Reactive collisions as a signature for meltinglike transitions in clusters

The formation of HCl molecules in collisions of H atoms with Cl(Ar)12 clusters is studied by classical trajectory calculations as a function of the cluster temperature. At sufficiently low impact energy (E=10 kJ/mol) a pronounced threshold effect with temperature is observed: In the solidlike regime of the cluster (T40 K) there is strong screening of the Cl atom by the icosahedral Ar solvation shell preventing H+Cl association. In the liquidlike phase of the aggregate (T45 K) a sharp onset of reactivity is found with a reactive cross section of 11.4×10-20 m2.