About the photoionization of methyl bromide (CH 3Br). Photoelectron and photoionization mass spectrometric investigation

Abstract

The threshold photoelectron (TPES) and the photoionization mass spectrometric study of CH 3Br 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 ˜ 2 E ( 2 A ′ – 2 A ″ ) of CH 3Br +. 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 ˜ 2 A and B ˜ 2 E 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 2 a 1 - 1 ionization and to double excitations described essentially by the 2 e - 2 4 a 1 1 and 1 e - 1 2 e - 1 4 a 1 1 configurations. The photoionization mass spectrometric study allowed us to investigate in detail the ionization and dissociation of CH 3Br + leading to CH 2 + , CH 3 + , Br + and CH 2Br + from threshold up to 20 eV photon energy. The experimental data are compared to ab initio dissociation energies. At the onset, the CH 3 + and CH 2Br + fragment ion production is correlated with the ground state of CH 3Br + and both fragment ions have to appear through dissociative autoionization from the ( 3 a 1 1 / 1 e 3 ) 6 s 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 ˜ 2 A 1 and B ˜ 2 E states of CH 3Br +, 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 CH 3 + the photoion-pair process is analyzed and detailed assignments are proposed on the basis of our latest VUV photoabsorption spectroscopic data.