Activation energies for thermal ionic and neutral desorptions from thin films of lithium halides

摘要

To clarify the mechanism of positive-ionic and neutral desorptions from heated lithium halide (LiX, X = F, Cl, Pr or I), a small amount (approx. 10(-12)-10(-7) mol) of LiX was deposited on a platinum plate (ca. 0.03-0.04 cm(2)) to prepare a thin film (theta(0) = 10(-1)-10(3) molecular layers at the start), and it was heated up to ca. 1500 K at a constant rate (beta = 0.4-140 K/s) in vacuum (approx. 10(-4) Pa) using our dual-ion source system which made it possible to measure simultaneously the desorption rates (D-0 and D+) of neutral molecule (LiX0) and ion (Li+). The temperature-programmed desorption spectra thus obtained were different in pattern from that observed previously with NaX where each of NaX0 and Na+ had only one peak. Namely, the high peaks (P-1(0) and P-2(+)) of LiX0 and Li+ appeared at a temperature generally lower and higher than the melting point (T-m) of each LiX, respectively, while low peaks (P-1(+), P-2(0), P-0(3) and P-3(+)) appeared usually above T-m. Theoretical analysis of the beta-dependence of peak appearance temperatures yields the activation energies (E-1(0+)-E-3(0+)) for desorption of LiX0 (or Li+) giving P-1(0+)-P-3(0+), respectively, and also the frequency factors (nu(1)(0+)-nu(2)(0+)) corresponding to respective peaks. With respect to LiF (theta(0) approximate to 13 molecular layers), for example, E-1(0) and E-1(+) were 220 and 167 kJ/mol, respectively, while nu(1)(0) and nu(1)(+) were respectively 1 x 10(17) and 8 x 10(10)/s. In conclusion, (1) each desorption obeys the first-order kinetics, (2) P-1(0)-(0)(3) originate from the desorption from LiX at the state of physical adsorption, crystal or chemisorption, (3) P-1(+)-P-3(+) are due to the desorption from active sites (high work function sites; e.g., 724 kJ/mol for LiCl)on the heterogeneous surface of LiX itself or Pt, and (4) the ionization efficiency (D+/D-0) even at P-1(+)-P-3(+) is usually less than 1% mainly because the fraction of the active sites is less than 1% of the desorbing surface area. (C) 2000 Elsevier Science B.V. All rights reserved. [References: 11]