Resolved high Rydberg spectroscopy of polyatomic molecules and van der Waals clusters

Using sub-Doppler double resonance excitation with Fourier-transform limited laser pulses and pulsed field ionization techniques we were able to resolve individual high n Rydberg states (45 < n < 110) below and above the lowest ionization energy of van der Waals clusters of benzene with the noble gases neon and argon. By choosing various selected J'K' intermediate rotational states we detected and assigned several Rydberg series with nearly vanishing quantum defect. They converge to different limits representing the rotational states in the vibrational states of the cluster cation. Even far above the ionization threshold sharp high-n Rydberg states with a width of 750 MHz are observed converging to intramolecular vibrational states located up to 800 cm-1 above the dissociation threshold of the cluster ion. This points to a slow dissociation rate of the cluster ion in the range of 3 x 10(5) s-1 < k < 5 x 10(8) s-1. In further studies single high Rydberg states of benzonitrile, a polyatomic molecule with an high dipole moment of 4.18 D, were detected in the range from n = 50 to 100. We plan to investigate the influence of the strong anisotropic dipole field of this molecule on the coupling between the high Rydberg electron and the molecular core.