State resolved rotational excitation in D2+H2 collisions

Abstract

In a crossed molecular beam experiment time-of-flight distributions of ortho D2 molecules scattered from normal H2 (nH2) and para H2 (pH2) have been measured in a center-of-mass angular range of 75° to 180°. The collision energies were 84.1 and 87.2 meV, respectively. In all spectra the rotational excitation of D2 from j=0 to j=2 has been resolved. With pH2 as secondary beam the same transition could also be observed for H2. The measurements show that the probability for rotational excitation of D2 depends on whether the scattering partner H2 is rotating (nH2) or not (pH2). In the first case the cross sections are larger by a factor of approximately 2. The reason for this behavior is the presence of an additional interaction term which is at long range distances, identical to the quadrupole–quadrupole interaction and which is absent if H2 is in the j=0 state. The experimentally derived differential cross sections for the rotational excitation of D2 and H2 are compared with theoretical results obtained by close coupling calculations based on the ab initio potential surface of Meyer and Schaefer. The comparison shows a remarkable agreement. However, small deviations in the positions of the diffraction oscillations of the elastic differential cross section curve suggest that the isotropic potential term has to be shifted to smaller distances. In order to maintain the relative position of the inelastic differential cross section curves which are well predicted by the ab initio potential the same shift has to be applied to the anisotropic potential terms.