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

To clarify the mechanism of positive-ionic and neutral desorptions from heated lithium halide (LiX, X = F, Cl, Br or I), a small amount (approx. 10−12–10−7 mol) of LiX was deposited on a platinum plate (ca. 0.03–0.04 cm2) to prepare a thin film (θ0 = 10−1–103 molecular layers at the start), and it was heated up to ca. 1500 K at a constant rate (β = 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 (D0 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 (P10 and P2+) of LiX0 and Li+ appeared at a temperature generally lower and higher than the melting point (Tm) of each LiX, respectively, while low peaks (P1+, P20, P30 and P3+) appeared usually above Tm. Theoretical analysis of the β-dependence of peak appearance temperatures yields the activation energies (E10+–E30+) for desorption of LiX0 (or Li+) giving P10+−P30+, respectively, and also the frequency factors (ν10+–ν20+) corresponding to respective peaks. With respect to LiF (θ0 ≈ 13 molecular layers), for example, E10 and E1+ were 220 and 167 kJ/mol, respectively, while ν10 and ν1+ were respectively 1 × 1017 and 8 × 1010/s. In conclusion, (1) each desorption obeys the first-order kinetics, (2) P10−P30 originate from the desorption from LiX at the state of physical adsorption, crystal or chemisorption, (3) P1+−P3+ are due to the desorption from active sites (high work function sites; e.g., 724 kJ/mol for LiC1) on the heterogeneous surface of LiX itself or Pt, and (4) the ionization efficiency (D+/D0) even at P1+−P3+ is usually less than 1% mainly because the fraction of the active sites is less than 1% of the desorbing surface area.