Experimental investigation on field evaporation of singly and doubly charged rhodium

Abstract

Field ion mass spectrometry and field ion appearance spectroscopy are used to analyze field evaporation processes of singly and doubly charged rhodium ions. Field strengths were ranged between 17 and 41 V/nm at tip temperatures between 600 and 100 K. The appearance energies of doubly charged species were found to increase with increasing field strengths from 29.5 ± 0.4 to 32.0 ± 0.2 eV, those of the singly charged species from 11.2 ± 0.1 to 12.4 ± 0.1 eV. Activation energies as determined from measured temperature dependences of the ion counting rates are equal for both kinds of ionic species and decrease from 1.5 to 0.05 eV with increasing field strengths. The observed field dependences cannot be predicted from the image hump model of field evaporation. Evaporation of Rh 1+is better understood using the charge exchange model. Generation of Rh 2+ is described by the postionization of Rh 1+ within the frame of a simple potential energy model from which the average interaction energy of rhodium surface atoms with their nearest neighbours may be derived.