Determination of the rates of both work function change and residual gas adsorption on a tungsten surface heated in a high vacuum

Abstract

The work functions ( φ + and φ e) effective for thermal positive-ionic and electronic emissions from a heated tungsten surface were measured as a function of the time ( t) elapsing immediately after the surface made essentially clean by flashing was adjusted to a working temperature (~ 1800 − 1500 K). The quantities ( Q) of residual gases (RG) adsorbed during t were also measured by flash desorption mass spectrometry of O + and CO + produced by electron impact of desorbed gases. Analysis of the data thus obtained yielded the rate expressions of Y n = A n- B nexp [− C n t], where Y 1-Y 3 are φ +, φ e and Q, respectively, and A n- C n are the constants corresponding to respective ns. Comparison among the expressions indicates that the work function increases (by up to ~ 0.7 eV) generally observed by the temperature decrease from ~ 2300 K to 1500 K is caused mainly by adsorption of oxygen and/or oxygen compounds included in RG.