Rotational excitation in the small-angle scattering of protons from diatomic molecules

Abstract

Data for the small-angle scattering of protons from several polar and nonpolar molecules in the energy range 10 to 30 eV indicate that the presence of a dipole moment in the neutral molecule greatly enhances the rotational excitation process in ion-molecule collisions. Within our experimental energy resolution, no rotational excitation was observable for proton collisions with H2, HD, D2, or N2. There is, however, some evidence for a small amount of rotational excitation of the dipolar molecule CO, and for HCl and HF large amounts of rotational excitation were observed. The inelasticity was found to be greater for HF than for HCl, consistent with the relative dipole moments and moments of inertia of the two molecules. The average excitation energies for HF measured at fixed scattering angle are qualitatively in agreement with a classical perturbation calculation for pure rotational excitation. For HCl, the classical perturbation calculation gives a rotational excitation energy much lower than the average observed translational energy loss, indicating that concurrent vibrational excitation, though not resolved in the presence of rotational excitation, may be relatively more important for HCl than for HF.