A Discussion on photoelectron spectroscopy - Molecular photoelectron spectroscopy

Abstract The principal features of helium 584 A photoelectron spectra of molecular vapours are outlined. Factors affecting the number of bands observed and their relation to the number of occupied electronic energy levels are discussed. The effect of the molecular translational velocity on the fundamental line widths attainable is considered and is shown to be significant though generally small compared with the much more common line broadening arising from ionic decomposition. Sharp lines are often to be associated with the presence of classical lone pairs but exceptions are noted. Structural effects in the lone pair ionization of chloro- and bromo-compounds are indicated. Some evidence is given for a correlation between the change in vibrational frequency produced upon ionization and the difference between adiabatic and vertical ionization energies. The use of such a correlation in the analysis of vibrational fine structure is exemplified by considering the examples of methylfluoride, 1,1-difluoroethylene, sulphur hexafluoride, ketene and pyrazine. A comparison between the p.e. spectra of butadiene, acrolein and glyoxal is used to indicate the utility of the method in creating energy level diagrams for related series of compounds.