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

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 (X) over tilde E-2 ((2)A'-(2)A) 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 (A) over tilde (2)A and (B) over tilde E-2 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(1)(-1) ionization and to double excitations corresponding essentially to the 2e(-2) 4a(1)(1) and 3a(1)(-1)2e(-1)4a(1)(1) configurations. The PIMS study allowed us to investigate in detail the ionization and dissociation of CH3I+ leading to CH2+, CH3+, 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 CH3+, 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 (X) over tilde E-2 state and autoionizing dissociation from the (E-2(3/2))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 (A) over tilde (2)A(1) and (B) over tilde E-2 states of CH3I+, autoionization is also suggested to contribute to the fragmentation in all decay channels.