Two-dimensional Penning ionization electron spectroscopy of CH₃I and CH₂I₂ by He*(2³S) metastable atoms

Penning ionization of iodomethane (CH₃I) and diiodomethane (CH₂I₂) upon collision with metastable He*(2 ³S) atoms was studied by collision-energy/electron-energy resolved two-dimensional Penning ionization electron spectroscopy (2D-PIES) over a wide range of collision energies (20 - 350 meV). Collision energy dependence of partial ionization cross sections (CEDPICS) indicates that the interaction with He*(2³S) atoms around the valence molecular orbitals is attractive at low collision energies except for the only σ_CH molecular orbital region of iodomethane which has a repulsive character in the normal collision energy range (70 - 350 meV). Large bands at lower electron energies (3.7 - 5.5 eV) in PIES spectra were ascribed to autoionization from a transient ion-pair. The related entrance ionization channel has an attractive character. The excitation transfer, which is responsible for the I** autoionization, takes place in regions close to carbon atoms and is repulsive towards He*(2³S) in the normal collision energy range. The superexcited complex of (He-CH₂I₂)** autoionizes via the excitation transfer occurring close to the σ_CI regions of the targets.