LiH state-to-state rotationally inelastic cross sections in collisions with HCl and DCl

A new technique is presented for the determination of state-to-state rotationally inelastic cross sections, which employs electric quadrupole state selection and laser fluorescence detection. Results for the scattering of 7LiH j=1 molecules by HCl and DCl scattering gas (?rel= 0.74 eV) are reported. With knowledge of the number density and scattering path length l and the observed invariance of the experimental cross sections with decreased l, absolute integral state-to-state cross sections σj=1→j′ have been obtained. For both 7LiH–HCl and 7LiH–DCl, the σj=1→j′ values are large (10–200 Å2), the j′=2 level being most probable; the cross sections fall monotonically with increasing j′ out to j′=6, the highest level probed. An unsuspected isotopic enhancement has been observed for the DCl system: All the σj=1→j′ values are ∼25% larger for 7LiH–DCl than for the corresponding transitions in the 7LiH–HCl system. Because both scattering partners have relatively large dipole moments, these systems are paradigms for rotational energy transfer induced by a dipole–dipole interaction. Several theoretical models, which have been widely used to interpret energy transfer in similar systems, are reviewed and compared to these detailed experimental results. It is found that the Born approximation gives cross sections which are far too large and predicts insufficient flux into the higher j′ states, even when higher-order electrostatic terms are included in the potential. The sudden approximation and the recently introduced adiabatically corrected sudden (ACS) approximation reproduce the experimental cross sections much better both in magnitude and in dependence on j′. The ACS method does significantly better and, in particular, successfully predicts the isotopic enhancement observed in the 7LiH–DCl system. The origin of this effect is shown to be due to energetic resonances: For dipole–dipole transitions of the type ja=1, jb→ja±1, jb∓1, the translational energy defect is less in DCl than HCl for those values of jb which made the largest contributions to the 298 °K Boltzmann rotational distribution of the hydrogen halides.