Structure of the benzene–Ar2 cluster from rotationally resolved ultraviolet spectroscopy

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Abstract
Rotationally resolved spectra of the two vibronic bands 610 and 1620 and a vibronic van der Waals band of the benzene–Ar2 cluster are presented, whose vibronic assignments are based on the analysis of their rotational structures. A fit to the rotational line positions in the symmetric top spectra yields an accurate set of rotational constants in the ground and the excited electronic state and the exact values for the band origins of the bands. From these values the spectral shift between corresponding cluster and monomer bands as well as the frequency of the van der Waals symmetric stretching vibration in the excited electronic state are precisely determined. The structure of the cluster is identified to be symmetric with one Ar atom located on the C6 axis on each side of the benzene ring at a distance of 3.58 Å in the S0 state and 3.52 Å in the S1 state. These bond lengths exactly agree with our recent values for benzene–Ar. From the result that the bond lengths are equal for the dimer and the trimer we conclude that there is no Ar–Ar interaction through the intermediate benzene ring plane.