R. Gomer

Desorption from Metal Surfaces by Low‐Energy Electrons

The effect of low‐energy (15–200‐eV) electrons on hydrogen, oxygen, carbon monoxide, and barium adsorbed on tungsten has been investigated by a field‐emission technique. Desorption cross sections σ were determined from work function and Fowler—Nordheim pre‐exponential changes and are significantly smaller than would be expected for comparable molecular processes. Marked variations in cross section with binding mode within a given system were found. Thus σH=3.5 10—20 cm2 and 5×10—21 cm2 for processes tentatively interpreted as the splitting of molecularly adsorbed H2 and desorption of H, respectively; σ0=4.5×10—19 cm2 for a loosely bound state and σ0≤2×10—21 cm2 for all other states; σBa<2×10—22 cm2 under all conditions. In the case of CO (reported in detail elsewhere), three binding modes observed previously could be confirmed and differentiated by their different cross sections: σvirgin=3×10—19 cm2; σβ=5.8×10—21 cm2, σα=3×10—18 cm2; conversion by electrons of virgin to β σvβ≥10—19 cm2. These results are ...

Theory of Field Desorption

The theory of field desorption proposed previously has been extended by a more detailed quantum‐mechanical calculation of the rate constants of desorption for various cases, including an attempt to evaluate matrix elements for the electronic transition when the latter is slow. A simple, generalized model of chemisorption on metals is presented and used in these calculations. It is found that two‐electron transitions lead to somewhat smaller rate constants than one‐electron transitions, but that the effect is not very large. In general, a one‐dimensional theory leads to rate constants with essentially normal pre‐exponential terms, except under conditions where adsorbate tunneling becomes important. A tentative explanation for the compensation effect, i.e., decreases in pre‐exponential term with decreasing activation energy observed experimentally in a number of different cases, is given in terms of changes of reaction path with temperature.

An experimental determination of absolute half-cell emf's and single ion free energies of solvation.

The concept of absolute half‐cell emf is discussed and defined as VMS‐φM for the reaction M→M+(solution)+e−(M), where VMS is the electrostatic potential difference between metal electrode and solution and φM the work function of metal in contact with solution. It is shown that this quantity is equal to VRS‐φR, where VRS is the electrostatic potential difference between a reference electrode in air above the solution and the solution, and φR the work function in air of this reference. The quantity VRS′, the potential difference between reference electrode and the solution surface, was found experimentally by the vibrating condenser method for a number of half‐cells, and φR was determined photoelectrically. It is shown from the variation of VRS′ with electrolyte concentration that the potential difference betwen the bulk of pure H2O and its air interface is ∼0.05 V, the surface being negative relative to bulk, and that this potential is increasingly screened out as electrolyte concentration increases. From ...

Adsorption of Potassium on Tungsten

The adsorption of potassium on thermally annealed and field‐evaporated tungsten surfaces was studied by field emission. The φ‐ vs‐θ curve has a minimum of 1.78 eV at θ=0.83, n=3.2×1014 atom/cm2 (which coincides with the minimum of φ110), and approaches φK at θ=2–3. Distinct minima also occur for other close‐packed planes, like 211. At θ=1, n=3.9×1014 atom/cm2, φ=1.95 eV the emission pattern is indistinguishable from that of clean tungsten. The average charge per K atom, q, varies from 0.27 at low θ to 0.2 electron charges at θ=0.8. The value of q on the close‐packed W planes at low coverage may be considerably higher.The activation energy of neutral desorption varied from 2.67 to 2.4 eV over the interval 0<θ<0.4 and fell rapidly at higher coverage, approaching the heat of sublimation of K, 1.0 eV, at θ=1.0. The activation energy of ionic desorption was measured over the interval 0<θ≤0.07, and indicates that ionic desorption occurs on regions of φ=5.3 eV. dφ/dθ on these regions is very high, ...

Adsorption of Potassium on Tungsten: Measurements on Single‐Crystal Planes

Work functions of the (110), (211), (100), and (111) regions of a thermally annealed tungsten field emitter were measured as a function of potassium coverage. Immobilely adsorbed layers yielded absolute dipole moments, permitting estimates of adsorbate charge. The results at low coverage are μ110=15.7 D, μ211=13.6 D, μ100=11.5 D, μ111=9.5 D, with μ defined as 2qd0=Δφ/2πn. At coverages θ≳0.6 there is evidence of higher layer formation and field enhancement. Work functions after thermal equilibration in combination with φ‐vs‐n data for immobilely adsorbed deposits yielded coverage anisotropies under equilibrium conditions. At θ≈0 n110/n=40,n211/n=0.35,n111/n<0.04,n100/n<0.01. As θ increases the anisotropies decrease, and for θ≳0.5 the ratios n→1 for all regions except 100. The corresponding differences in heats of adsorption at θ≈0 are less than 0.4 eV for all regions except possibly 100, with binding strongest on 110. A quantum‐mechanical model is briefly sketched, and a quasistatistical treatment ...

