Direct Access to the Dipole-Forbidden nπ^∗T₁ State of p-Benzoquinone by Photodetachment Photoelectron Spectroscopy

Abstract

In this work we show that photodetachment photoelectron (PD-PE) spectroscopy on radical anions allows direct access to dark states, such as dipole-forbidden nπ^∗ triplet and singlet states. p-benzoquinone (BQ) was chosen as model molecule because it is experimentally and theoretically well investigated and its lowest electronic states are electric dipole-forbidden nπ^∗ triplet and singlet states. Because of the high density of electronic states we observe a dense structure with many transitions in our PD-PE spectrum. We identify the first peak at 2.3 eV above the S₀ onset as the T₁ state origin, in good agreement with previous data. We are not able to resolve the splitting between the states T₁ (1³B_1g) and T₂ (1³A_u) as well as the corresponding S₁ (1¹A_u) and S₂ (1¹B_1g) states, but perform a tentative assignment of the singlet-triplet splitting on the basis of literature data. The fact that the spectral features in our PD-PE spectrum cover a broad energy range is taken as a strong experimental evidence for the presence of the T₃ππ^∗ state, which has been predicted to lie in this energy range. Our investigations on BQ show that in principle, by starting spectroscopy in radical anions most of the selection rules, valid in neutral BQ molecules, can be circumvented and nπ^∗ states, triplet states and other dipole-forbidden states become directly accessible with bright intensity. Much higher electron energy resolution is required to allow unambiguous assignments and make PD-PES of radical anions a valuable method for the spectroscopy dark states of neutral closed-shell molecules.