ACTIVATION-ENERGIES FOR THE DESORPTION OF NEUTRAL MOLECULES AND POSITIVE-IONS FROM ALKALI-HALIDE LAYERS HEATED ON A METAL-SURFACE

To determine the activation energies (E+ and E-0) and the frequency fa ctors (nu(+) and nu(0)) for thermal positive-ionic and neutral desorpt ion from an ionic crystalline surface in high vacuum, a thin film (the ta(0) approximate to 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 (beta approximate to 2- 80 K s(-1)), and the absolute desorption rates of the ions (M+) and ne utral molecules (MX0) were simultaneously measured by using our therma l ionization type dual-ion source system operated by a temperature-pro grammed desorption method. This system had so high a detection sensiti vity thai even desorption rates of ca. 10(5) ions s(-1) and ca. 10(10) molecules s(-1) were readily measured with reasonable accuracy. In ea ch of the desorption spectra obtained with sodium halides, NaX0 and Na + showed single sharp peaks at low and high temperatures corresponding to ca. 40% and < 0.4% of theta(0), respectively. Theoretical analysis of the relation between beta and each desorption peak-appearance temp erature (T-p approximate to 700-1300 K) furnished the following result s; NaCl (E-0 = 193-176 and E+ = 290 kJ mol(-1); nu(0) = 10(10.7) and n u(+) = 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 th e 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 LiCl0 and Li+ yielded ((a) 145 and 291, (b) 277 and 234; (a) 10(9.6) and 10(12.6), (b) 10(11.7) and 1 0(9.0)), respectively. Physico-chemical analysis of these data supplie d a substantial clue to the ionic and neutral desorption processes and mechanisms.