An experimental and theoretical study of the valence shell photoelectron spectra of thiophene, 2-chlorothiophene and 3-chlorothiophene

Abstract

The valence shell photoelectron spectra of thiophene, 2-chlorothiophene (2-Cl-Th) and 3-chlorothiophene (3-Cl-Th) have been investigated theoretically and experimentally. The third-order algebraic-diagrammatic construction approximation scheme for the one-particle Green’s function has been employed to evaluate the vertical ionisation energies and spectral intensities. The ground state geometrical parameters of the three molecules have been optimised at the level of the second order Møller–Plesset perturbation theory, and standard cc-pvDZ basis sets have been used throughout. The results for the outer valence region of thiophene agree well with available experimental and theoretical data. Very satisfactory agreements have been obtained between the theoretical predictions for the photoelectron spectra of 2-Cl-Th and 3-Cl-Th and the corresponding experimental data recorded in the present study. Assignments have been proposed for the major spectral structures. In all three molecules the breakdown of the molecular orbital ionisation picture associated with the π 1 molecular orbital is discussed. Synchrotron radiation has been used to record the complete valence shell photoelectron spectra of 2-Cl-Th and 3-Cl-Th. Photoelectron angular distributions and branching ratios have been determined in the photon energy range 13–115 eV, and those for the chlorine and sulphur 3p orbitals show effects that can be attributed to Cooper minima. A high resolution photoelectron spectrum of the outer valence shell of 2-Cl-Th has been recorded using HeI radiation, and vibrational structure has been observed and analysed in the X 2 A ′′ , A 2 A ′′ , B 2 A ′ , C 2 A ′′ and I 2 A ′ state bands.