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 built onto the 6¹₀ transition of the bare molecule are reported for the complexes C₆H₆⋅Ar, C₆D₆⋅Ar, and C₆H₆⋅⁸⁴Kr. The rotational structure of most of the bands is identified as that of a perpendicular transition with Coriolis coupling constants nearly the same as those of the 6¹₀ band of the respective complex. We therefore conclude that the excited van der Waals modes of the three complexes have a₁ 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 explained by a genuine perpendicular nor a genuine parallel transition. This situation will be analyzed in detail in accompanying work and the final vibronic assignments deduced. The rovibronic lines in all the spectra show a linewidth of 130 MHz that is solely due to the laser linewidth and to residual Doppler broadening in the molecular jet. It is concluded that the excited vibronic combination states of intramolecular and van der Waals vibrations do not predissociate on the nanosecond time scale of our experiment. Two of the reported spectra show irregularities in the rotational structure that are explained by coupling to adjacent combination states.