Surface Ionization of Some Rare Earths on Tungsten

The surface ionization of Gd, Er, and Yb atomic beams on a clean (112) tungsten surface has been studied in an ultrahigh-vacuum mass spectrometer. For the first time, the absolute ionization efficiencies have been determined as a function of surface temperature. The degree of ionization α was found to follow the general form of the Saha-Langmuir equation A α=Ae^-E/kT. The magnitudes of A and E, however, are not consistent with the predicted values. Values of A and E are summarized below: The Saha-Langmuir theory was found to be inadequate for the surface ionization of complicated atoms in beams on single-crystal tungsten surfaces. This work makes clear the necessity for more detailed theoretical studies of the process. An auxiliary measurement of the work function of the (112) surface of tungsten yielded 4.85±{}0.07 eV.