Chemisorption studies of nitrogen on polycrystalline tungsten

The effect of nitrogen adsorption on the thermionic emission from a polycrystalline tungsten ribbon has been measured in the accelerating region both at high and low applied fields. The results have yielded information on the behavior of the work function of (100) and (111) planes towards nitrogen adsorption. The thermionic-emission data have been compared with average work-function changes measured using a diode operating in the space-charge-limited region. The work-function data combined with adsorption kinetics and with thermionic-emission changes during desorption have confirmed that on chemisorption: (i) (100) planes show a decrease in the work function and have a higher sticking probability and lower heat of adsorption than (111) planes. (ii) Conversely, (111) planes show an increase in the work function and have a lower sticking probability than (100) planes. At high temperatures nitrogen desorbs preferentially from (111) planes, and moreover the order of desorption from the two planes is different. An initial adsorption at coverages less than 4×10¹³ atom/cm², occurring on highly reactive sites tentatively associated with defects and crystallite edges has been found to increase the work function.