Coherent oscillations in the charge-transfer system 4-dimethylamino-benzonitrile

4-Dimethylamino-benzonitrile was excited in the gas phase at 270 nm into the S2 (La) state and probed by femtosecond time-resolved photoionization at 2 μm. Coherent oscillations were detected in the transient ion signal. We claim that from S2, the molecule relaxes through a conical intersection and, going backwards along the charge-transfer (CT) reaction coordinate, enters into the S1 (Lb) well, where it vibrates along the amino-group twist and inversion. Probably mainly the last slope stimulates these oscillations. We conclude that the conical intersection – and hence also the CT state – is displaced from the Lb minimum in a direction containing the twist and inversion as components and that inversion is part of the nonadiabatic coupling vector.