A Field Ion Microscope Study of Adsorption of Methane on Iridium

Abstract

Field ion specimens of iridium were exposed to methane at low pressure for controlled periods of a few seconds, with the field removed, and then imaged in neon or in helium. With neon microscopy, adsorption was clearly visible after exposure to 2×10-9 Torr for 5–20 seconds. The rate of adsorption was measured on several different crystallographic planes. The <110> zones appeared to be very resistant to adsorption. Field desorption was done by raising the specimen voltage in controlled increments, and the field required to desorb the adsorbate was measured for several different planes. Some field-induced corrosion was seen on the {311} planes. With helium microscopy, adsorption was not visible except after rather larger exposures to 1×10-8 Torr for 25 seconds. However, field-induced corrosion was seen after smaller exposures, suggesting that adsorption had occurred but that the adsorbate had been removed by the field. The {311} planes were particularly susceptible to corrosion.