MODEL OF THE CONTAMINATION EFFECT IN ION-INDUCED ELECTRON-EMISSION

Ion-induced electron emission yields from contaminated surfaces are well known to be enhanced relative to the yields from atomically clean surfaces. Under the bombardment of energetic ions, the surfaces become sputter-cleaned with time and the yields from the samples are reduced accordingly. The time dependent reduction of yields observed are shown to be due to various effects such as desorption of contaminant atoms and molecules by incident ions and adsorption of residual gas onto previously clean sites. Experimental results obtained in the present work show the lower, saturated yield (gamma(s)) to be a function of residual gas pressure (P) and the fluence (phi(i)) of the ion. We present a dynamic equilibrium model which explains the increase in yields for surface gas contamination, the decrease in yields for contaminant desorption, and the pressure/fluence dependence in the time required to reach gamma(s). The predictions of the model agree well with the observations of gamma(s) as a function of the ratio of gas flux to ion flux, and the electron yields of clean and gas covered surfaces.