The vacuum ultraviolet absorption spectrum 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 (VUV) absorption spectrum of norbornadiene (NBD) is reported, and the extensive vibrational structure obtained has been analyzed. The previously known 5b(1)3s-Rydberg state has been reinterpreted by comparison with our recent highresolution photoelectron spectral analysis of the (XB1)-B-2 ionic state. Additional vibrational details in the region of this Rydberg state are observed in its VUV spectrum when compared with the photoelectron B-2(1) ionic state; this is attributed to the underlying valence state structure in the VUV. Valence and Rydberg state energies have been obtained by configuration interaction and time-dependent density functional theoretical methods. Several low-lying singlet valence states, especially those that arise from pi pi* 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 0(0) band positions have been made from these fits. The anomaly of the observed UV absorption by the (1)A(2) state of NBD is attributed to HT effects. Generally, the HT components are less than 10% of the FC terms. The calculated 5b(1)3s lowest Rydberg state also shows a low level of HT components. The observed electron impact spectra of NBD have been analyzed in detail in terms of triplet states. Published under an exclusive license by AIP Publishing.

Automated summary

A synchrotron-based vacuum ultraviolet (VUV) absorption spectrum of norbornadiene (NBD) is reported, revealing extensive vibrational structures. The study compares different spectral states of NBD to analyze valence and Rydberg state energies, focusing on low-lying singlet valence states arising from pi pi* excitations. The analysis of the VUV spectrum includes investigation of Franck-Condon (FC) and Herzberg-Teller (HT) profiles.