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.     

Work function of stepped tungsten single crystal surfaces

The work function of regularly stepped tungsten single crystal planes has been investigated as function of terrace width and step orientation. The terraces were always formed by (110) planes. One set of samples exhibited step edges running parallel to the [001] direction but different terrace widths. Another set with almost equal terrace width showed step edges of different directions with correspondingly different edge structures. The work function measurements were performed using thermionic emission in the temperature range 2000–2800 K. The work function decreases linearly with step density for a given step orientation. Different step orientations give rise to slightly different work function reductions. The results are interpreted by attributing additional dipole moments to edge atoms. Different edge structures lead to different dipole moments. These findings are consistent with Smoluchowski's “smoothing effect” of the electron charge distribution caused by the structural arrangement of surface atoms. The temperature coefficient of the work function also depends strongly on step density and step orientation.     

Local modifications of the surface work function by potassium adsorption

We show that at low potassium coverage the K/Ni(111) surface has two kinds of sites: one whose local work function is close to that of Ni(111) and another whose local work function is much smaller. The evidence is provided by the metastable quenching spectrum of K/Ni(111) and Xe covered K/Ni(111).     

Work function in field emission — the (110) plane of tungsten

Field emission for the (110) plane of field evaporated and thermally annealed tungsten has been restudied with particular reference to the significance of interpretation of the work function. It is concluded that the results obtained in field emission are consistent with results from non-field emission methods where the important variable of the local field is adequately considered. Possible contribution of other factors to measurement of the clean surface work function is also discussed.     

The adsorption of CO on the (100) plane of tungsten; thermal and work function measurements

Thermal desorption and work function measurements indicate that a largely molecular layer, with some dissociation, is formed at 80–100 K, with an increase in work function of 0.55 eV. The coverage in this layer is 11.5 × 10¹⁴ molecules/cm², or CO/W = 1.15. On heating, equal amounts of a β precursor, possibly dissociated, and a molecular α species are formed at ≈300 K, with abundances of 5 × 10¹⁴ molecules/cm² each. The α desorption is complete at 360 K. The β precursor evolves on heating without desorption in the range 400–700 K as indicated by work function decreases, to β-CO, which is almost certainly dissociated. This change occurs at lower temperatures for low coverages. Thermal desorption shows 3 peaks, which have been traditionally labelled β_{1, β2}, and β₃ at 930, 1070, and 1375 K. Of these only β₃ corresponds to a well defined state. Readsorption after heating to 950 or 1150 K results in a doubly peaked spectrum at 1070 and 1375 K. The β₁ and β₂ peaks obey complex desorption kinetics, probably corresponding to desorption and rearrangement. The coverage of β₃ is 2.5 × 10¹⁴ molecules/cm², suggesting that the c(2 × 2) LEED pattern corresponds to occupany of every other unit cell by a C or an O atom. For coverages ? 1.5 × 10¹⁴ molecules/cm² β₃ desorption obeys second order kinetics with an activation energy of 83 ± 3 kcal/mole. For β₃ the work function decreases from the clean W value by 0.1 eV, suggesting adsorption of C and O in the center of the W unit mesh, below the surface layer of W atoms. Readsorption on β and β precursor layers leads to formation of electropositive α-CO, with a multiply peaked thermal desorption spectrum, indicating the existence of different binding sites. Adsorption-heatingreadsorption, -heating-readsorption sequences indicate that additional changes in the α desorption spectrum occur, suggesting reconstruction in the β layer.     

Lithium adsorption on hot oxygen covered tungsten surfaces

The mean residence times τ of lithium particles on oxygen covered tungsten surfaces were measured accurately over a wide temperature range (1200 < T < 1900 K) by the beam modulation technique with a lock-in analyzer. A predominant monocrystalline W(100) structure was obtained by recrystallization of a polycrystalline tungsten ribbon. The residence time was determined as a function of the oxygen coverage θ and the temperatureT of the surface. The desorption energy l and the preexponential factor τ₀, calculated from the Arrhenius equation, are not only dependent on the amount of adsorbed oxygen but also on the oxygen structure. Apparently the desorption parameters l and τ₀ are correlated. An increasing desorption energy l is connected to a decreasing “vibration period” τ₀ whereby the influence on the residence time is partly compensated.     

Kinetic isotope effect in hydrogen adsorption on tungsten

A kinetic isotope effect in the rate of adsorption of hydrogen on polycrystalline tungsten has been sought, following the report of such an effect on a (100)-oriented tungsten crystal. However, no difference between the isotopes could be detected. The reason for these discordant results may lie either in an unidentified experimental artefact or in a genuine difference between oriented and polycrystalline surfaces.     

