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 t wo-photon ionization to Rydberg levels close to various ionization thr esholds. The field ionized threshold ions are monitored and separated from the non-energy-selected ions in a reflecting field mass spectrome ter with high mass resolution. The appearance of threshold ions at the daughter ion mass indicates the onset of a dissociation process. Daug hter ions are first observed for the 16(1)6(1)(3/2) level of the two i nvestigated dimers. This leads to an upper limit of the dissociation e nergy of benzene-Ar of 340 cm-1 which is probably higher than the true dissociation energy. For the first time threshold ions are observed f or large internal energies of some 5 eV in the core indicating that hi gh Rydberg states maintain their long lifetime even if the core is ele ctronically or vibrationally excited by several eV.