Role of angular orientation of dipoles on work function during caesium deposition on a metal surface – a phenomenological model
It has been experimentally proven that adsorption of alkali metals like Caesium (Cs) on tungsten reduces tungsten’s effective work function. The work function of pure W surface is 4.55eV and that of pure Cs is 2.14 eV ; however with fractional monolayer (0.6 – 0.7) of Cs coverage on W surface makes the effective work-function of the W surface as low as 1.5eV . It is well known from previous experiments that chemisorption of foreign atoms on metal surfaces give rise to a strong double-layer. This may be positive outwards or negative outwards according to the nature of adsorbed atoms. The ionization potential of Cs (3.85 eV) is smaller than the work function of polycrystalline tungsten (4.5 eV). Consequently, the electropositive Cs will give up its valence electron to the metal, forming an electrical double layer with positive outwards and hence reducing the effective work function. The theory of adsorbate-induced work function change (WFC), is to a large extent based on the classical dipole model given by Langmuir , which had been further modified by Miller, by a simple point dipole depolarization model . However, this experimentally observed anomalous variation of work function during Caesiation has not been explained before using classical model for the full fractional range of monolayer formation (0≤Ѳ≤1). In the present work, it is shown analytically that the orientation of dipoles in the classical model indeed plays a significant role and explains the experimental trend of anomalous variation of work function. Fractional monolayer can be achieved by time-varying controlled deposition of Cs atoms on a metal surface.
The existing classical model is based on a 2-D square lattice of parallel point dipoles with an axis perpendicular to the line joining them. At a low coverage of adatoms, surface dipoles are non-interacting owing to which they tend to form an array of dipoles, all identically normal to the surface. However, at high coverage(ϴ>.5) in addition to the point depolarization, we propose that the repulsive interaction among the neighboring dipoles due to their closeness may lead to change in the orientation of the dipoles. Thus by including an effective angular orientation factor (AOF) of mutually interacting dipoles, we have made an analytical approach to extend the existing classical model. This phenomenological simple model reasonably explains the experimental results of anomalous variation of work function during Cs deposition on W surface.
 Herbert B. Michaelson. IBM Corporation, Armonk. New York 10504 (1977).
 V.M. Gavrilyuk, A.G. Naumovets, A.G. Fedorus, Soviet Phys.-JETP, 24 (1967).
 J.B. Taylor, I. Langmuir, Phys. Rev. 44, 423 (1933).
 A. R. Miller, Proc. Cambridge Philos. Soc. 42, 292 (1946).