Gert Ehrlich

Atomic View of Surface Self‐Diffusion: Tungsten on Tungsten

Surface diffusion of tungsten adatoms on several smooth, low‐index planes of the tungsten lattice has for the first time been followed by direct observation of individual atoms in the field‐ion microscope. Contrary to expectation, the mobility at room temperature is found to increase in the order (211) > (321) ∼ (110) > (310) ∼ (111). Migrating atoms are reflected at the boundaries of the (110), (211), and (321) planes; on the latter two, motion along atomic rows is favored over diffusion across lattice steps. From quantitative determinations of the rate of change of the mean‐square displacement, diffusion coefficients are obtained as follows: (110), D=3×10−2exp(−22 000/RT)cm2/sec; (321), 1×10−3exp(−20 000/RT); (211), 2×10−7exp(−13 000/RT). Differences in diffusion on the (211) and (321), planes of very similar structure, suggest a weakening of interatomic forces at lattice edges.

MODERN METHODS IN SURFACE KINETICS. FLASH DESORPTION, FIELD EMISSION MICROSCOPY, AND ULTRAHIGH VACUUM TECHNIQUES

Publisher Summary This chapter describes modern methods in surface kinetics. The tremendous advance in the development and perfection of a variety of experimental techniques has been made possible, in large measure, by the reduction of ultrahigh vacuum techniques. It has become easy to specify exactly the environment at an interface and to perform meaningful measurements on samples with a surface area of less than a square centimeter. Flash desorption, although still dependent upon macroscopic wire samples, has made it possible to measure the rate processes involving the transfer of molecules between the gas phase and the solid. Even the dependence of surface kinetics on atomic structure could be established by studies on macroscopic samples. The power of the flash desorption technique lies in its ability to give a rapid, direct count of the number of adsorbed species on a surface. However, information on the properties of the adsorbed layer is obtained only indirectly by deduction from adsorption and desorption measurements. To supplement these indirect studies, there are needed techniques, such as field electron microscopy that yield information on the properties of the adsorbed material by direct observation of the gas layer.

Chemisorption on Single‐Crystal Planes: Nitrogen on Tungsten

The effects of surface structure on the adsorption of nitrogen have been examined by contact‐potential measurements on macroscopic planes cut from single‐crystal tungsten. On the (100), room‐temperature chemisorption lowers the work function by 0.4 eV, confirming previous field‐emission results. Flash desorption reveals only one state, β, in which nitrogen is bound as adatoms. On the considerably less reactive (111), the work function φ rises by 0.15 eV. Two states contribute: α, which is probably molecular, and β; both raise φ. At higher concentrations, a second β state is isolated by flash desorption. Nitrogen does not chemisorb at all on the (110) at 300°K and p∼10−7 mm. Adsorption of γ nitrogen can, however, be observed by lowering the temperature to T∼130°K or raising the pressure above 10−3 mm. On the (110), this state is atomic and lowers φ. On other planes, γ nitrogen has different properties and forms because of the heterogeneity induced in the surface layer by the presence of adatoms. Macroscopi...

Kinetic and Experimental Basis of Flash Desorption

Techniques are developed for deriving both qualitative and quantitative information on the kinetics of gas desorption from measurements at continuously changing temperature. First‐ and second‐order processes can be distinguished immediately by the constancy of the end point of the former. Quantitative values for activation energy and frequency factors are deduced from the experimental time‐temperature curve and the instantaneous slopes of the evolution curve, even for systems with concentration‐dependent rate parameters. It is shown that for multiple desorption peaks, qualitative detection is simplified by slow heating, but may result in interconversion. The experimental basis of desorption measurements using the Bayard‐Alpert gauge is also analyzed, together with artifacts arising from negative pressures, bistable gauge operation, formation of new species in the gauge, and the delay in sensing density pulses transmitted through tubes.

Trapping and Energy Transfer in Atomic Collisions with a Crystal Surface

The condensation of atoms on a solid is examined by calculating the critical energy for trapping of a particle of arbitrary mass and force constant colliding with a linear lattice. In the harmonic approximation and using classical mechanics, the maximum kinetic energy for trapping is found to depend strongly upon the well depth, in qualitative agreement with experiment. For a gas atom colliding with its own lattice (that is, force constant ratio β=1 and mass ratio μ=1) capture occurs at translational energies up to 25 times the binding energy. For β=0.2, this critical energy has diminished to 1.3 times the dissociation energy Q of the homogeneous lattice. The dependence of the critical energy on force constant is not monotonic, however—there is a maximum at β∼0.75. At β=0.75 and μ=1, for example, the incident energy must exceed ∼31Q to prevent trapping.Most of the energy exchange occurs during the repulsive part of the collision. For this part of the collision interval, the presence of a trapping potentia...

