An experimental and theoretical study of the valence shell photoelectron spectra of 2-bromothiophene and 3-bromothiophene

The valence shell photoelectron spectra of 2-bromothiophene and 3-bromothiophene have been studied, both experimentally and theoretically, in order to characterise the main bands due to single-hole states and to assess the importance of electron correlation in the formation of satellite states. Synchrotron radiation has been employed to measure photoelectron angular distributions and branching ratios in the photon energy range 13-115 eV, and the results indicate that the photoionisation dynamics of the (12a ')(-1) (B) over tilde (2)A ', (2a ")(-1) (C) over tilde (2)A " and (11a ')(-1) (D) over tilde (2)A ' states are affected by Cooper minima. The experimental data demonstrate that the 12a ' and 2a " orbitals, corresponding to the Br 4p lone pairs, retain their atomic properties to a substantial degree, and that the same is true, although to a lesser extent, for the 11a ' orbital which is related to the S 3p subshell. The results are compared with similar measurements on chlorothiophene and bromobenzene. The vertical ionisation energies and spectral intensities of the entire valence shell photoelectron spectrum have been computed using the third-order algebraic-diagrammatic construction approximation scheme for the one-particle Green's function. These theoretical predictions have allowed assignments to be proposed for all the prominent structure observed in the experimental spectra. The outer valence Green's function method has also been used and the calculated ionisation energies show good agreement with the experimental values. Mulliken atomic populations have been computed for some of the outer valence orbitals, and the calculated Br 4p and S 3p content possessed by the 12a ', 2a " and 11a ' molecular orbitals is in accordance with the strength of the Cooper minimum observed in the photoelectron asymmetry parameters associated with these orbitals. (C) 2001 Elsevier Science B.V. All rights reserved.