Vibrations of atoms in surfaces and adsorbed species

The substrate (or adsorbent) was modelled in one dimension by a linear chain of five carbon atoms, (We could have chosen any other atom.) A terminal carbon atom represents a surface atom and the remaining atoms the bulk. There is only one force constant, the C-C stretching force constant (C—C of C2 12.16 mdyn A ). The dynamics were treated by the Wilson GF method. The substrate then had four vibrations similar to longitudinal acoustic modes (LAM) ( 10.1.2) LAM1,170 cm LAM2, 330 cm LAM3, 455 cm LAM4, 535 cm. The calculated INS spectrum comprised four bands of equal, weak intensity. 

We model hydrogen adsorption by attaching a hydrogen atom to the surface atom (terminal carbon) an extra band is produced, the stretching mode of hydrogen against the surface, C—H. This is usually a high 

The reader should be aware that the above discussion is a simplified version of an extraordinarily complex dynamical problem. Our purpose has been to draw attention to certain aspects of surface vibrations which may be revealed by INS spectroscopy. While specific riding nodes can sometimes be associated with motions of the surface atoms of the 

Experimental methods are described in Chapter 3 ( 3.5.5). Here we refer to certain aspects of INS experimental methods which require special attention in measurements on catalysts the mass of sample required if we are to obtain an acceptable signal-to-noise ratio, background subtraction, and the pre-treatment of the catalyst. 

To obtain the INS spectrum of the surface of a catalyst or an adsorbed species (or both together) we have to subtract a backgroimd spectrum. The background may be the spectrum of the sample container, or the container with catalyst prior to any desired treatment or dosing. The spectrum after backgroimd subtraction is the difference spectrum. In this chapter the spectra discussed and presented will be difference spectra unless otherwise stated. The spectrum of a substance to be adsorbed is determined separately and represents the spectrum of a model compound used in the interpretation of the data ( 5.1). Note that in INS experiments, because the spectrum is determined at 20 K, the reference state of a substance is the solid. 

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