Herzberg-Teller active Van der Waals modes in p-difluorobenzene-Ar detected by rotationally resolved UV spectroscopy

Rotationally resolved UV spectra of the 000 (S1 from S0) band and two van der Waals vibronic transitions of para-difluorobenzene-Ar are analyzed. The rotational structure of the transition at 34 cm(-1) to the blue of the origin is of a-type and d iffers from the parallel c-type structure of the origin and of the band at 42 cm(-1) involving the stretching fundamental. For the a-type band the transition dipole moment is perpendicular to the plane of the aromatic ring, representing clear experimental proof for Herzberg-Teller coupling induced by a van der Waals mode. Possible pathways for the intensity borrowing and consequences for the assignment in other dimers are discussed.