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

4-Dimethylamino-benzonitrile was excited in the gas phase at 270 nm into the S-2 (L-a) state and probed by femtosecond time-resolved photoionization at 2 mum. Coherent oscillations were detected in the transient ion signal. We claim that from S-2, the molecule relaxes through a conical intersection and, going backwards along the charge-transfer (CT) reaction coordinate, enters into the S-1 (L-b) 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 L-b minimum in a direction containing the twist and inversion as components and that inversion is part of the nonadiabatic coupling vector. (C) 2003 Elsevier B.V. All rights reserved.