It is observed that relatively large positive ion currents can be thermally desorbed from W, Ta, and Mo surfaces even several days after bombardment of the gas-covered surface with low-energy positive nitrogen ions or electrons. No measurable ion current is desorbed if the surface has not previously been bombarded. The dependence of the desorption current on bombarding current and time, on nitrogen pressure, and on the desorption time and temperature is presented. It is postulated that ions or possibly excited neutral particles are captured in traps of several different depths on the metal surface and that as the surface temperature is increased the traps of least depth are emptied first.

• Received 12 April 1957

DOI:https://doi.org/10.1103/PhysRev.107.994