Book contents
- Frontmatter
- Contents
- Preface
- Chapter 1 Introduction to surface processes
- Chapter 2 Surfaces in vacuum: ultra-high vacuum techniques and processes
- Chapter 3 Electron-based techniques for examining surface and thin film processes
- Chapter 4 Surface processes in adsorption
- Chapter 5 Surface processes in epitaxial growth
- Chapter 6 Electronic structure and emission processes at metallic surfaces
- Chapter 7 Semiconductor surfaces and interfaces
- Chapter 8 Surface processes in thin film devices
- Chapter 9 Postscript – where do we go from here?
- Appendix A Bibliography
- Appendix B List of acronyms
- Appendix C Units and conversion factors
- Appendix D Resources on the web or CD-ROM
- Appendix E Useful thermodynamic relationships
- Appendix F Conductances and pumping speeds, C and S
- Appendix G Materials for use in ultra-high vacuum
- Appendix H UHV component cleaning procedures
- Appendix J An outline of local density methods
- Appendix K An outline of tight binding models
- References
- Index
Chapter 4 - Surface processes in adsorption
Published online by Cambridge University Press: 06 July 2010
- Frontmatter
- Contents
- Preface
- Chapter 1 Introduction to surface processes
- Chapter 2 Surfaces in vacuum: ultra-high vacuum techniques and processes
- Chapter 3 Electron-based techniques for examining surface and thin film processes
- Chapter 4 Surface processes in adsorption
- Chapter 5 Surface processes in epitaxial growth
- Chapter 6 Electronic structure and emission processes at metallic surfaces
- Chapter 7 Semiconductor surfaces and interfaces
- Chapter 8 Surface processes in thin film devices
- Chapter 9 Postscript – where do we go from here?
- Appendix A Bibliography
- Appendix B List of acronyms
- Appendix C Units and conversion factors
- Appendix D Resources on the web or CD-ROM
- Appendix E Useful thermodynamic relationships
- Appendix F Conductances and pumping speeds, C and S
- Appendix G Materials for use in ultra-high vacuum
- Appendix H UHV component cleaning procedures
- Appendix J An outline of local density methods
- Appendix K An outline of tight binding models
- References
- Index
Summary
Chemi- and physisorption
A qualitative distinction is usually made between chemisorption and physisorption, in terms of the relative binding strengths and mechanisms. In chemisorption, a strong ‘chemical bond’ is formed between the adsorbate atom or molecule and the substrate. In this case, the adsorption energy, Ea, of the adatom is likely to be a good fraction of the sublimation energy of the substrate, and it could be more. For example, in chapter 1, problem 1.2(a), we found that in a nearest neighbor pair bond model, Ea = 2 eV for an adatom on an f.c.c. (100) surface when the sublimation energy L0 = 3 eV. In that case the atoms of the substrate and the ‘adsorbate’ were the same, but the calculation of the adsorption stay time, τa, would have been valid if they had been different. Energies of 1–10 eV/atom are typical of chemisorption.
Physisorption is weaker, and no chemical interaction in the usual sense is present. But if there were no attractive interaction, then the atom would not stay on the surface for any measurable time – it would simply bounce back into the vapor. In physisorption, the energy of interaction is largely due to the (physical) van der Waals force. This force arises from fluctuating dipole (and higher order) moments on the interacting adsorbate and substrate, and is present between closed-shell systems. Typical systems are rare gases or small molecules on layer compounds or metals, with experiments performed below room temperature. Physisorption energies are ∼50–500 meV/atom; as they are small, they can be expressed in kelvin per atom, via 1 eV ≡ 11604 K, omitting Boltzmann's constant in the corresponding equations.
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- Introduction to Surface and Thin Film Processes , pp. 108 - 143Publisher: Cambridge University PressPrint publication year: 2000