PENNING IONIZATION ELECTRON-SPECTROSCOPY OF DICHLOROBENZENES

He*(2(3)S) Penning ionization electron spectra (PIES) and He I ultraviolet photoelectron spectra (UPS) of omicron-, m-, and p-dichlorobenzenes were measured under the same experimental conditions. The analysis of the relative band intensity of PIES was shown to give useful information on the spatial electron distribution of individual molecular orbitals. On the basis of the characteristics of PIES, all the bands in the UPS of dichlorobenzenes were assigned. Since the Penning ionization process is regarded as an electrophilic reaction of a metastable atom with a molecule, the relative intensity of the bands in PIES is related to the reactivity of the corresponding orbitals. It was confirmed that Penning ionization electron spectroscopy is a useful method for probing the reactivity of n, pi, and sigma-orbitals and the steric shielding effect of some orbitals upon the impact of an electrophilic reagent.