The vacuum ultraviolet absorption spectra of norbornadiene: vibrational analysis of the singlet and triplet valence states of norbornadiene by configuration interaction and density functional calculations

A synchrotron-based vacuum ultraviolet absorption spectrum (VUV) of norbornadiene (NBD) is reported, and the extensive vibrational structure obtained has been analysed. The previously known 5b₁3s-Rydberg state has been reinterpreted by comparison with our recent high resolution photoelectron spectral (PES) analysis of the X²B₁ ionic state. Additional vibrational detail in the region of this Rydberg state is observed in its VUV spectrum, when compared with the PES ²B₁ ionic state; this is attributed to underlying valence state structure in the VUV. Valence and Rydberg state energies have been obtained by configuration interaction (CI) and time dependent density functional theoretical methods (TDDFT). Several low-lying singlet valence states, especially those which arise from ππ* excitations, conventionally termed NV₁ to NV₄, have been examined in detail. Their Franck-Condon (FC) and Herzberg-Teller (HT) profiles have been investigated and fitted to the VUV spectrum. Estimates of the experimental 0⁰ band positions have been made from these fits. The anomaly of observed UV absorption by the ¹A₂ state of NBD is attributed to HT effects. Generally the HT components are less than 10% of the FC terms. The calculated 5b₁3s lowest Rydberg state also shows a low level of HT components. The observed electron impact spectra of NBD have been analysed in detail in terms of triplet states.