Low density observations of Rb and Cs chains along the liquid–vapour coexistence curves to the critical point in relation to quantum-chemical predictions on the metal-insulator transitions in Li and Na rings

Recent ab initio predictions concerning the metal-insulator (MI) transition in rings of the light alkali atoms, Li and Na, are compared and contrasted with experimental facts concerning diluted Rb and Cs alkalis. The main focus here is on the local coordination number as a function of density as these two heavy alkali metallic fluids are taken along the liquid–vapour coexistence curve towards the critical point, which in these cases coincides with the MI transition. Also recorded are the results of experiments in which Cs chains are observed at large interatomic spacing outside semiconducting substrates of InSb and GaAs.     

Desorption kinetics and surface diffusion of potassium,rubidium and cesium on a silicon(111)7 × 7-surface

Using pulsed atomic beam technique and a surface ionization ion microscope, the desorption kinetics and the surface diffusion of the alkalis potassium, rubidium and cesium were investigated on a Si(111)7 × 7-surface at extremely low alkali coverages. In the temperature range 1120 … 800 K, the mean adsorption lifetime τ(T) = τ₀ · exp(E_desⁱ/kT) and the mean diffusion length x(T) - defined in the equilibrium between adsorption, diffusion and desorption - were measured. From these data the diffusion constant D(T) = D₀ · exp(-E_diff/kT) was obtained as D = x^?2/τ. For temperatures T ? 750 K, the diffusion constant was calculated from nonstationary alkali concentration profiles using the Boltzmann-Matano method. From the temperature dependence of these quantities the parameters of desorption (E_des,ⁱ τ₀) and surface diffusion (E_diff, D₀) for K, Rb and Cs on Si(111) were obtained. The values of E_diff and D₀ are comparably high and may be interpreted by non-localized diffusion according to a model proposed by Bonzel (Surf. Sci. 21 (1970) 45).     

Broadening mechanisms for the adsorbate-induced resonance in the Na/Cu(1 1 1) and Cs/Cu(1 1 1) systems

Adsorption of alkali atoms on the (1 1 1) and (1 0 0) noble metal surfaces has been shown recently to induce long-lived resonances located inside the surface projected band gap. However, the width of these resonances, as it appears in two-photon photo-emission experiments, is much larger than the inverse of their lifetime. We report on a theoretical study of some broadening mechanisms of these resonance lines in the Na/Cu(1 1 1) and Cs/Cu(1 1 1) systems at low coverage, including the homogeneous natural line broadening and the inhomogeneous statistical broadenings due to the distribution of adsorption heights associated to the quantal vibration of the alkali adsorbate and to the lateral disorder of alkali adsorption on the surface. The inhomogeneous mechanisms are shown to induce a very large broadening of the resonance line, in quantitative agreement with experimental results. The most important broadening effect appears to be the effect of the distribution of alkali adsorption heights.     

Systematics of the metallization of low-Z and alkali fluids

Hydrogen, nitrogen, oxygen, cesium, and rubidium undergo nonmetal–metal (NMM) transitions in the degenerate warm fluid phase. It is quite likely that all these fluids are monatomic or very nearly so. For N, O, and H, these NMM transitions occur under quasi-isentropic compression to ～100?GPa (1?Mbar) pressures and densities of ～10 times initial liquid density in the case of H. These conditions were achieved with a two-stage gun. In the cases of Cs and Rb, these transitions occur at only ～0.01?GPa in the expanded fluid at 2000?K. These NMM transitions are Mott transitions. The values of the minimum metallic conductivities are essentially the same for all five because minimum metallic conductivity depends weakly on density of metallization and number of conduction electrons per atom. In contrast, the density dependences of the semiconductivities are very different. In the spirit of Mott, quantum mechanical wave functions of the free atoms are used to estimate the densities at which semiconductivies are appreciable. The radial extents of the charge-density distributions are well correlated with the Mott-scaled density dependences of the semiconductivities. These radial extents depend on the degree to which the filled-electron core screens the valence electron(s) from the nuclear Coulomb force. This simple picture gives a qualitative explanation for the density dependences of the semiconductivities of all five and for the Herzfeld criterion, which predates quantum mechanics.