The electronic states of thiophene studied by optical (VUV) absorption, near-threshold electron energy-loss (EEL) spectroscopy and ab initio multi-reference configuration interaction calculations

A reinvestigation and extension of the observed VUV and EEL spectrum of thiophene has been carried out, and the spectra assigned by means of high level multi-reference multi-root CI studies. It is concluded that each of two strong absorption bands centred around 5.5 and 7.0 eV respectively owe their intensity to excitation of two ¹pp^* states, one each of symmetry ¹A₁ and ¹B₂. The range of calculated valence states of pp^*-type has been extended to include those from both 3b₁ and 2b₁. A wide range of Rydberg states derived from the five least bound orbitals has been obtained. The adiabatic IPs for the lowest series of cationic states with C_2v symmetry, obtained by open shell SCF/CI studies, occur in the order: 1a₂^-1< 3b₁^-1< 11a₁^-1< 7b₂^-1< 2b₁^-1. The equilibrium structures for the cations, when compared with the ground state under the same procedures, show that the order of the bond lengths C₂C₃ < C₃C₄ is reversed in the ²A₂ cation, while the SC bond length is markedly increased in the ²B₂ and ²B₁ (2b₁^-1) cations. Significant distortions of the ring skeleton in the higher ions suggest that lower symmetry states are formed on ionisation. The present Green's function, TDA and CI calculations all lead to the same order of vertical IPs for the first nine IPs.