Water ionization and desorption of protonated water clusters in high electric fields were studied on a Pt emitter tip in ultrahigh vacuum. Ramped field desorption (RFD) experiments were carried out at 109 and 145 K. As the tip potential was linearly ramped, crystalline ice adsorbed at low temperatures (145 K) was ionized and emitted from the tip as protonated water clusters H+(H2O)(n). Emitted ions were mass resolved using a Wien filter for n up to 7. The onset of ionization occurred at a higher field for the 109 K RFDs (0.44 V Angstrom(-1)) than for the 145 K RFDs (0.32 V Angstrom(-1)). All observed ions for each temperature were observed simultaneously in the onset flash. However, clusters n = 2 and 3, which were dominant at 109 K, were not observed at 145 K. These results, coupled with previous low temperature (109-150 K) ice field desorption data, and higher temperature (170-300 K) field adsorbed ion emission data, suggest a two step dissociative ionization/desorption mechanism in which the strongly temperature dependent, dissociative ionization event determines emission onset at low temperatures, while the field dependent, ion emission event determines emission onset at higher temperatures. In addition, local enhancement of the ionization event was observed by directly imaging ion cluster emission using field ion microscopy. (C) 2003 Elsevier Science B.V. All rights reserved.