Collision-energy-resolved Penning ionization electron spectroscopy of bromomethanes (CH₃Br, CH₂Br₂, and CHBr ₃) by collision with He*(2³S) metastable atoms

The collision-energy-resolved Penning ionization electron spectroscopy was used for investigating the occurrence of ionization of bromomethanes upon collision with metastable He*(2³S) atoms. Lone-pair (n _Br) orbitals of Br_4p characters were found to have larger ionization cross sections than σ_c-Br orbitals. The negative collision-energy dependence of the partial ionization cross sections associated to the S* band indicated an attractive interaction for the entrance channel. The results of the collision-energy dependence of the partial ionization cross sections shows that the interaction potential between the molecule and the He excited state atom is highly anisotropic.