Penning ionization of 1-bromoadamantane and bromocyclohexane by collision with He*(2³S) metastable atoms: Spin-orbit coupling effect and anisotropic interaction around bromine atom

The He I ultraviolet photoelectron spectra and Penning ionization electron spectra for 1-bromoadamantane and bromocyclohexane are measured and assigned on the basis of the outer valence Green's function calculations. Spin-orbit splitting is observed clearly for the lone pair orbitals localized on the Br atoms. Slope parameters of collision energy dependence of partial ionization cross sections for two split branches in the bromocyclohexane spectra are nearly equal. Two possible explanations are discussed by a strong spin-orbit coupling effect and the coexistence of two isomers. The former breaks down the spatial electron distribution picture of molecular orbital in electron energy spectra; the latter is supported by the calculated results of anisotropic interaction around the Br atom using the unrestricted second-order Møller-Plesset perturbation theory with the 6-31+G(d,p) basis set. By comparison of the anisotropic interactions around the Br and Cl atoms, a shielding effect by the hydrocarbon group which leads to decreasing attractive interactions around the substituted Br atom is found to be relatively weak even for the large bromohydrocarbons.