Two-dimensional Penning Ionization Electron Spectroscopy of CH3I and CH2I2 by He*(2S) Metastable Atoms

Penning ionization of iodomethane (CH3I) and diiodomethane (CH2I2) upon collision with metastable He(2S) 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*(2S) atoms around the valence molecular orbitals is attractive at low collision energies except for the only σCH 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(2S) in the normal collision energy range. The superexcited complex of (He–CH2I2)** autoionizes via the excitation transfer occurring close to the σCI regions of the targets.