Effect of hydrogen on the evaporation field of metals

Abstract

Field ion images of Fe, Ni, Cu and Pd were observed in a Ne and a H 2 atmosphere and the applied voltage at which the atomic planes of the specimen surface were field-evaporated at a contant rate was measured in a temperature range from 20 to 150 K. The voltage-temperature relation for Fe and Ni was almost linear in Ne and the field evaporation was discussed in terms of the charge exchange model. A large reduction of the evaporation voltage by adsorbed hydrogen was observed with these metals. This reduction was attributed to the decreased binding energy of surface atoms and the substrate by the chemisorbed H atoms. It was also observed that there were two distinct stages in the voltage-temperature relation with the transition temperature of about 85 K in hydrogen adsorbed Fe and Ni. Assuming the same evaporation mechanism in Ne and in H 2, the decrease in the binding energy in these stages was estimated. A significant reduction of the evaporation voltage by hydrogen was also observed with Cu.