Surface composition

Under normal conditions, a surface exposed to a gas is constantly bombarded with molecules and a freshly prepared surface is covered very quickly. Just how quickly can be estimated using the kinetic model of gases and the expression (eqn 21.14) for the collision flux:

ZW=

A practical form of this equation is

ZW= with Z₀=2.63×10²⁴ m⁻²s⁻¹

where Mis the molar mass of the gas. For air (M ≈ 29 g mol⁻¹) at 1 atm and 25°C the collision flux is 3 × 1027 m⁻² s⁻¹. Because 1 m2 of metal surface consists of about 1019 atoms, each atom is struck about 108 times each second. Even if only a few collisions leave a molecule adsorbed to the surface, the time for which a freshly prepared surface remains clean is very short.

The obvious way to retain cleanliness is to reduce the pressure. When it is reduced to 10−4 Pa (as in a simple vacuum system) the collision flux falls to about 1018 m−2 s−1, corresponding to one hit per surface atom in each 0.1 s. Even that is too brief in most experiments, and in ultrahigh vacuum (UHV) techniques pressures as low as 10⁻⁷ Pa (when ZW =1015 m⁻² s⁻¹) are reached on a routine basis and as low as 10−9 Pa (when ZW=1013m⁻²s⁻¹) are reached with special care. These collision fluxes correspond to each surface atom being hit once every 105 to 106 s, or about once a day. The layout of a typical UHV apparatus is such that the whole of the evacuated part can be heated to 150–250°C for several hours to drive gas molecules from the walls. All the taps and seals are usually of metal so as to avoid contamination from greases. The sample is usually in the form of a thin foil, a filament, or a sharp point. Where there is interest in the role of specific crystal planes the sample is a single crystal with a freshly cleaved face. Initial surface cleaning is achieved either by heating it electrically or by bombarding it with accelerated gaseous ions. The latter procedure demands care because ion bombardment can shatter the surface structure and leave it an amorphous jumble of atoms. High temperature annealing is then required to return the surface to an ordered state. We have already discussed three important techniques for the characterization of surfaces: scanning electron microscopy (Impact I8.1), which is often used to observe terraces, steps, kinks, and dislocations on a surface, and scanning probe microscopy (Impact I9.1), which reveals the atomic details of structure of the surface and of adsorbates and can be used to visualize chemical reactions as they happen on surfaces (Fig. 25.7). In the following sections, we describe additional techniques that comprise the toolbox of a surface scientist.

Fig. 25.7 Visualization by STM of the reaction SiH₃ → SiH₂ + H on a 4.7 nm × 4.7 nm area of a Si (001) surface. (a) The Si (001) surface before exposure to Si2H6(g). (b) Adsorbed Si2H6 dissociates into SiH2(surface), on the left of the image, and SiH3(surface), on the right. (c) After 8 min, SiH3(surface) dissociates to SiH2(surface) and H(surface). (Reproduced with permission from Y. Wang, M.J. Bronikowski, and R.J. Hamers, Surface Science 64, 311 (1994).)

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مواضيع ذات صلة

Surface growth

Experimental results

Electron tunnelling

Controlling chemical reactions with lasers

Spectroscopic observation of the activated complex

Quantum mechanical scattering theory

Classical trajectories

Attractive and repulsive surfaces

The direction of attack and separation

Potential energy surfaces

State-to-state dynamics

Experimental probes of reactive collisions