattered LiF correspond to a Boltzmann distribution with the same vibrational temperature (within ±30°K) as the unscattered beam. With an H2O beam at ∼300°K, weak relaxation is observed. With target beams of SF6, NH3, ND3, or CHFCl2, marked vibrational relaxation is observed. Comparison with the concurrent elastic scattering gives rough lower bound estimates for the differential cross sections for inelastic scattering: ≳ 0.02 Å2/sr for H2O and NH3 at a center-of-mass scattering angle θ∼45° and ≳ 0.05 Å/sr for CHFCl2 and SF6 at θ∼25°. In each case, the vibrational distribution of the scattered LiF (elastic + inelastic) remains approximately Boltzmann. For the SF6 case, the LiF vibrational temperature varies with the target beam temperature: Tv(LiF)∼800°K with the SF6 at ∼300°K and Tv(LiF)∼950°K with SF6 at ∼500°K.