Application of Penning ionization electron spectroscopy to assignments of electron spectroscopic bands of anthracene

Abstract

The He*(2³S) Penning ionization electron spectrum and He^I u.v. photoelectron spectrum of anthracene were measured. Most observed bands were assigned to various π- and σ-orbitals on the basis of the recently developed concept of exterior electron density (EED), which predicts theoretically the branching ratio of Penning ionization, i.e. the relative intensity of Penning ionization spectral bands. A critical comparison between the He*(2³S) Penning ionization electron spectrum (PIES) and He^I u.v. photoelectron spectrum (UPS) was also employed for the assignment. Thus, the ionization potentials, ranging from 7 to 16 eV, of 13 σ-orbirals as well as of seven π-orbitals were deduced. The π-orbitals give very strong bands in PIES. The σ-orbitals can be classified into three types, those mainly distributed over adjacent hydrogen atoms, those separately distributed on hydrogen atoms, and those mainly localized on C–C bonds, which give strong, medium, and weak Penning bands, respectively.