Neutral and Ionic Desorption of Cesium from Tungsten

Rates of neutral and ionic desorption of Cs from tungsten field emitters have been measured as functions of temperature and coverage, and the corresponding activation energies and frequency factors of desorption determined. The results for neutral desorption agree extremely well with the data of Taylor and Langmuir for coverages from near zero to 1.4×1014 atom/cm2, but fall below the heats of these authors at higher coverages. The combination of neutral and ionic heats yields a value of Φ=5.3 eV for the zero coverage work function of the regions from which ionic desorption occurs, in agreement with previous results for potassium. It is shown that surface equilibrium leads to a different averaging than independent evaporation, so that the above result is to be expected. The phenomenon of phase segregation observed by Taylor and Langmuir at very low coverages under steady‐state conditions is discussed and shown to be a kinetic rather than a thermodynamic effect.