Collision-energy-resolved Penning ionization electron spectroscopy of bromomethanes (CH3Br, CH2Br2, and CHBr3) by collision with He*(23S) metastable atoms.

Ionization of bromomethanes (CH₃Br, CH₂Br₂, and CHBr₃) upon collision with metastable He^*(2 ³S) atoms has been studied by means of collision-energy-resolved Penning ionization electron spectroscopy. Lone-pair (n_Br) orbitals of Br_4p characters have larger ionization cross sections than σ_C-Br orbitals. The collision-energy dependence of the partial ionization cross sections shows that the interaction potential between the molecule and the He^*(2 ³S) atom is highly anisotropic around CH₃Br or CH₂Br₂, while isotropic attractive interactions are found for CHBr₃. Bands observed at electron energies of ∼2 eV in the He^*(2 ³S) Penning ionization electron spectra (PIES) of CH₂Br₂ and CHBr₃ have no counterpart in ultraviolet (He I) photoionization spectra and theoretical (third-order algebraic diagrammatic construction) one-electron and shake-up ionization spectra. Energy analysis of the processes involved demonstrates that these bands and further bands overlapping with σ_C-Br or π_CH2 levels are related to autoionization of dissociating (He⁺-Br^-) pairs. Similarly, a band at an electron energy of ∼1 eV in the He^*(2 ³S) PIES spectra of CH₃Br has been ascribed to autoionizing Br** atoms released by dissociation of (unidentified) excited states of the target molecule. A further autoionization (S) band can be discerned at ∼1 eV below the lone-pair n_Br bands in the He^*(2 ³S) PIES spectrum of CHBr₃. This band has been ascribed to the decay of autoionizing Rydberg states of the target molecule (M^**) into vibrationally excited states of the molecular ion. It was found that for this transition, the interaction potential that prevails in the entrance channel is merely attractive. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]