DISSOCIATION-ENERGY OF NEUTRAL AND IONIC BENZENE-NOBLE GAS DIMERS BY PULSED-FIELD THRESHOLD IONIZATION SPECTROSCOPY

Delayed pulsed field threshold ionization of clusters excited to high long-lived Rydberg states is used to study their dissociation behavior. Benzene-Ar and benzene-Kr dimers are excited by resonance enhanced two-photon ionization to Rydberg levels close to various ionization thresholds. The field ionized threshold ions are monitored and separated from the non-energy-selected ions in a reflecting field mass spectrometer with high mass resolution. The appearance of threshold ions at the daughter ion mass indicates the onset of a dissociation process. Daughter ions are first observed for the 16(1)6(1)(3/2) level of the two investigated dimers. This leads to an upper limit of the dissociation energy of benzene-Ar of 340 cm-1 which is probably higher than the true dissociation energy. For the first time threshold ions are observed for large internal energies of some 5 eV in the core indicating that high Rydberg states maintain their long lifetime even if the core is electronically or vibrationally excited by several eV.