CONTROLLING THE INTERNAL ENERGY CONTENT OF SIZE-SELECTED CLUSTER IONS - AN EXPERIMENTAL COMPARISON OF THE METASTABLE DECAY-RATE AND PHOTOFRAGMENTATION METHODS OF QUANTIFYING THE INTERNAL EXCITATION OF (H2O)N-A

The metastable decay rates of (H2O)n- clusters are found to be strongly dependent on source conditions and are correlated to the distribution of photofragments. We relate these variations to changes in the internal energy content of the clusters and analyze the effects to provide two independent determinations of the internal energy variation. The two methods are found to be in surprisingly good agreement. Measurements on larger clusters are carried out using photofragmentation, and changes on the order of 0.30 eV are easily affected by changing the pressure of argon backing the expansion. An interesting result of this work is that the cooling efficiency increases with cluster size.