Time-of-flight measurements of the rotational excitation of para- and ortho-H2 by collisions with Li+ ions. I. Angular dependent transition probabilities for the j=0→2, 2→0, 2→4, and 1→3 transitions at Ec.m.=0.6 eV

Time-of-flight spectra for the scattering of Li+ ions from para-H2 in the j=0 and j=2 rotational states and from ortho-H2 in the j=1 state have been measured for center of mass scattering angles between 17° and 50° at a center of mass energy of 0.6 eV. In all the spectra two or more maxima were observed. For 100 °K para-H2 they could be attributed to the j=0→0 and j=0→2 transitions. For para-H2 at 300 °K additional maxima due to the j=2→0 and the j=2→4 transitions were observed. By using normal H2 and subtracting off the known contributions from para-H2 the transition probability for the j=1→3 transition was also obtained. The angular resolution was sufficient to resolve the ’’fast’’ undulations as well as the rainbow maximum for both the j=0→0 and j=0→2 transitions. The results are compared directly with differential cross sections calculated by Schaefer and Lester [J. Chem. Phys. 62, 1913 (1975)] using a close coupling procedure with the inclusion of a number of closed channels for the best ab initio CI-type potential hypersurface. The comparison reveals a noticeable discrepancy in the angular location of the rainbow. This difference suggests that the ab initio potential well is too shallow in the experimentally probed region by about 17% to 35%.