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 (1)pi pi* states, one each of symmetry (1)A(1) and B-1(2). The range of calculated valence states of pi pi*-type has been extended to include those from both 3b(1) and 2b(1). 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-2 upsilon symmetry, obtained by open shell SCF/CI studies, occur in the order: 1a(2)(-1) < 3b(1)(-1) < 11a(1)(-1) < 7b(2)(-1) < 2b(1)(-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 (2)A(2) cation, while the SC bond length is markedly increased in the B-2(2) and B-2(1) (2b(1)(-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. (C) 1999 Elsevier Science B.V. All rights reserved.