Rotationally Resolved Spectra of Benzene-Noble Gas Complexes

Abstract

Extremely narrow bandwidth pulsed tunable UV laser ($\Delta v = 100 MHz$) excitation combined with the sensitive technique of mass-selected two-photon ionization was used to obtain rotationally resolved UV spectra of benzene-X clusters, where $X= Ar^{a}, Ne$, or Kr. The $6^{1}{0}$ and $6^{1}{0}1^{1}{0}$ vibronic band in the $S{1}\leftarrow S_{0}$ transition of each of $C_{6}H_{6}-Ar, C_{6}H_{6}-Ar, C_{6}H_{6}^{20}Ne, C_{6}H_{6}-^{22}Ne, C_{6}H_{6}-^{84}Kr$, and $C_{6}H_{6}-^{86}Kr$ were measured with a resolution of 130 MHz. An accurate set of rotational constants has been obtained for each band. In all cases, the rotational spectrum is unambiguously modelled as a symmetric rotor with first order Coriolis coupling, as for the corresponding bands for the monomer. From the precise rotational constants, we can conclude that: a) The structure of benzene itself, in both $S_{0}$ and $S_{1}$, remains unchanged. b) The noble gas atoms are located on the $C_{6}$ symmetry axis and their distance from the benzene ring is determined for both electronic states. c) For all of the noble gas solute atoms, the van der Waals bond length decreases upon electronic excitation, as anticipated by the increased polarizability and expansion of the benzene ring. d) Even at several times the van der Waals binding energy in benzene vibrational energy, no dissociation within 10 ns is evident.