Temperature and field dependence of protonated water cluster emission from field adsorbed water layers on platinum

Water ion cluster formation in high electric fields was characterized on a Pt emitter tip over temperatures ranging from 170 to 300 K. Ion clusters emitted from the field-adsorbed water layer were mass resolved using a Wien filter. Two series of tests were conducted to characterize the ion cluster formation. In the first, tip temperature and water pressure were held constant while the applied field at the tip was ramped linearly with time. As the field was increased, water ion clusters H+(H2O)n were emitted, beginning with large n clusters (up to 7) and proceeding through each lower n cluster in turn. The ion emission onset field was found to depend on the value of n and is lower for larger n clusters. The onset of ionization for each cluster was observed to be relatively constant as a function of temperature, however, ion signal intensity was temperature dependent. For the second series of tests, tip temperature was cyclically ramped while the applied field was held constant. The cyclical ramping experiments provided Arrhenius graphs showing thermal deactivation energies for ion emission from the tip. The observed deactivation corresponds to thermal desorption of the nth solvating water molecule of the ion cluster, i.e. the energy associated with the solvation of the proton by the nth solvating water molecule. These energies were found to be 0.85, 0.76 and 0.55 eV for n=3, 4 and 5, respectively.