Electronic Spectroscopy of Ovalene: Reassignment of the S₂(B_3u)– S₀(A_g) Transition

Abstract

As part of our experiments to characterize solid para-hydrogen (para-H 2 ) as a matrix host for electronic spectroscopy with potential applications in the ongoing quest for the carriers of the diffuse interstellar bands (DIB), we studied dispersed fluorescence and fluorescence excitation spectra of ovalene (C 32 H 14 ), a planar polycyclic aromatic hydrocarbon (PAH) of D 2h symmetry. Although generally in good agreement with previously reported data for jet-cooled C 32 H 14 , our results, in conjunction with quantum-chemical calculations, indicate that the observed spectral progressions are associated with the S 2 (B 3u )−S 0 (A g ) electronic transition instead of the originally assigned S 1 −S 0 transition for C 32 H 14 in a supersonic jet, and that the previously reported origin band was misassigned and should be located at ∼21050 cm −1 . The reassignment is further supported by the comparably long fluorescence lifetime of ∼1.7 μs. From an analysis of spectral features located >1600 cm −1 in the fluorescence excitation spectrum, we estimate an S 2 (B 3u )−S 3 (B 1g ) energy gap of ∼350 cm −1 .