HERZBERG-TELLER ACTIVE VAN-DER-WAALS MODES IN P-DIFLUOROBENZENE-AR DETECTED BY ROTATIONALLY RESOLVED UV SPECTROSCOPY

Rotationally resolved UV spectra of the 0(0)(0) (S-1<--S-0) band and t wo van der Waals vibronic transitions of para-difluorobenzene-Ar are a nalyzed. The rotational structure of the transition at 34 cm(-1) to th e blue of the origin is of a-type and differs from the parallel c-type structure of the origin and of the band at 42 cm(-1) involving the st retching fundamental. For the a-type band the transition dipole moment is perpendicular to the plane of the aromatic ring, representing clea r experimental proof for Herzberg-Teller coupling induced by a van der Waals mode. Possible pathways for the intensity borrowing and consequ ences for the assignment in other dimers are discussed.