Sputtering of Chemisorbed Nitrogen with Ar+

Flash filament techniques and mass spectrometry have been used to make sputtering measurements for nitrogen which is chemisorbed on a tungsten surface and bombarded with Ar+ ions. The sputtering rate for a given energy between 20 eV and 300 eV is described by the equation dN/dt=−S(N/)N0)ni, where N is the surface coverage, N0 the saturation surface coverage, ni the ion flux, and S the sputtering ratio measured at saturation. The sputtering ratio for nitrogen was found to be ∼0.02 at 17 eV, indicating a low threshold energy for sputtering of the strongly bound nitrogen atom. This result suggests that the well-known experimental relationship between sublimation energy (∼7 eV/nitrogen atom) and threshold energy may not hold for chemisorbed gases. For all energies between 100 and 300 eV the measured sputtering ratio for nitrogen was approximately twice that found for tungsten from atomically clean tungsten. This is particularly surprising when one considers that nitrogen covers less than 20% of the geometrical surface area. This result may, indicate a large sputtering ratio even for gases with large binding energies.