High n Rydberg spectroscopy of benzene: Dynamics, ionization energy and rotational constants of the cation

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Abstract
In a high resolution uv-uv double resonance experiment with two Fourier-transform limited nanosecond laser pulses high Rydberg states up to n>100 were selectively excited and detected by ionization in a pulsed electric field. We were able to identify 64 Rydberg series in benzene C6D6 and 20 series in benzene C6H6 converging to different rotational levels of the ionic molecular core. Their assignment by application of a crosscorrelation analysis yields accurate rotational constants and a precise value for the lowest rotationless ionization energy of 74556.57(5) cm−1 (C6H6) and 74583.51(5) cm−1 (C6H6). All observed series show surprisingly low quantum defects below 0.01 and no strong local perturbations at their crossings points. This points to a weak intramolecular coupling of the series in the absence of an external electric field. In a second series of experiments the influence of external fields on the intensity, the position and dynamics of single n Rydberg peaks is investigated. It is shown that the peak shape and peak position of a single Rydberg peak depends strongly on the applied electric field. Within the envelope of one Rydberg peak different decay times of the pulsed field ionization signal can be found.