Sample temperature and introduced gas pressure dependences of the thermal positive and negative ion emissions from thick layers of potassium fluoride

Abstract

Using a dual ion source developed by us, the thermal emission rates ( n ± and n )) of either positive or negative ions (K + or F -) and neutral molecules (KF 0) desorbing from ≈ 10 3 molecular layers of KF deposited on a platinum disk ( ≈ 0.2 cm 2) were measured simultaneously as a function of sample temperature ( T, 650–750 K) in a high vacuum ( ≈ 0.2 μTorr). The desorption energies of K +, F - and KF 0 were about 2.9, 2.8 and 2.4 eV, respectively. The ionization efficiencies ( n ±/ n 0) for K + and F - were ≈ 10 -7-10 -6 and 10 -9-10 -8, respectively. Theoretical analysis of these data using our simple model of “dissociative self-surface ionization” yielded that the effective work functions (φ + and φ -) of the active layer sites (only ≈ 5% and 0.03% of the total emitting surface area) from which K + and F - were emitted were about 6.3 and 1.6 eV, respectively, while KF 0 was emitted from the inactive site (≈ 95% of the total area) and the apparent roughness factor for the polycrystalline sample layer surface was ≈ 5. At T ≈ 700 K, introduction of N 2 had little effect upon φ ±, but that of O 2 up to ≈ 10 μTorr decreased both φ + and φ - by up to ≈ 0.3 eV, on the contrary to the work function increase (≈ 0.3 eV) simultaneously observed with a platinum filament (1260 K) used to detect KF 0 by “molecular beam surface ionization”.