ROTATIONALLY RESOLVED VIBRONIC SPECTRA OF THE VAN-DER-WAALS MODES OF BENZENE-AR AND BENZENE-KR COMPLEXES

Rotationally resolved vibronic spectra of eight van der Waals bands bu ilt onto the 6(0)(1) transition of the bare molecule are reported for the complexes C6H6 . Ar, C(6)Dg(6) . Ar; and C6H6 .Kr-84. The rotation al structure of most of the bands is identified as that of a perpendic ular transition with Coriolis coupling constants nearly the same as th ose of the 6(0)(1) band of the respective complex. We therefore conclu de that the excited van der Waals modes of the three complexes have al symmetry. Precise rotational constants are fitted to the large number of unblended lines assigned in each spectrum. In contrast, the lowest energy van der Waals bands of both C6H6 . Ar and C6D6 . Ar display a completely different rotational structure which can neither be explain ed by a genuine perpendicular nor a genuine parallel transition. This situation will be analyzed in detail in accompanying work and the fina l vibronic assignments deduced. The rovibronic lines in all the spectr a show a Linewidth of 130 MHz that is solely due to the laser linewidt h and to residual Doppler broadening in the molecular jet. It is concl uded that the excited vibronic combination states of intramolecular an d van der Waals vibrations do not predissociate on the nanosecond time scale of our experiment. Two of the reported spectra show irregularit ies in the rotational structure that are explained by coupling to adja cent combination states. (C) 1996 American Institute of Physics.