A new crossed beam approach to vibrationally inelastic scattering in neutral collision systems. State-to-state cross sections for He+I*2 (υ′=15, 25, 35) at Ec.m.=89 meV

Abstract

Details are given of a new type of crossed beam experiment that is designed to study state-resolved vibrationally inelastic scattering of neutral atoms from neutral diatomics or polyatomics. The molecule is initially excited to a selected vibrational level in an excited electronic state by laser pumping. Dispersed fluorescence from molecules that have undergone inelastic scattering is used to observe the final vibrational state distributions. The technique is used here to characterize the vibrationally inelastic scattering of He from each of three initial levels υ′=15, 25 and 35 within the B0+u state of I2 . The center-of-mass collision energy is 89 meV (720 cm−1). Thirty cross sections (relative, not absolute) for vibrationally inelastic scattering are obtained. The cross sections characterize processes with vibrational quantum changes extending to Δυ=±3 for the initial level υ′=15, to Δυ=±5 for υ′=25, and to Δυ=±7 for υ′=35. Relative cross sections for pure rotationally inelastic scattering within the initially pumped υ′ levels are also reported. The data analysis incorporates much new information about I2 predissociation, spectroscopy, and Franck–Condon factors that has become available within the last ten years. An error analysis of the cross sections has been performed so that they are suitable benchmarks for theoretical calculation. Procedures are given for estimating fluorescence signal intensities in experiments of this type. The signal expectations are consistent with the I2 observations and suggest that extension to certain polyatomics is quite feasible.