Structure-sensitive chemisorption: The mechanism of desorption from tungsten

Abstract The validity of measurements on evaporated tungsten films has been examined and found in doubt; the flash-filament technique has therefore been extended to yield data on the stability of binding states of gases adsorbed on metals. Qualitative results are presented for the adsorption of nitrogen, hydrogen, and oxygen on tungsten. Just as for nitrogen at room temperature, two states of binding have been isolated for both hydrogen and oxygen; for the latter two gases the pressure-measuring device seriously affects the adsorption. In the absence of atomic hydrogen produced by the ionization-gauge filament, in experiments with hydrogen only a single state of adsorption is found, which desorbs continuously from 100 to 540°K. The methods for deriving quantitative data on rates of desorption from pressure/temperature curves obtained by the flash-filament technique are presented, and are applied to a study of the evolution of β-nitrogen from tungsten. The desorption is found to obey second-order kinetics, indicating that the nitrogen is dissociated and exists on the surface as adatoms. Although the heat of desorption is constant for all coverages examined (η β =10−8O×10 12 molecules cm 2 ) , the kinetics differ from sample to sample; heats varying between 58 and 81 kcal mole have been measured, and a correlation between high heats and high rates of adsorption has been observed. Conditions for the maintenance of surface equilibrium are derived, and from these and the experimentally observed continuous evolution of β-nitrogen, the heterogeneities primarily responsible for variations in the heat of desorption are identified as being of a small scale, possibly associated with the presence of lattice steps. The implications of structural effects in the interpretation of the decrease in the heat of adsorption with increasing cover, reported in the past for other systems, are analysed, and it is shown that in a mobile adlayer only interactions between adatoms can bring about such a diminution. The possible structure-sensitivity of chemisorption also invalidates past attempts to establish the nature of the surface bond from measurements of the heat of adsorption on different metals; an examination of the available data shows that a simple correlation between d -band structure of the solid and activity in chemisorption is not warranted.

Interaction of Rare Gases with Metal Surfaces. I. A, Kr, and Xe on Tungsten

The interactions of argon, krypton, and xenon with a tungsten surface have been examined in the field emission microscope. Bombardment by rare gas ions results in a penetration of the lattice and an apparent diminution of the work function; bombardment damage is partly annealed between 79° and 300°K. Adsorption lowers the work function by 1.38 ev for Xe, on saturation at 79°K, 1.18 ev for Kr at 20°K, and 0.87 ev for A at the same temperature. The work function diminishes monotonically with Xe coverage even after more than a single layer is formed.Binding to the surface is structure sensitive—it is strongest around the 100 pole, in the vicinity of the 116 and 130 planes, and weakest at the 111. This sequence of binding energies corresponds to that calculated for different lattice sites on the assumption of dispersion forces. The lowering of the work function caused by adsorption is interpreted as a polarization of the gas atoms by the dipole layer of the metal surface; formally, the interactions at a metal...

Molecular Dissociation and Reconstitution on Solids

The dependence of heterogeneous reactions on the nature of the solid surface is explored by analyzing the elementary rate processes involved. The relations between the rates of adsorption, surface diffusion, and evaporation, and the depth and distribution of binding sites on the surface are presented. Empirical generalizations concerning the binding energy of adatoms and the cohesive energy of solids are examined; Paulings additivity rule is found to fail for most metal hydrides and thus does not provide a reliable base for estimating the covalent contribution to the heat of adsorption.Rates of association of atoms and dissociation of molecules on solids are estimated from a knowledge of the experimental heat of adsorption — ΔH and the postulate that the activated complex for dissociative chemisorption of a diatom has a configuration resembling two noninteracting atoms, one in its equilibrium binding position on the surface, the other in the saddle‐point position for surface migration. For chemisorption ...

Low‐Temperature Chemisorption. I. Flash Desorption of Nitrogen

Flash desorption (desorption by continuous temperature displacement) is applied to a study of the low‐temperature interactions of N2 with an initially clean tungsten surface. Molecular nitrogen is found to dissociate into atoms on adsorption even at 115°K, at a rate which diminishes with increasing temperature, but is initially independent of surface concentration. Formation of this atomically bound β nitrogen is hindered at low temperatures (T∼115°K) by competitive growth of an additional state γ, in which nitrogen, at ∼2½ times the concentration in the β state, is bound as molecules with an energy of 9 kcal mole—1, resulting in a total surface concentration of 600×1012 molecules cm—2. The population in this γ state depends sensitively upon the arrangement of atoms in the β state. Preadsorption at T∼300°K equalizes the populations in β and γ; annealing at T∼1000°K at impingement rates of 7×1015 molecules cm—2 min—1, further lowers nγ/nβ and brings about rearrangement of the tungsten surface as well, with...

The mechanism of chemisorption on metals

Abstract The available experimental data on the adsorption of diatomic molecules on a bare metal surface are analysed to establish the kinetic processes leading to chemisorption. It is shown that, with the possible exception of hydrogen, molecules physically adsorbed on the surface form a reservoir from which chemisorption can occur. Kinetic mechanisms involving such a physically adsorbed precursor are developed, both for reaction on a uniform surface as well as for reaction at surface singularities, such as grain boundaries or lattice steps. Analysis of the rate of adsorption of nitrogen on tungsten reveals that during the initial stages the rate of chemisorption is diffusion controlled, and occurs preferentially at heterogeneities of atomic dimensions, which are tentatively identified with lattice steps on the surface.