Evaluation of the effective work functions of a binary salt using thermochemical data on its two constituent elements

The effective work functions (Φ⁺ and Φ⁻) for the emission of positive and negative ions (M⁺ and X⁻), respectively, from an inhomogeneous surface of a binary salt (MX) such as an alkali-metal halide are investigated on the basis of experimental results achieved in our previous work. Each of the work functions is generally expressed as a function of the thermochemical values of the constitutive elements. Φ⁺ = I + D + D⁰ − 2D⁺ Φ⁻ = A − D − D⁰ + 2D⁻ where I is the first ionization energy of the electropositive atom M, A the electron affinity of the electronegative atom X, D the dissociation energy of the molecule MX, D⁰ the desorption energy (or the heat of sublimation) of MX, and D^±the desorption energy of M⁺ or X⁻.