Collision-energy-resolved Penning ionization electron spectroscopy of bromomethanes (CH_3Br,CH_2Br_2,and CHBr_3) by collision with He~*(2~3S) metastable atoms

摘要

Ionization of bromomethanes (CH_3Br,CH_2Br_2,and CHBr_3) 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 sigma_(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 CH_3Br or CH_2Br_2,while isotropic attractive interactions are found for CHBr_3.Bands observed at electron energies of approx 2 eV in the He~*(2 ~35) Penning ionization electron spectra (PIES) of CH_2Br_2 and CHBr_3 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 sigma__C_Br) or pi_(CH_2) levels are related to autoionization of dissociating (He~+-Br~-) pairs.Similarly,a band at an electron energy of approx 1 eV in the He~*(2 ~3S) PIES spectra of CH_3Br 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 approx 1 eV below the lone-pair n_(Br) bands in the He~*(2 ~35) PIES spectrum of CHBr_3.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.