ONSET AND EVOLUTION OF MAGIC NUMBERS IN MASS-SPECTRA OF MOLECULAR CLUSTERS

We have calculated the evaporation rate for every cluster size of Ar(n)+ and Xe(n)+ in the microcanonical ensemble (as a function of its internal energy) up to n = 37. This allows us to perform numerical experiments simulating the evolution of the size and energy distributions in a cluster beam up to arbitrary long times. We find that there is a time lag for the onset of magic numbers (especially abundant cluster sizes) of molecular clusters. This time is required for the clusters to cool down by evaporation and become solid-like and it may differ by several orders of magnitude for different species thus explaining apparently contradictory experiments. For longer times we find, in agreement with theoretical predictions but contrary to simple intuition, that the relative abundance of magic/nonmagic clusters decreases with time.