Activation energies for the desorption of neutral molecules and positive ions from alkali-halide layers heated on a metal surface

Abstract To determine the activation energies ( E + and E 0 ) and the frequency factors ( ν + and ν 0 ) for thermal positive-ionic and neutral desorption from an ionic crystalline surface in high vacuum, a thin film ( θ 0 ≈ 70–1400 molecular layers composed of 2.5–50 nmol) of alkali halide (MX) deposited on a platinum plate (ca. 0.03 cm 2 ) was heated up to ca. 1500 K at a constant rate ( β ≈ 2–80 K s −1 ), and the absolute desorption rates of the ions (M + ) and neutral molecules (MX 0 ) were simultaneously measured by using our thermal ionization type dual-ion source system operated by a temperature-programmed desorption method. This system had so high a detection sensitivity that even desorption rates of ca. 10 5 ions s −1 and ca. 10 10 molecules s −1 were readily measured with reasonable accuracy. In each of the desorption spectra obtained with sodium halides, NaX 0 and Na + showed single sharp peaks at low and high temperatures corresponding to ca. 40% and θ 0 , respectively. Theoretical analysis of the relation between β and each desorption peak-appearance temperature ( T p ≈ 700–1300 K) furnished the following results; NaCl ( E 0 = 193−176 and E + = 290 kJ mol −1 ; ν 0 = 10 10.7 and ν + = 10 11.6 −10 12.3 s −1 ), NaBr (112 and 257; 10 6.7 and 10 11.0 ), NaI (139 and 234; 10 9.4 and 10 9.9 ). RbCl, having two peaks of Rb + alone at adjacent (a) low, and (b) high temperatures, afforded the data; (154 and (a) 239, (b) 205; 10 9.9 and (a) 10 11.7 , (b) 10 9.1 ), while LiCl with two peaks of both LiCl 0 and Li + yielded ((a) 145 and 291, (b) 277 and 234; (a) 10 9.6 and 10 12.6 , (b) 10 11.7 and 10 9.0 ), respectively. Physico-chemical analysis of these data supplied a substantial clue to the ionic and neutral desorption processes and mechanisms.