Thermodynamic properties of alkali-chloride films

To elucidate the thermodynamic properties of a thin film (θ0≈1–103 molecular layers (ML) at the start) of alkali chloride (MCl, MLi, Na, K, Rb or Cs), a small amount (∼10−10–10−7 mol) of MCl deposited on a platinum plate (∼0.03 cm2) was heated up to ∼1500 K at a constant rate (∼1–80 K s−1) in vacuum. The desorption rates of ion (M+) and neutral molecule (MCl0) were measured simultaneously with our dual-ion source system. Theoretical analysis of the temperature-programmed desorption spectra thus obtained yields the following results: (1) as θ0 increases from ∼70 up to 1500 ML of NaCl, for example, the neutral desorption energy of NaCl0 increases from 1.82 up to 2.26 eV, the latter of which agrees with the heat of sublimation (2.29 V) of NaCl; (2) in contrast, the desorption energy of Na+ remains constant at 3.00 eV; (3) the chloride layers prepared at θ0≤102 ML are generally in a physical adsorption state rather than a crystalline state; (4) the desorption rate of M+ from each MCl becomes maximum at a temperature above each melting point (Tm), where the sample layers are usually less than ∼10% of θ0; (5) the ionization efficiency above Tm ranges from ∼10−2 to 10−4 according to θ0; (6) M+ is desorbed from those active sites (less than ∼10% of the desorbing surface area) with high work function; (7) the chloride layers prepared at θ0≤10 ML and heated above Tm are useful for efficiently producing M+ (∼1 μA cm−2).