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

The valence shell photoelectron spectra of furan, pyrrole and thiophene hav e been studied using synchrotron radiation and particular attention has bee n paid to the bands associated with the pi orbitals. In each of these molec ules 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) a nd pi (3) constitute the two outermost orbitals in all three molecules. How ever, the location of the most tightly bound pi (1) orbital within the elec tronic configuration remains uncertain. In the present work the photoelectr on bands due to the pi orbitals have been investigated through angular dist ribution measurements. Photoelectron asymmetry parameters and branching rat ios have been determined using monochromated synchrotron radiation in the p hoton energy range 15-120 eV. The spectral behaviour of asymmetry parameter s associated with pi orbitals is expected to differ from that of asymmetry parameters associated with sigma orbitals. These differences provide an exp erimental means of distinguishing between the two types of orbitals. The ma ny-body Green's function method has been employed to evaluate the ionisatio n energies and pole strengths of all valence states. These theoretical pred ictions have proved equally important in identifying the pi (1) photoelectr on 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 ob served in the inner valence region. (C) 2001 Elsevier Science BN. All right s reserved.