Theoretical considerations associated with the production of negative ions in sputter-type ion sources

Processes which are important to the optimum formation of negative ions by sputtering in the presence of an alkali metal are discussed. Negative ion generation probabilities calculated from several theoretical models including the tunneling, Norskov and Lundqvist, ''freezing'' approximation of Overbosch and Los as well as modified surface ionization theories are compared with experimental observation. An analytical expression is introduced which reproduces with good accuracy observed maximum work function changes for electropositive adsorbates on metallic adsorbants. The expression is extremely useful for computing the value of the work function at its minimum which in turn can be used in the appropriate model for computing optimum negative ionization probabilities