Resolved high Rydberg (45 ≲ n ≲ 110) spectroscopy of benzene cation core vibrations

High Rydberg states located in the ionization continuum of the polyatomic molecule benzene are resolved up to 1070 cm(-1) excess energy above the adiabatic ionization energy. This is achieved using a sub-Doppler double resonance excitation with Fourier-transform limited laser pulses and a special pulse scheme for the separation and suppression of prompt ions produced during laser excitation. The Rydberg states in the 45 less than or similar to n less than or similar to 110 range are well below the Inglis-Teller limit and only weakly perturbed by residual electric stray fields. Their series limits found by computer assisted automated cross correlation procedure represent individual rovibrational states of the benzene cation. In this way rotational resolution within various vibrational states of different symmetry in the benzene cation is achieved and vibrational frequencies are given with an accuracy of +/-0.06 cm(-1). The widths of the individual Rydberg states are discussed in terms of the Stark splitting yielding lower limits for the time constants of an intramolecular vibrational dynamics. (C) 2000 Elsevier Science B.V. All rights reserved.