The electronic states of pyrrole 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 pyrrole has been carried out, and the spectra assigned by means of high level multi-reference multi-root CI studies. A similar reasssessment of the UV-photoelectron spectrum leads to the conclusion that our previous assignment of IP₃ to ionisation of the inner p-level (1b₁) is correct and that most other assignments to ionisation of a s-electron are incorrect. This is supported by the calculation of a wide range of Rydberg states derived from the five least bound orbitals, in the IP order: 1a₂^-1 < 2b₁^-1 < 1b₁^-1 < 9a₁^-1 < 6b₂^-1 < 5b₂^-1 < 8a₁^-1 < 7a₁^-1 < 4b₂^-1. It is concluded that each of the two strong absorption bands centred around 6 and 7.5 eV respectively owe their intensity to excitation of two ¹pp^* states of symmetry ¹A₁ and ¹B₂. The range of calculated valence states of pp^*-type has been extended to include those from 1b₁.