The photoionization dynamics of methyl iodide (CH3I): a joint photoelectron and mass spectrometric investigation

Abstract

Threshold photoelectron (TPES) and photoionization mass spectrometric (PIMS) studies of CH3I in the 8–20 eV photon energy range are presented. The interpretation and assignments are supported by ab initio calculations. The TPES study shows many new discrete features in the Jahn–Teller split ground 2E (2A′–2A″) state of CH3I+. A new continuous band starting at about 11.7 eV is detected. These observations are essentially correlated with autoionizing transitions. This interpretation is supported by constant ion state (CIS) spectroscopy. A large enhancement of the transitions to the 2A and 2E ionic states is ascribed to large autoionizing contributions. Based on the present calculations, the weak to very weak bands in the 18.0–23.0 eV photon energy range are mainly assigned to 2a−11 ionization and to double excitations corresponding essentially to the 2e−2 4a11 and 3a−112e−14a11 configurations. The PIMS study allowed us to investigate in detail the ionization and dissociation of CH3I+ leading to CH+2, CH+3, I+ and CH2I+ from the threshold up to 20 eV photon energy. The experimental data are compared to ab initio calculated dissociation energies. The threshold of appearance of CH+3, I+ and CH2I+ fragments is concentrated in the 12.2–12.7 eV photon energy range. All three exit channels are correlated with the ground state of CH3I+ via non-adiabatic transitions. All three fragment ions have to appear through predissociation of the ionic 2E state and autoionizing dissociation from the (2E3/2)6p Rydberg state. This interpretation is strongly supported by the photoabsorption spectrum measured recently in the same photon energy range. At higher energies, besides direct or predissociation of the 2A1 and 2E states of CH3I+, autoionization is also suggested to contribute to the fragmentation in all decay channels.