Heat capacities of freely evaporating charged water clusters

We report on evaporation studies on positively charged water clusters (H$^+$(H$_2$O)$_N$) and negatively charged mixed clusters(X$^-$(H$_2$O)$_N$) with a small core ion $X$ ($X$=O$_2$,CO$_3$ orNO$_3$), in the size range $N=5$ to $300$. The clusters were produced by corona discharge in ambient air, accelerated to 50 keV and mass selected by an electromagnet. The loss of monomers during the subsequent 3.4 m free flight was recorded. The average losses are proportional to the clusters\u27 heat capacities and this allowed the determination of size-dependent heat capacities. The values are found to increase almost linearly with clusters size for both species, with a rate of 6-8 $k_B$ per added molecule. For clusters with $N21$ the values areintermediate between the bulk liquid and the solid water 0 $^o$C values