Adsorbed atoms and molecules

Adsorbed atoms and molecules can also diflfiise across terraces from one adsorption site to another [33]. On a perfect terrace, adatom diflfiision could be considered as a random walk between adsorption sites, with a diflfiisivity that depends on the barrier height between neighbouring sites and the surface temperature [29]. The diflfiision of adsorbates has been studied with FIM [14], STM [34, 35] and laser-mduced themial desorption [36]. 

K. Tanaka and M. Ikai, Adsorbed atoms and molecules destined for a reaction, Top. Catal., 2002, 20, 25. 

The chapter Electron Spin Resonance in Catalysis by Lunsford was prompted by the extensive activity in this field since the publication of an article on a similar subject in Volume 12 of this serial publication. This chapter is limited to paramagnetic species that are reasonably well defined by means of their spectra. It contains applications of ESR technique to the study of adsorbed atoms and molecules, and also to the evaluation of surface effects. The application of ESR to the determination of the state of transition metal ions in catalytic reactions is also discussed. 

Note that gtrans is given per degree of freedom, implying that the total translational partition function for an adsorbed molecule is given by (Qtrms)2- Also, the total partition functions for vibration and rotation are the products of terms for each individual vibration and rotation, respectively. Table 2.2 gives values for the partition functions for adsorbed atoms and molecules at 500 K. Vibrational partition functions are usually close to one, but rotational and translational partition functions have larger values. 

The growth rate of silicon crystals by either MBE or CVD is relatively slow, and so there is ample time for adsorbed atoms and molecules to diffuse to energetically more favorable sites. Experimental rates of diffusion of silicon on silicon and the activation barriers, however, are not known well. Experimental estimates of the activation barrier for silicon atoms diffusing on the Si(lll) surface have ranged from 58 kcal/mole when the pyrolysis of silane is used to produce surface silicon atoms , to 4.6 kcal/mole for the direct deposition of silicon atoms under ultrahigh vacuum Furthermore, a comparison of these... 

One of the most interesting observations of "surface science is the ordered arrangement of adsorbed atoms and molecules (see, for example, refs. 80-84). Numerous LEED experiments showed that the diffraction patterns for partially covered surfaces exhibit some additional spots (additional compared with the absence of adsorbate). These spots correspond to new... 

The data in Table 2.2 are for natural samples. It is notoriously difficult to prepare and maintain a clean solid surface, since any freshly created surface quickly becomes contaminated with adsorbed species even with a rather good laboratory vacuum. Any naturally occurring solid material must be considered to have a surface extensively populated by adsorbed atoms and molecules rather than a pristine surface. Such surfaces are what are examined in most laboratory experiments (e.g., those reported in Table 2.2), and, of coruse, just such surfaces are geochemically relevant for noble gas adsorption. It is interesting to note, however, that in other situations, noble gas adsorption can be rather a stronger effect. Thus, for example, Bernatowicz et al. (1983) examined Xe adsorption on a vacuum-crushed lunar rock and concluded that a small part of the freshly created surface had an adsorption potential as high as 14 kcal/mole but that in a few days at 10 8torr this surface was rendered inaccessible to Xe by other chemical species that were better competitors for the sorbent surfaces. 

A surface is illuminated with a high-intensity laser, and photons are generated at the second-harmonic frequency through non-linear optical process. For many materials only the surface region has the appropriate symmetry to produce a SHG signal. The non-linear polarizability tensor depends on the nature and geometry of adsorbed atoms and molecules. 

Another class of techniques monitors surface vibration frequencies. High-resolution electron energy loss spectroscopy (HREELS) measures the inelastic scattering of low energy ( 5eV) electrons from surfaces. It is sensitive to the vibrational excitation of adsorbed atoms and molecules as well as surface phonons. This is particularly useful for chemisorption systems, allowing the identification of surface species. Application of normal mode analysis and selection rules can determine the point symmetry of the adsorption sites./24/ Infrarred reflectance-adsorption spectroscopy (IRRAS) is also used to study surface systems, although it is not intrinsically surface sensitive. IRRAS is less sensitive than HREELS but has much higher resolution. 

Rostov, M.K., Cole, M.W., Lewis, J.C., et al. (2000). Many-body interactions among adsorbed atoms and molecules within carbon nanotubes and in free space. Chem. Phys. Lett., 332, 26-34. 

Bradshaw A M 1982 Vibrational spectroscopy of adsorbed atoms and molecules Appl. Surf. Sci. 11/12 712-29... 

PSD Photon Stimulated Desorption X-Rays Surfaces with adsorbed species Par u.v. light E> 10 eV Ions - analyzed with mass spectrometer 0.1-2 nm SIruclure and desorption kinetics ol adsorbed atoms and molecules 22... 

We will discuss the surface-structure sensitivity of chemical binding of adsorbed atoms and molecules when we review the properties of the surface chemical bond in Chapter 6. 

Surfaces are heterogeneous on the atomic scale. Atoms appear in flat terraces, at steps, and at kinks. There are also surface point defects, vacancies, and adatoms. These various surface sites achieve their equilibrium surface concentrations through an atom-transport process along the surface that we call surface diffusion. Adsorbed atoms and molecules reach their equilibrium distribution on the surface in the same way. This view of surface diffusion as a site-to-site hopping process leads to the random-walk picture, in which the mean-square displacement of the adsorbed particle along the. r-component of the coordinate is given by... 

Laser desorption that bums a hole in the adsorbed monolayer is utilized to measure the diffusion rate of adsorbed atoms and molecules. Describe the technique and discuss the results of one recent investigation 1109, 110]. 

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