1184 Surface processes in adsorption

with respect to coverage, and various terms related to the atomic vibrations (as you would by now expect).

4.3.2Adsorption out of equilibrium with the gas phase

The examples of physisorption, discussed above, are typically, but not always, in equilibrium with the gas phase. In these cases the state of the system depends on T, and also on p. But at low T, exchange with the gas phase can be extremely, even in®nitesimally, slow. Phase diagrams which use u and T as axes are favored by experimentalists in chemisorption, and more generally at low T, where the pressure goes exponentially to zero. Often in these diagrams the pressure is not known, and there may thereby be some uncertainty about the true nature of the equilibrium. In this case, which can occur for physisorption and frequently occurs for chemisorption, the gas pressure is not only immeasurably low, but is irrelevant for discussion of the behavior of the system.

Typically such systems are treated as closed 2D systems, the equilibrium (or lack of it) with the 3D gas being ignored. This is reasonable for dissociative chemisorption at low and moderate temperatures, owing to the very high adsorption energy of the atoms: they are literally con®ned to the surface layer. A metal±metal chemisorption example where the equilibrium with the gas is taken into account at higher temperature is the AES and work function (Df) data for Au/W(110) (Kolaczkiewicz & Bauer 1984). In this data, AES is sensitive to the total Au coverage u within the ®rst ML, but Df depends on whether the atoms are in the form of large islands (f higher) or as isolated adatoms (f lower). Thus the data are suYcient to map out the 2D gas±2D solid phase equilibrium on a u±T plot.

Two examples from the recent physisorption literature will be suYcient to illustrate these various points. There have been several sets of experiments where sub-ML amounts of Xe have been condensed onto metal surfaces. One of these involved STM experiments at liquid helium temperatures (4 K), where the STM tip was used to move the Xe atoms over the surfaces and construct the impressive if somewhat predictable IBM (Eigler & Schweizer 1990). Xe/Ni(110) is a typical physisorption system, yet at 4 K the atoms stay where they are pushed/put for hours, and never leave the surface during the duration of the experiment, unless one engages in (again non-equilibrium) experiments to pick them up and transport them with the STM tip.

A second example is the detailed T-dependent study of Xe/Pt(111) (Horch et al. 1995). Good STM pictures could be produced below about 30 K, where nuclei of solid ML Xe were shown to grow; above this temperature, however, STM pictures were blurred, due to the motion of Xe atoms over the surface. This temperature is well below that needed for Xe to desorb from the surface ± only then is the full equilibrium state obtainable. Note that observations of the average structure are then quite possible with diVraction techniques, but that observation of the local structure by STM is impossible. At low T, what we are observing is really the ®rst stage of Xe crystal growth, rather than equilibrium adsorption. Another way to look at this is to note that we can have a local equilibrium within the 2D system at lower temperature than that needed