Lecture notes by John A. Venables. Lecture given 20 Feb 96. Notes updated 11 Oct 09, ex 11 Dec 96.
References: Refs: Zangwill, part 2 is entitled simply ‘Adsorption’, comprising chaps 8-16. Here, however, I am defining adsorption somehwat more narrowly; the distinctions between physi- and chemi-sorption are spelt out by Zangwill in chaps 8 and 9. J.G. Dash, Films on Solid Surfaces, (Academic, 1975) is useful, as are many, but not all, Statistical Mechanics books. In section C2, I am following T.L. Hill, Introduction to Statistical Thermodynamics, Chapters 7-9, reasonably closely.
Physisorption is weaker, and is often described as implying that no chemical interaction is present. This can't really be true, because if there were no attractive interaction, then the atom wouldn't stay on the surface for any measurable time- it would simply bounce back into the vapor. A better distinction is that in physisorption, the energy of interaction is largely due to the van der Waals force. This force is due to fluctuating dipole (and higher order) moments on the interacting adsorbate and substrate, and is present between closed-shell systems. Typical systems are rare gases on layer compounds and other similar systems. Physisorption energies are of order 50-500 meV/atom. One can see that these energies are comparable to the sublimation energies of rare gas solids, as given in section 1.3, Table 1.Click here to cross-reference to Table in Section 1.3
Adsorption of molecules often proceeds in two stages. A first, precursor stage, has all the characteristics of physisorption, but this state is metastable. In this state the molecule may re-evaporate, or it may stay on the surface long enough to transform irreversibly into a chemisorbed state. This transition is rather dramatic, usually resulting in splitting the molecule and adsorbing the individual atoms: dissociative chemisorption. The adsorption energies for the precursor phase are similar to phyisorption of rare gases, but may contain additional contributions from the dipole, quadrupole, etc moments of the molecules. The dissociation stage can be explosive- literally. The heat of adsorption is given up suddenly, and can be imparted to the resulting adatoms. An example is O2/Al(111), which we will discuss briefly in section C3. O2 and N2 can be condensed at low T as (long-lived) physisorbed molecules on many substrates. Bulk solid F2 is however quite dangerous, and has an alarming tendency to blow up by reacting dissociatively with its container.
Calculating some of these limits, and making comparisons between the different models is a suitable topic for a mini-project. One anharmonic model which aims to have the correct low T limit, and to be useful at high T, is the quantum cell model.
Note added 10 December 1996: in Spring 1996, one student started on a adsorption project of this type, but then realised that it would be too timeconsuming. I am hoping to build up some resources in this area in due course. Another student did an essay on the (integral and fractional) quantum Hall effects; although the literature in this field is rather different, there are some overlaps which enable one to view them similarly to ‘surface or 2D’ physics.
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