About the photoionization of methyl chloride: A threshold photoelectron spectroscopic and photoionization mass spectrometric investigation Original Research Article

The threshold photoelectron spectroscopic (TPES) and the photoionization mass spectrometric (PIMS) study of methyl chloride (CH3Cl) 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 2E (2A′–2A′′) state of CH3Cl+ and a new continuous band peaking at 13.4 eV. These were assigned to both direct ionization and autoionizing transitions. The transitions to the à 2A and to the B 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 2a1−1 ionization and to double excitations described essentially by the 2e−2, 4a11 and 1e−12e−14a11 configurations. The PIMS study allowed us to investigate in detail the ionization and dissociation of CH3Cl+ into CH2+, CH3+, CHCl+ and CH2Cl+ from threshold up to 20 eV photon energy. At the onset, the CH3+, CHCl+ and CH2Cl+ fragment ion production is correlated to the ground state of CH3Cl+ 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 à 2A1 and B 2E states of CH3Cl+ autoionization also contributes to the fragmentation in all decay channels. For CH3+ the photoion-pair process is analyzed and detailed assignments are proposed on the basis of recent PAS data.