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 Original Research Article

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 1ππ∗ states, one each of symmetry 1A1 and 1B2. The range of calculated valence states of ππ∗-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 C2v 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 Greenʹs function, TDA and CI calculations all lead to the same order of vertical IPs for the first nine IPs.