Ballistic mechanism for vibrational and rotational energy transfer in Ar + CsI collisions

Velocity and angular distributions have been measured for scattering from crossed beams of Ar and CsI. The Ar beam was generated by the seeded-nozzle technique, using dilution with H2 and varying the source temperature to obtain collision energies from 0.35 to 1.1 eV. The CsI beam was generated by thermal effusion at ∼ 1000 ° K. Mass spectrometric analysis of the scattered CsI was carried out for both the parent CsI+ ion and the fragment Cs+ ion. The velocity spectra for Cs+ show very pronounced peaks located near the ArCsI centroid. These peaks correspond to an extremely inelastic, ``ballistic'' process in which most of the initial relative translational energy goes into vibrational or rotational excitation. The velocity spectra for CsI+ agree with Cs+ in the elastic region but show almost no inelastic peak. This is consistent with the ballistic process, since highly vibrationally excited molecules are likely to be readily fragmented by electron bombardment. Kinematic analysis of the data indicates the fractional energy transfer Δ E/E> 90 % for the wide-angle inelastic scattering. The energy transfer decreases appreciably at smaller angles. The intensity of inelastic scattering into the forward hemisphere is roughly a factor of 2 larger than that into the backward hemisphere. The angular distribution of elastic scattering also has an unusual shape, with a pronounced minimum apparently due to attenuation by the inelastic scattering. The total cross section for ballistic energy transfer is about 20 Å2. An optical model treatment shows the angular distributions are consistent with an inelastic transition probability which is low for small impact parameters and high for large impact parameters. Vibrational and rotational energy transfer are shown to be comparable for impact parameters that give the maximum transition probability, near b ∼ 2.7 ± 0.7 Å. At smaller b the excitation is primarily vibrational, at larger b primarily rotational. The Δ E/E predicted for a completely impulsive hard-sphere collision (determined solely by mass ratios) is only 40%. This suggests the ballistic mechanism differs qualitatively from Landau-Teller; it may involve a resonant or quasibound ArCsI complex.