About the photoionization of methyl chloride A threshold photoelectron spectroscopic and photoionization mass spectrometric investigation

摘要

The threshold photoelectron spectroscopic (TPES) and the photoionization mass spectrometric (PIMS) study of methyl chloride (CH_3Cl) in the 8-20 eV photon energy range is presented. The interpretation and assignments are supported by ab initio calculations. The detailed analysis of the TPES shows numerous features in the Jahn-Teller split ground (X-tilde)~2E (~2A'-~2A") state of CH_3Cl~+ and a new continuous band peaking at 13.4 eV. These were assigned to both direct ionization and autoionizing transitions. The transitions to the (A-tilde)~2A and to the (B-tilde)~2E states undergo a large enhancement ascribed to important autoionizing contributions. Based on the present calculations, the weak to very weak signals in the 19-26 eV photon energy range were mainly assigned to 2a_1~(-1) ionization and to double excitations described essentially by the 2e~(-2), 4a_1~1 and 1e~(-1)2e~(-1)4a_1~1 configurations. The PIMS study allowed us to investigate in detail the ionization and dissociation of CH_3Cl~+ into CH_2~+, CH_3~+, CHCl~+ and CH_2Cl~+ from threshold up to 20 eV photon energy. At the onset, the CH_3~+, CHCl~+ and CH_2Cl~+ fragment ion production is correlated to the ground state of CH_3Cl~+ and all fragment ions have to appear through dissociative autoionization from the 2e → 3p Rydberg state. This interpretation is supported by the photoabsorption spectrum (PAS) measured recently in the same photon energy range. At higher energies, beside direct dissociation of the (A-tilde)~2A_1 and (B-tilde)~2E states of CH_3Cl~+ autoionization also contributes to the fragmentation in all decay channels. For CH_3~+ the photoion-pair process is analyzed and detailed assignments are proposed on the basis of recent PAS data.