Molecular beam surface ionization detection. I

Absolute ionization coefficients β have been measured for K on a 'low work function' PtW surface by the field reversal method. The modified Langmuir-Saha equation has been fitted by a least-squares procedure to the static surface ionization values as a function of temperature, and ionization coefficients have been computed from the best fit parameter values. The agreement between the two methods is satisfactory at low temperatures, within 9% at β=0.7-0.8, with the largest error probably lying in the coefficients from static ionization. By a comparison with other methods used for determinations of β<1 the field reversal method is shown to be as accurate as the apparently most reliable method used for such work, the microbalance method, first used by Schroen 1963. The Richardson plots for the probably graphite coated metal surface are curved. It is shown, that the curavature does not depend on the choice of the spectral emissivity or other possbile experimental errors. The only explanation for the curved plots, which appears to be consistent with the experiments, is a true non-linear temperature dependence of the work function, probably due to the semiconducting surface layer. Non-liner least-squares fits have been used to fit the theoretical expressions to the electron emission current, the surface ionization current and also to a combined quantity, in which the temperature dependence of the work function has been effectively removed. The nature of the PtW surface and some earlier, inconsistent investigations of this surface are also discussed. (Auth.)