Resolved high Rydberg spectroscopy of benzene⋅rare gas van der Waals clusters: Enhancement of spin–orbit coupling in the radical cation by an external heavy atom

Abstract

Individual high n Rydberg states in van der Waals dimers of benzene and noble gas atoms are resolved after double resonance excitation with two Fourier-transform limited narrow band UV laser pulses. For a selected rovibronic intermediate state several Rydberg series appear converging to different rotational states of the cation. Their position is determined by an automated cross correlation (CRIES) of the experimental with a theoretical Rydberg spectrum with an accuracy of 0.02 cm−1. Analysis of the resolved rotational states of the cationic clusters in their vibrational ground state yields precise information on the ionization energies, the structure and average van der Waals distances. Rotational analysis provides clear spectroscopic evidence for an enhancement of spin–orbit coupling between the orbital angular momentum and the spin of the remaining unpaired electron in the ionic benzene by the external heavy atoms Ar and Kr. The resulting effect of the spin–orbit coupling on the rotational energy levels is fully resolved in these cases and we present accurate values for the spin–orbit coupling constants of the benzene+⋅Ar (a=0.51 cm−1) and benzene+⋅84Kr (a=2.89 cm−1) cluster cations.