About the photoionization of methyl bromide (CH3Br). Photoelectron and photoionization mass spectrometric investigation

The threshold photoelectron (TPES) and the photoionization mass spectrometric study of CH3Br in the 8-20 eV photon energy range is presented. The interpretation and assignments are supported by ab initio calculations. The TPES shows several new discrete features in the Jahn-Teller split ground state X˜2E(2A′-2A″) of CH3Br+. An additional continuous band starts at about 11.8 eV. These observations are both correlated with direct ionization and autoionizing transitions. This is supported by constant ion state (CIS) spectroscopy. A large enhancement of the transitions to the A˜2A and B˜2E states is ascribed to important autoionizing contributions. Based on the present calculations, the weak to very weak bands in the 17.5-22.0 eV photon energy range were mainly assigned to 2a1-1 ionization and to double excitations described essentially by the 2e-24a11 and 1e-12e-14a11 configurations. The photoionization mass spectrometric study allowed us to investigate in detail the ionization and dissociation of CH3Br+ leading to CH2+, CH3+, Br+ and CH2Br+ from threshold up to 20 eV photon energy. The experimental data are compared to ab initio dissociation energies. At the onset, the CH3+ and CH2Br+ fragment ion production is correlated with the ground state of CH3Br+ and both fragment ions have to appear through dissociative autoionization from the (3a11/1e3)6s or 5s Rydberg state. This interpretation is supported by the photoabsorption spectrum measured recently in the same photon energy range. At higher energies, beside a likely direct (pre)dissociation of the A˜2A1 and B˜2E states of CH3Br+, autoionization also contributes to the fragmentation in all decay channels. Avoided crossings in a manifold of 2A′ states are likely to be involved. This is supported by ab initio calculations. For CH3+ the photoion-pair process is analyzed and detailed assignments are proposed on the basis of our latest VUV photoabsorption spectroscopic data.