Thermochemical and thermionic properties of ionic crystalline films

Abstract

To clarify the thermochemical and thermionic properties of various layers of alkali halide (MX) such as LiF, LiI, NaBr, KCl, RbCl or CsCl, a thin film [ θ ̄ 0≃1−10 4 molecular layers (ML) at the start] of MX was prepared on a metal plate (∼0.04 cm 2), and the absolute desorption rates (n + and n 0 ) of both positive ion (M +) and neutral molecule (MX 0) were measured simultaneously in a vacuum as a function of sample temperature increased stepwise (by ∼5 K, approx. every 20 min) or constantly (∼0.3–80 K/s) by using our dual-ion source system. Theoretical analysis of the experimental data yields the following conclusions. (1) Below the melting point ( T m), more than 90% of MX are desorbed as MX 0. (2) Its desorption energy ( E 0) increases up to the heat of sublimation as θ ̄ 0 increases, suggesting that the film prepared at θ ̄ 0 ⪡ 10 3 ML is partly amorphous rather than crystalline. (3) Ionic desorption, on the other hand, occurs mainly above T m giving 1–3 peaks in a temperature-programmed desorption spectrum. (4) The desorption energy of M + from MX is much larger than E 0 and depends upon the species of X. (5) The film surface is very inhomogeneous in thermionic properties, and M + is desorbed from those active sites having a high work function (e.g. 724 kJ/mol for LiCl). (6) The fractional surface area of the active sites over the film surface is as small as 10 –2–10 –5. (7) The ionization efficiency (∼ n +/ n 0) is less than 10 –2, usually increasing with a decrease in θ ̄ 0 .