Rotational excitation of H₂molecules by Li⁺impact in the energy range from 50-400 eV

With an ion energy-loss spectrometer, of which ultimate energy resolution was improved to be as good as 7.0 meV (FWHM), rotational excitation of H 2 molecules by Li + impact was studied. Above 150 eV, rotational excitation occurs in strongly forward scattering and integral cross sections could be determined from the energy-loss spectra obtained at a scattering angle of 0 degrees . The cross sections for the transitions, J=0 to 2, 1 to 3, 2 to 4, 3 to 5, 2 to 0, 3 to 1, and 1 to 5 were determined. The cross sections with Delta J=2 transitions are of the magnitude of 10 -16 cm 2 and the cross section with the Delta J=4 transition is about two orders of magnitude smaller than the Delta J=2 transition. Rotational excitation and de-excitation cross sections were found to depend strongly on the initial rotational states. This dependence was compared with the first Born approximation and the agreement between experiment and theory is fairly good.