The adsorption of beryllium on the tungsten (211) surface

The adsorption of beryllium on a W(211) surface was investigated by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), work function change (Δφ) measurements and thermal desorption spectroscopy (TDS). Measurements were made at room temperature before and after annealing of the adsorbed layer. The work function change depends on the thermal history of the sample and can be explained in terms of a temperature-dependent adsorption process involving double layer formation.     

The adsorption of sulfur on the tungsten (110) surface

The adsorption and desorption of S on a W(110) surface is studied by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), work function change (δφ) measurements and thermal desorption spectroscopy (TDS). The evolution of the structure with coverage θ is quite different from that reported for S on Mo(110) and — at low coverages — from that of Te on W(110). At low coverages the structure indicates complex lateral interactions. The bonding changes with coverage similar to S on W(100). Evidence for more complex desorption kinetics than assumed in the past is presented.     

Surface photoemission from ultrathin potassium films adsorbed on tungsten

The initial stage of formation of ultrathin potassium films on W(100) is studied by threshold photoemission spectroscopy using p-and s-polarized light in a photon energy range of 1.6–3.5 eV. It is found that the photoemission current spectrum depends on the surface coverage by the alkaline atoms. Mathematically, this shows up as the dependence of the matrix elements responsible for photoemission excitation on surface coverage. The matrix elements vary because the photoelectron escape depth is small; hence, the emission comes from the surface layer under irradiation by both p-and s-polarized light.     

Effect of Cu adsorption on the tungsten (100) surface resonance

It has recently been observed that adsorbed Au and Hg do not quench the W (100) surface resonance up to monolayer coverage. We report here that Cu does quench the W (100) surface resonance at θ≧0.5 monolayers. This suggests that the effective potential of Cu differs more strongly from that of W than do the effective potentials of Au or Hg.     

The influence of potassium on the adsorption of hydrogen on iron

Preadsorption of potassium on Fe(100) and (111) surfaces causes an increase of the adsorption energy of hydrogen by about 2 kcal/mole and a lowering of the sticking coefficient for hydrogen adsorption by about a factor of two. Work function measurements indicate the existence of electronic interactions.     

The adsorption and nucleation of zirconium on tungsten field emitters

The adsorption of zirconium on clean tungsten, at residual gas pressures below 1 × 10⁻¹⁰ Torr, has been studied by field emission microscopy. It was necessary to outgas the Zr above 2400 °K to obtain reproducibility. The gases evolved during degassing of the zirconium were predominantly CO, CH₄ and H₂. Mass spectra of the evaporant flux showed no tungsten or oxide contamination after thorough outgassing. It was found that heating below the -β phase transition temperature of Zr (1135 °K) resulted in the formation of large cap-shaped nuclei on the high index planes, while above this temperature the deposit became smoothed on the {100} planes. A model is proposed involving a closepacked square array of Zr atoms on the W(100). We conclude that the above two temperature regions are eminently suitable for studying epitaxy following the initial growth processes of nucleation or of monolayer addition. The presence of contamination impeded both surface migration and nucleation and lowered the work function.     

Field emission studies of selenium adsorption on tungsten and molybdenum

The adsorption of selenium on tungsten and molybdenum field emitters has been studied. Despite the topographical similarity of the two substrates there are marked differences in the migration and localised adsorption of the first two monolayers of selenium. Anomalous behaviour of the emission current and Fowler-Nordheim parameters is found for adsorption on both substrates. Measurements on single crystal planes indicate that this behaviour is not caused by localised growth or clustering effects. It is suggested that the model proposed by Nicolaou and Modinos to explain similar effects during germanium adsorption on tungsten is also applicable in the present case.     

ESCA study of carbon monoxide and oxygen adsorption on tungsten

The chemisorption of both CO and O₂ on a clean tungsten ribbon has been studied using an ultrahigh vacuum X-ray photoelectron spectrometer. For CO, the energy and intensity of photoemission from O(1s) and C(1s) core levels have been studied for various adsorption temperatures.At adsorption temperatures of ～100 K., the “virgin”-CO state was the dominant adsorbed species. Conversion of this state to more strongly-bound β-CO is observed upon heating the adsorbed layer to ～320K. Thermal desorption of CO at 300?T?640 K causes sequential loss of α₁-CO and α₂-CO as judged by the disappearance of O(1s) and C(1s) photoelectron peaks characteristic of these states.Oxygen adsorption at 300K gives a single main O(ls) peak at all coverages, although at high oxygen coverages there exist small auxiliary peaks at ～2eV lower kinetic energy. The photoelectron C(1s) and O(1s) binding energies observed for these adsorbed species are all lower than for gaseous molecules containing C and O atoms. For CO adsorption states there is a systematic decrease in photoelectron binding energy as the strength of adsorption increases. These observations are in general accord with expectations based on electronic relaxation effects in condensed materials.