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

Using sub-Doppler double resonance excitation with Fourier-transform limite d laser pulses and pulsed field ionization techniques we were able to resol ve individual high n Rydberg states (45 < n < 110) below and above the lowe st ionization energy of van der Waals clusters of benzene with the noble ga ses neon and argon. By choosing various selected J(K')' intermediate rotati onal states we detected and assigned several Rydberg series with nearly van ishing quantum defect. They converge to different limits representing the r otational 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 locate d up to 800 cm(-1) above the dissociation threshold of the cluster ion. Thi s 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 investig ate the influence of the strong anisotropic dipole field of this molecule o n the coupling between the high Rydberg electron and the molecular core.