The vibrational structure of the S₀ → S₁ transition of anthracene

An ab initio calculation of the energy, geometry and vibrational frequencies of anthracene in the ground and first excited singlet state is presented, and compared with recent matrix isolation fluorescence results. It is found that a single configuration can reproduce the experimental data for S₁ reasonably well: the 0,0 transition's energy is within 3% of the experimental value, and most observed vibronic bands in S₁ can be assigned. The calculation helps to clarify some peculiarities of previous assignments, particularly for modes of the b_1g symmetry. Some bands observed in the supersonic jet and in the matrix, and showing anomalously long decay times and unusual matrix shifts are found to fit with the calculated b_3g modes. It is tentatively suggested that they are due to Herzberg-Teller vibronic coupling with a B_1u⁺ state, whose presence is indicated in a recent synchrotron fluorescence excitation study.