The identification of the outer valence shell π-photoelectron bands in furan, pyrrole and thiophene

The valence shell photoelectron spectra of furan, pyrrole and thiophene have been studied using synchrotron radiation and particular attention has been paid to the bands associated with the pi orbitals. In each of these molecules three doubly occupied pi -type molecular orbitals (denoted pi (1), pi (2) and pi (3)) are formed, and previous work has established that pi (2) and pi (3) constitute the two outermost orbitals in all three molecules. However, the location of the most tightly bound pi (1) orbital within the electronic configuration remains uncertain. In the present work the photoelectron bands due to the pi orbitals have been investigated through angular distribution measurements. Photoelectron asymmetry parameters and branching ratios have been determined using monochromated synchrotron radiation in the photon energy range 15-120 eV. The spectral behaviour of asymmetry parameters associated with pi orbitals is expected to differ from that of asymmetry parameters associated with sigma orbitals. These differences provide an experimental means of distinguishing between the two types of orbitals. The many-body Green's function method has been employed to evaluate the ionisation energies and pole strengths of all valence states. These theoretical predictions have proved equally important in identifying the pi (1) photoelectron bands because they indicate whether ionisation from a particular orbital should give rise to a main-line, or whether the single-particle model has broken down, interpretations have been proposed for most of the features observed in the inner valence region. (C) 2001 Elsevier Science BN. All rights reserved.