Spin-orbit coupling effect and intramolecular orbital interactions: Penning ionization of CH₂BrCl, CHBrCl₂, and CH₂BrCN by collision with He*(2³S) metastable atoms

He*(2³S) Penning ionization electron spectra (PIES) as well as He I ultraviolet photoelectron spectra have been measured for CH₂BrCl, CHBrCl₂, and CH₂BrCN. For the lower ionization-potential bands, spin-orbit split states are assigned in the spectra for CH₂BrCl and CH₂BrCN, whereas valence-orbital, ionic states are assigned for CHBrCl₂. These assignments are based on the fact that the intramolecular orbital interactions in CHBrCl₂ are much stronger than those in the other molecules, whereas the spin-orbit coupling effects predominate for the other two molecules. Collision energy dependence of partial Penning ionization cross sections and the PIES indicate that the magnitudes of the attractive interactions in the electron distribution regions of the lone pair orbitals (n) of the Cl and Br atoms and the N atom in the CN group and π_CN orbitals are in a sequence of n_N ≫ n_Br ∼ π_CN > n_Cl.