Current fluctuations from small regions of adsorbate covered field emitters: A method for determining diffusion coefficients on single crystal planes

A novel method is presented for determining surface diffusion coefficients of adsorbates on single crystal planes of field emitters in terms of the time correlation function of current fluctuations. The method is based on the fact that current fluctuations are related to adsorbate density fluctuations, whose time correlation is governed by relaxation times simply related to diffusion coefficients. A general formalism is presented, some idealized cases are worked out analytically, and the case of a circular aperture is treated in detail numerically. Switching between different adsorption states, in addition to but not affected by diffusion, is also included. The forms of the correlation function are shown to be complicated, with a 1t tail at large t, and cannot be represented by simple exponential decay.

Mobility and Adsorption of Hydrogen on Tungsten

The surface diffusion of hydrogen on W field emitters has been investigated by making use of the low vapor pressure and high sticking coefficient of H2 at liquid helium temperature to obtain unilateral contamination. Average values of the energy of desorption of H2 have also been obtained. It is found that spreading occurs in three distinct phases. At very high initial coverages a moving boundary migration occurs below 20°K, corresponding to migration of physically adsorbed H2 on top of the chemisorbed layer, precipitation at its edge, and further migration over the newly covered region. For initial deposits corresponding to ∼1 monolayer spreading sets in at ∼180°K with a boundary moving radially outward from the close‐packed 110 faces. The activation energy of this process is ∼6 kcal and corresponds to migration over the smooth (close‐packed) regions of the tip followed by precipitation of chemisorbate at trap sites on the rough regions. At coverages too low to permit saturation of traps diffusion out of...

A Monte Carlo study of surface diffusion coefficients in the presence of adsorbate–adsorbate interactions. II. Attractive interactions

Chemical, jump, and tracer diffusion coefficients, D, Dj, and D*, respectively, for the case of nearest‐neighbor repulsive interactions have been determined by Monte Carlo simulations on a fixed square lattice. D was determined by the fluctuation autocorrelation function method and from the Kubo–Green equation, using values of 〈(δN)2〉/〈N〉 determined from isotherms. It was found that the two methods give essential agreement over the entire coverage and temperature range examined, 0.73≤T/Tc≤7.33, 0.1≤θ≤0.8. The changes in activation energies for D, D*, and Dj as function of T and θ were determined and can be understood in terms of the phase diagram of the system.

Mobility of oxygen on the (110) plane of tungsten

Abstract The correlation function of field emission current fluctuations from the (110) plane of tungsten field emitters covered with oxygen has been measured in the range 0.15 ⩽ O/W ⩽ 0.56 for 300 ⩽ T ⩽ 800 K. At intermediate temperatures the experimental correlation functions agree with a theoretical model based on single particle diffusion, and yield diffusion coefficients and activation energies of diffusion of O atoms on this plane. Edif = 14 kcal for O/W ⩽ 0.20 and then increases to 22 kcal for O/W = 0.56. The preexponential D0 = 10−7 cm2 sec−1 at low coverage and increases to 10−4 cm2 sec−1 at high coverage. At high temperatures (600–700 K, depending on θ) the decay of the correlation function is slower than predicted by theory. It is suggested that this results from dynamical correlations, or multi-particle density fluctuations, which can occur when several adsorbate particles are simultaneously in motion. At low temperatures (300–500 K, depending on θ) exponential decay of the correlation function is observed, and explained as a prediffusive flip-flop of O atoms between binding locations. The temperature dependence of the mean square fluctuations in the diffusive regime indicates mean repulsive interactions of 5–8 kcal between O atoms.

Field Emission in Liquids

Field emission from tungsten emitters into liquid H2, D2, He, Ar, N2, O2, and C6H6 has been carried out in modified field emission tubes, which permitted switching from ultrahigh vacuum to very pure liquids and back to vacuum. In all cases space‐charge‐limited behavior was found at >10−9 A, from which the following mobilities (in centimeter2 volt−1·second−1/units) could be estimated: 3 × 10−2 cm2 V−1·sec−1 for H2 at 21°K, 0.9∼10−2 for D2 at 21°K, 4 × 10−2 for He at 4.2°K, 0.36 for He at 1.8°K, 8 × 10−3 for O2, 8 × 10−3 for N2, and 4 × 10−3 for C6H6. In the case of Ar a limiting velocity of 5 × 106 cm/sec rather than a field‐dependent mobility was observed. For He and Ar onset of high emission (10−9 A) is abrupt, indicating electron avalanching with concomitant gas‐bubble formation at the tip which leads to sputtering. In the case of O2 electrochemical etching occurred even at low currents. For H2 and D2 a true Fowler–Nordheim field emission regime could be observed for i≤10−10 A, from which an energy barr...