Resolved high Rydberg spectroscopy of benzene center dot rare gas van der Waals clusters: Enhancement of spin-orbit coupling in the radical cation byan external heavy atom

Individual high n Rydberg states in van der Waals dimers of benzene and nob le gas atoms are resolved after double resonance excitation with two Fourie r-transform limited narrow band UV laser pulses. For a selected rovibronic intermediate state several Rydberg series appear converging to different ro tational states of the cation. Their position is determined by an automated cross correlation (CRIES) of the experimental with a theoretical Rydberg s pectrum with an accuracy of 0.02 cm(-1). Analysis of the resolved rotationa l states of the cationic clusters in their vibrational ground state yields precise information on the ionization energies, the structure and average v an der Waals distances. Rotational analysis provides clear spectroscopic ev idence for an enhancement of spin-orbit coupling between the orbital angula r momentum and the spin of the remaining unpaired electron in the ionic ben zene by the external heavy atoms Ar and Kr. The resulting effect of the spi n-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(+).Kr-84 (a = 2.89 cm( -1)) cluster cations. (C) 1999 American Institute of Physics. [S0021-9606(9 9)00412-2].