[en] Photoionization mass spectrometry ; Threshold photoelectron spectroscopy ; Constant ion state spectroscopy ; Dissociative ionization ; Autoionization ; Predissociation ; Ion-pair process ; Ab initio calculations ; Jahn-Teller splitting
[en] The threshold photoelectron (TPES) and the photoionization mass spectrometric (PIMS) 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 X2E(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 A2A and B2E 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 ionization and to double excitations described essentially by the 2e-24a11 and 1e-12e-14a11 configurations. The photoionization mass spectrometric study allows 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 (3a11/1e1)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 A2A1 and B2E states of CH3Br+, autoionization also contributes to the fragmentation in all decay channels. Avoided crossings in a manyfold 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.
Laboratoire de Dynamique Moléculaire, BESSY
EU Contract n° ERBFMGE-CT97-0123 and R II -CT-2004-506008, ARC Contract n° 99/04-245 and PAI Coontract n° P4/03.