ENHANCEMENT OF METALLIC SILVER MONOMER EVAPORATION BY THE ADHESION OFPOLAR-MOLECULES TO SILVER NANOCLUSTER IONS

We have compared the metallic evaporation channels from metastable [Ag -X=5,Ag-7,Ag-11(AgI)(Y=1-4)](+) clusters in the first field free regio n of a double focusing mass spectrometer with that of the correspondin g pure metallic clusters, [Ag-X=5,Ag-7,Ag-11](+). It is found that the presence of the polar AgI molecules increases the rate of silver mono mer evaporation relative to that of silver dimer evaporation. Using th ermodynamic expressions for the heat of evaporation of the different e vaporation processes and assuming the absence of reverse activation en ergies, an expression for the difference between the activation energy of silver monomer and dimer evaporation is derived. It is shown that dipole/induced-dipole forces resulting from the presence of AgI polar molecules lead to an enhancement of silver monomer evaporation if the polarizability of the pure metallic cluster ions increases with the nu mber of Jellium electrons. Our theoretical calculations of the static polarizabilities of [Ag-x](+), using time dependent density functional theory within the local density approximation, shows a smooth increas e in the polarizabilities with the number of the Jellium electrons in these clusters. Finally, we observe that the enhancement of Ag monomer evaporation per AgI needed is smaller for clusters with an even numbe r of AgI molecules than with an odd number of them. This was proposed to result from the contribution of configurations with dipole ''pairin g'' of the AgI molecules in clusters with an even number of AgI molecu les, Dipole pairing would decrease the average dipole/induced-dipole i nteraction between the AgI molecules and the metallic part of these '' mixed'' clusters.