Excited electronic states of thiophene: high resolution photoabsorption Fourier transform spectroscopy and ab initio calculations

The recently introduced synchrotron radiation-based Fourier transform spectroscopy has been employed to study the excited electronic states of thiophene. A highly resolved photoabsorption spectrum has been measured between ∼5 and 12.5 eV, providing a wealth of new data. High-level ab initio computations have been performed using the second-order algebraic-diagrammatic construction (ADC(2)) polarization propagator approach, and the equation-of-motion coupled-cluster (EOM-CC) method at the CCSD and CC3 levels, to guide the assignment of the spectrum. The adiabatic energy corrections have been evaluated, thereby extending the theoretical study beyond the vertical excitation picture and leading to a significantly improved understanding of the spectrum. The low-lying π → π* and π → σ* transitions result in prominent broad absorption bands. Two strong Rydberg series converging onto the 2A2 state limit have been assigned to the 1a2 → npb11B2 and the 1a2 → nda21A1 transitions. A second, and much weaker, d-type series has been assigned to the 1a2 → ndb11B2 transitions. Excitation into some of the Rydberg states belonging to the two strong series gives rise to vibrational structure, most of which has been interpreted in terms of excitations of the totally symmetric ν4 and ν8 modes. One Rydberg series, assigned to the 3b1 → nsa11B1 transitions, has been identified converging onto the A 2B1 state limit, and at higher energies Rydberg states converging onto the 2A1 state limit could be identified. The present spectra reveal highly irregular vibrational structure in certain low energy absorption bands, and thus provide a new source of information for the rapidly developing studies of excited state non-adiabatic dynamics and photochemistry.