Work function of adsorption systems potassium on tantalum and on molybdenum

Work functions φ_hkl of thermally annealed and potassium covered tantalum and molybdenum as a function of potassium surface density on (011), (112), (100) and (111) planes of these metals have been measured using a field emission microscope. The measurements have been performed at liquid nitrogen temperature (immobile layers). The work function decreases linearly at first, then more slowly, passes through a minimum, and then attains a constant value. Quantitative data on the dependence of φ_hklon surface density of potassium, N_hkl, for tantalum, molybdenum and tungsten have been compared. The principal results of the observations are: (i) for K on Ta, Mo, and W the work function minimum exhibits no distinct dependence on the type of substrate, however, it proves to depend on crystallographic direction of the latter; (ii) the values of the high coverage limit work function are approximately equal for one type of metal planes; (iii) the values of the high coverage limit surface densities of potassium adsorbed on Ta(011), Mo(011) and W(011) surfaces are approximately equal to the surface density of the (011) plane of bulk potassium crystal.