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

• Published in: Thermochimica acta Vol. 299; no. 1; pp. 59 - 65
• Main Authors: Kawano, Hiroyuki, Zhu, Yongfa, Kamidoi, Susumu, Shimizu, Hironari, Udaka, Masayuki
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 19-09-1997
• Subjects: Activation energy; Alkali-halide layers; Frequency factor; Ionic and neutral desorption rates; Thermal desorption processes; Thermal desorption processes; Alkali-halide layers; Frequency factor; Ionic and neutral desorption rates; Activation energy
• ISSN: 0040-6031; 1872-762X
• DOI: 10.1016/S0040-6031(97)00137-8

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 < 0.4% of
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.