To elucidate the thermodynamic properties of a thin film (theta (0) approximate to 1-10(3)) molecular layers (ML) at the start) of alkali chloride (MCl, M=Li, Na, K, Rb or Cs), a small amount (similar to 10(-10)-10(-7) mol) of MCl deposited on a platinum plate (similar to0.03 cm(2)) was heated up to similar to 1500 K at a constant rate (similar to1-80 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 theta (0) increases from similar to 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 theta (0) less than or equal to 10(2) 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 (T-m), where the sample layers are usually less than similar to 10% of theta (0); (5) the ionization efficiency above T-m ranges from similar to 10(-2) to 10(-4) according to theta (0); (6) M+ is desorbed from those active sites (less than similar to 10% of the desorbing surface area) with high work function; (7) the chloride layers prepared at theta (0) less than or equal to 10 ML and heated above T-m are useful for efficiently producing M+ (similar to1 muA cm(-2)).