A Simple Method of Simultaneous Detection of Thermal Ions and Neutral Molecules Emitted from Thick Layers of Unitary Alkali Halide on a Metal Surface

Abstract

A simple ion source was designed and developed to study the thermal ionic and neutral emission from thick layers of unitary alkali halide(MX)deposited on a metal surface. Besides a platinum ribbon(F1, 0.02 cm2 )functioning as the substrate metal, this ion source has a second platinum filament(F2)as a neutral beam detector and also a grid(G)between F1 and F2 in order to retard the ions emitted from F1 or F2. A part of the neutral molecules(MX1°)evaporating from F1 is ionized with F2 after passing through G. In this work with potassium fluoride, the ion source was found to be quite useful as an instrument by which the primary and secondary positive ions(M1+and M2+)emitted from F1 and F2, respectively, were separated without employing any additional separator. The collection efficiency of M1+ was so high (about 41%), and hence such a weak positive ion emission from F1 as about3×103 ions/sec 0.02 cm2 at about 350°C could be detected with a vibrating reed electrometer with a maximum sensitivity of 1×10-16 A per scale division. The detection efficiency(γ2+ )of MX1° converted into M2+ under a residual gas pressare of 2× 107 Torr was(4.1±2.0)×105, which was determined by a convenient method developed in this work. Consequently, the lower detectable limit of MX1° with another electrometer having the same sensitivity as above was usually about2×107 molecules/sec.0.02 cm2. In this way the emission rates of both M1+ and MX1° could be simultaneously measured as a function of the surface temperature(T1)and the mean sample layer thickness(θ1)on F1 over the ranges of 350-460°C and 103 -1 molecular layers, respectively. The characteristics of F2 as a neutral beam detector were examined under various conditions in order to study the probable sources responsible for the large error in γ2+. Negative ion emission from F1could be detected at T1≥500°C, where 77, was no longer kept constant. The effectiveness of the present instrument and method to investigate the mechanism of thermalion emission from thick alkali halide layers on a metal surface is discussed on the basis of the results obtained in this preliminary experiment.