Time-of-flight studies of rotational quantum excitation of H2 in collision with protons at Ec.m.=3.7 eV

Abstract

In a crossed beam apparatus, a monoenergetic pulsed proton ion beam is scattered from a nozzle beam of n‐H₂ or p‐H₂ in their rotational ground states. The energy loss of the scattered ions is analyzed using a time‐of‐flight technique. Rotational quantum transitions corresponding to Δ j up to 20 could be resolved in the angular region of 0<ϑ?55°, which extends up to the rainbow angle. Only a small amount of vibrational excitation is obtained at E_c.m.=3.7 eV. The measured time‐of‐flight spectra have been transformed into the center of mass system to provide distributions of relative transition probabilities. These agree qualitatively with the predictions of Giese and Gentry based on Monte Carlo averaged classical trajectories for 10 eV. The measured probability distribution has a maximum for elastic scattering (Δj=0) at all angles. It drops to between 5% and 35% for Δj=2 transitions, and for most angles decreases only gradually with increasing Δj. At center of mass angles of 15° and 23°, the rotational probability distribution shows a second distinct maximum at Δj=8. At angles greater than 27°, the transition probability distribution falls off nearly monotonically. The first moment of the energy loss distribution shows two maxima at about 23° and 30°. The largest maximum at 30°, which is roughly one‐half the rainbow angle, was predicted by Giese and Gentry.