Experimental and theoretical study of rotationally inelastic polar molecule collisions: 7LiH–HCN

Abstract

This article describes the study of rotational energy transfer in the strongly polar LiH–HCN system. A supersonic beam of LiH, rotationally state selected in ja=1 by an electric quadrupole field, is scattered by HCN gas at room temperature. Laser fluorescence detection is used to determine integral cross sections for ja=1→j′a LiH transitions. The measured cross sections (in Å2) are 245±30 (2σ), 519±88, 222±47, 125±26, 64±17, and 41±12 for j′=0,2,3,4,5, and 6, respectively. The large magnitudes of the cross sections reflect the strong, long-range dipolar coupling. A comparison is made with the predictions of various theoretical models, based on the approximate solution of the time-dependent classical-path equations of motion for rectilinear trajectories. The usual Born approximation significantly overestimates the cross sections for the dipole allowed (ja→ja±1) transitions, even when statistical microreversibility is imposed. Worse, extension of the Born approximation to second order appears to introduce even larger errors. Better agreement with experiment is obtained within the sudden approximation, although the predicted ja=1→j′a=0,2 cross sections are still too large, due to the breakdown of this approximation at large impact parameter for this long-range system. This can be remedied in the adiabatically corrected sudden (ACS) approximation, which we have recently developed, by the introduction of an effective dephasing frequency into the sudden action integral. In a confirmation of our earlier study of the LiH–HCl, DCl systems, we find that the ACS cross sections are in good agreement with the experimental values, especially for the strongest transitions.