The torsional dynamics of the 9-(N-carbazolyl)-anthracene (C9A) molecule is investigated by means of time-independent (1) and time-dependent (2) quantum-mechanical simulations in a diabatic representation. The study includes effects of surface crossing of the bright S₁ state with a dark state.

1. The intensity pattern of the S₀->S₁ fluorescence excitation spectrum is used to fit an effective one-dimensional Hamiltonian with a single-minimum potential for the dark state together with diabatic couplings to the double well potential of the bright state.

2. Based on this Hamiltonian, first predictions for a pump-probe scheme are made. In the pump process the molecules are excited to the S₁ state followed by competing torsions in the bright state and diabatic curve crossings to the dark state, depending on the pump frequency. Assuming the probe process to be an ionization from the bright state, the interfering effects of the dark state on the dynamics in the bright state can be monitored in a directly time-dependent way on a fs - ps time scale