MAGIC NUMBERS IN THE MASS-DISTRIBUTION OF THE BENZENE(+)-ARGON(N) IONS - EVAPORATION DYNAMICS AND CLUSTER STRUCTURE

The intensity distribution of benzene+-Ar(n) cluster ions formed by laser ionization of neutral clusters has been investigated: two main intensity anomalies (magic numbers at n = 20 and 45) have been observed in the 15-60 size range. The evaporation dynamics of these species in the 2-50 microsecond time window following ionization has been studied using the electrostatic mirror of a reflection time-of-flight mass spectrometer as a kinetic energy analyser capable to distinguish parent and daughter ions. The magic numbers are interpreted in terms of size dependent evaporation behaviors: beyond n = 20, a sudden decrease of the evaporation energy is observed; in the n = 45-47 size range, the magic number is accounted for by the specific dynamics of the n = 46 and 47 clusters, in particular the possible loss of two argon atoms for n = 47 within the experimental time window. These results and their implications on the cluster structure are discussed in the light of the evaporative ensemble model and compared to the evaporation characteristics of similar species, in particular the neat rare gas clusters.