La Dynamique de Photoionisation de l'Iodure de Methyle (CH3I): une étude par spectroscopie photoélectronique et par spectrométrie de masse.

The threshold photoelectron (TPES) and the photoionization mass spectrometric (PIMS) study of CH3I in the 8-20 eV photon energy range are presented. The interpretation and assignments are supported by ab initio calculations. The TPES shows many new discrete features in the Jahn-Teller split ground X 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 A 2A and B 2E ionic states is ascribed to large autoionizing contributions. Based on the present calculations, the weak to very weak bands in the 18.0 eV-23.0 eV photon energy range are mainly assigned to 2a1-1 ionization and to double excitations corresponding essentially to the 2e-2 4a11 and 3a1- 1 2e-1 4a11 configurations. The photoionization mass spectrometric 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 are concentrated in the 12.2-12.7 eV photon energy range. All three exit channels are correlated with the ground state of CH3I+ via nonadiabatic transitions. All three fragment ions have to appear through predissociation of the ionic X 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 [1]. At higher energies, beside direct or predissociation of the A 2A1 and B 2E states of CH3I+, autoionization is also suggested to contribute to the fragmentation in all decay channels