The temperature dependence of the intensity, I, of a diffracted beam of electrons from plane 100 of PbS was studied as a function of slight impurities on the surface. For specimens in which the residual gases were frozen, log I decreased linearly as the temperature increased from -100 to 200 degrees. But, in the presence of residual gases of about 10-anion5 mm Hg, log I at temperatures smaller than 0 degrees varies sufficiently to be an indication of the purity of the surface. Author
Edited trans. of Leningrad Univ. Vestnik (USSR) v26 n4, Seriya Fizika i Khimii, n1 p143-146 1971, by Charles T. Ostertag, Jr.