R. Błaszczyszyn

Work function of the adsorption system of potassium on tungsten

Abstract The work functions φhklof tungsten single crystal planes as functions of the surface densities Nhkl of the adsorbed potassium have been measured by means of the field emission method. Sealed-off field emission tubes with a Faraday collector and rotatable emitter were used. Another, special tube was made in order to determine the surface density of potassium. The unequilibrated adlayers on {110}, {112}, {100} and {111} tungsten single crystal planes have been investigated. For all the planes investigated the φhkl(Nhkl) dependence exhibited a distinct minimum. An attempt has been made to compare the experimental results with the theoretical models suggested recently.

Interactions of sulfur with nickel surfaces: Adsorption, diffusion and desorption

The kinetics of adsorption, surface diffusion and thermal desorption of sulfur on Ni surfaces have been studied using field electron emission microscopy methods. The sticking probability for elemental sulfur sublimed onto a Ni specimen is approximately unity for Ni substrate temperatures from 77 to 530 K. The maximum average work function for ~ monolayer coverage of S ranged from 4.7 to 5.05 eV in different experiments; for fractional monolayer doses of S, surface diffusion was rapid at T 400 K, with activation energies ranging from 15 to 28 kcalmol. For multilayer adsorption of sulfur, diffusion occurs without motion of a sharp boundary, and there is evidence of extensive surface reaction between S and Ni (emission from small “crystallites” is evident in the field emission patterns). Sulfur desorbs from Ni at temperatures above 1500 K.

Coverage dependence of fe flicker noise: Spectral density functions due to potassium on W(112) and W(111)☆

Abstract Field emission flicker noise spectral density functions W (ƒ) have been determined for potassium on W(112) and W(111) single planes. The coverage dependence of the spectral densities W (ƒ j ) shows pronounced maxima and minima, whereas the slopes ϵ obtained from double logarithmic plots of W (ƒ) ~ ƒ −ϵ vary considerably. Minima and maxima of W (ƒ j ) are assumed to be due to coherent and disordered adlayers, respectively, and the behaviour of the exponents ϵ supports further the proposed observation of order-disorder transitions of the potassium adsorbate. LEED results for W(112)K and W(111)K are in fair agreement with the corresponding coverages from noise measurements.

Comparison of experimental noise spectral densities of K-covered tungsten emitters with field-emission flicker-noise theories

Abstract Spectral-density functions W(f) (f is the frequency) of the field-emission flicker noise were investigated for various emission currents with sealed-off field-emission microscopes of the Muller type. The tungsten emitters were covered with potassium in order to observe the surface diffusion-induced flicker noise. The constancy of the coverage for each set of measurements was secured with special precautions. The main object of the experiments is the exponent e of the spectral density W(f) ∼ f-e. An f-1.5 dependence as required by the theory of Timm and Van der Ziel and an increase of e with the emission current could not be detected. Therefore, the experimental results seem to be in better agreement with the surface-diffusion model of diffusion-induced field-emission flicker noise as suggested by Kleint.

The interaction of water with surfaces of Pt and Ir field emitters

Abstract Some results of a study of the adsorption and thermal desorption of H 2 O on Pt and Ir surfaces, using field emission methods, are presented. At a substrate temperature of 78 K H 2 O admolecules reduced the total field emission current both from the Pt and Ir tips in a wide range of coverages whereas the local emission current, e.g. from the (001) and (113) faces of Ir, was increased. Water was desorbed from thermally cleaned Pt in the temperature range 140–160 K with an average desorption energy of E des = 54kJ/mol. The possible behavior of H 2 O molecules on the crystallographically diversified surfaces of Pt and Ir under the conditions of field emission is discussed.

Correlation of field emission flicker noise and work function for tungsten single crystal planes with adsorbed potassium

Abstract The flcker noise power of the (112), (110), (100), (111) and (122) planes as well of (110) vicinals of a tungsten field emitter has been measured as a function of potassium coverage. Distinct minima of the noise power for coverages that correspond exactly to the work function minima could be found. It seems that this coincidence might be explained — apart from big lateral interactions of the adparticles — by the stronger adsorbate-substrate interaction in a coherent adiayer than in an incoherent one and/or by an adlayer transition to the “metallic” state resulting in compensation of the ionic adsorbate charge by a common electron gas.

Interaction of water with field emitter tips

The contribution of high-field techniques to solid-surface studies of the interaction between water and metal surfaces is reviewed. Particular attention has been paid to certain aspects of studies that concern the influence of high electric field on the adsorption layer of water in the multilayer and submonolayer coverage range. Recent results of investigations of the diffusion of water on metal surfaces are discussed.

Ni adsorption on the principal faces of a tungsten crystal : work function changes and adsorbate distribution

Abstract The field electron microscope was used to study work function changes as a function of the amount of Ni deposition on a W tip at 78 K and annealed at various temperatures from 78 to 870 K. The work function changes were measured for the (110), (112), (111) and (100) faces of the W crystal. Strong differences were found in the work function changes for these four faces. The surface density of the adsorbate corresponding to the extreme of work function change for each face was equal to the reticular density of the substrate surface. On the basis of this, the distribution of the Ni adatoms on the surface of the W single crystal under the conditions of thermal equilibrium was determined. The results are compared with already known properties of the Ni/W adsorption system.

EFFECT OF PREADSORBED NI ATOMS UPON THE POTASSIUM DIFFUSION ON THE W(112) PLANE

Diffusion of potassium on the W(112) plane with preadsorbed nickel was investigated by means of the spectral analysis of field emission flicker noise (FEFN). The analysis in terms of the Timm and Van der Ziel model was used which gives the surface diffusion coefficient, D, of potassium. Derived from its temperature dependence the diffusion activation energy Ed is presented for some Ni coverages at a constant potassium dose (\gqk = 0.4). The Ed dependence on the nickel coverage — first reduction and then an increase of Ed with increasing Ni coverage — is in agreement with the curve obtained from cross-correlation FEFN measurements of the same system. The reduction is understood to be a result of the smoothing effect caused by Ni atoms on the W(112) plane.

Influence of surface roughness on adsorbate mobility

Field-emission fluctuations of the coadsorption system W(112)Ni-K were investigated by the cross-correlation technique in two perpendicular crystallographic directions at various temperatures. In- and cross-channel movements show a considerable reduction ofτmax which is the time delay corresponding to the cross-correlation function maximum with increasing Ni coverage at a constant potassium probe concentration.τmax, its activation energy and the “signal velocity” are discussed in terms of the well-known W(112)Ni surface structure.