Work function of metallic surfaces bombarded with cesium ions

Abstract

Reduction of work function due to the build-up of cesium coverage on beryllium, molybdenum, and tungsten caused by cesium ion bombardment was investigated for incident energies between 5 eV and 1 keV. Surface characterization was performed as a function of incident ion energy and dose using Auger spectra and work function measurements. A steady state surface coverage is achieved in all cases for a dose of less than 10 16 ions/cm 2. The coverage is strongly dependent on target mass. Beryllium has nearly monolayer coverage at all beam energies, while above 150 eV molybdenum has less than 50% coverage and tungsten has less than 20% coverage. The minimum steady state work function at 60% coverage can be achieved on molybdenum and tungsten at 100 and 45 eV, respectively. The effects of hydrogen coadsorption are also examined. The development of the surface coverage occurs due to the effects of implantation, reflection, and sputtering. A Monte Carlo calculation shows that a surface coverage of cesium on beryllium inhibits sputtering, which should be true for other light elements.