Halogen adsorption on Fe(100): I. The adsorption of Cl2 studied by AES,LEED, work function change and thermal desorption

The initial sticking probability of chlorine on Fe(100) at room temperature is calculated to be 0.13, and there is evidence to suggest that the chlorine adsorbs into a short lived mobile precursor state above the surface. The work function change, Δφ, is proportional to coverage and reaches a maximum value of 1.43 eV at saturation. At this coverage a c(2 × 4) LEED pattern is formed. On heating, chlorine is lost from the surface, but the mechanism is such that no detectable loss is incurred at a constant elevated temperature. The c(2 × 4) pattern is shown to be a coincidence structure formed from a $\text{(}\text{1}\text{2}\text{3}\text{?1}\text{2}\text{3}\text{)}$ net of chlorine atoms on the Fe(100) substrate. This structure is a special case of the more general $\text{(}\text{1}\text{2}\text{tan}\text{α}\text{?1}\text{2}\text{tan}\text{α}\text{)}$ structure formed at lower concentrations of chlorine. The c(2 × 4) is formed when α = 56.31°, which gives the chlorine atoms a hard sphere diameter of 0.345 nm and a concentration of 0.75 atoms per four-fold site.     

A theory of order-disorder transition during crystallization of binary systems2

A theory is presented describing the order-disorder transition in binary crystals growing on the condition of supersaturation from nonsolid phases. The theory applies to systems that crystallize with an almost perfectly ordered structure if exposed to conditions close to thermodynamic equilibrium. Based on a model that assumes incorporation and detachment of single atoms to occur at the kink sites of mono-atomic steps existing at an otherwise smooth crystal surface a kinetic master equation for the time dependence of configuration probabilities has been formulated. Several simplifying assumptions have been employed. Any solution for the steady-state conditions depends on a roughness parameter λ, the far-order parameter η, the incorporation frequency ω₊ and a parameter q, related to the atomic interaction energies. The solutions are discussed for conditions prevailing near equilibrium with η 1 and within the range of order-disorder transition with η 0. The analysis reveals that for different incorporation frequencies different critical values of the parameter q exist for which a transition from an ordered to a disordered phase is predicted to occur. Each of these critical values of q corresponds to a critical transition temperature T_t.