Desorption of atoms

The rate of desorption of atoms per unit area is proportional to the concentration (JVa), of adatoms, and the constant of proportionality is the rate coefficient d1, i.e. 

The dependence of d, on temperature is given by the Arrhenius-type relation 

---

Figure 3.14. Microscopic pictures of the desorption of atoms and molecules via mobile and immobile transition states. Ifthe transition state resembles the ground state, we expect a prefactor of desorption of the order of 10 s h Ifthe adsorbates are mobile in the transition state, the prefactor increases by one or two orders of magnitude. For desorbing...

Secondary ion mass spectrometry (SIMS) is by far the most sensitive surface technique, but also the most difficult to quantify. When a surface is exposed to a beam of ions (Ar", 0.5-5 keV), energy is deposited in the surface region of the sample by a collisional cascade. Some of the energy will return to the surface and stimulate the ejection (desorption) of atoms, ions, and multi-atomic clusters. In SIMS, positive or negative secondary ions are detected directly with a quadrupole mass spectrometer. 

Hydrogen adsorbs dissociatively on almost all metals. At 500 K the H atoms diffuse freely over the surface. Desorption occurs associatively by recombination of two H atoms, while desorption of atomic hydrogen can be ignored. 

Taylor and Langmuir (SO) followed the desorption of Cs ions and atoms from a W surface by a thermionic method. At low coverage the rate of desorption of positive ions was measured by the positive ion current. At higher coverages, the rate of desorption of atoms was determined by allowing a calculated fraction of the total number of atoms to impinge on an adjacent incandescent filament where they became ionized then the rate of evaporation of atoms from the original surface was calculated from the increase in the ion current. The electron emission was measured simultaneously so that the rate of evaporation of ions and atoms could be ex-... 

Although the FABMS (Fast Atom Bombardment Mass Spectrometry) technique has only been in use for a few short years (since 1981), it traces its roots back well over a century (1). It has been observed that the bombardment of a surface by energetic ions produces the desorption of atoms and molecules from the surface of the target. This process, known as Sputtering, produces a yield (number of atoms sputtered per incident ion) which generally increases with the energy, the mass and the incidence angle of bombardment (2). 

The most prominent feature in luminescence of Xe, Kr and Ar - the so-called M-band (Fig.la) - is formed by 1,3SU+— Xg+ transitions in (R2 ) excimer M-STE (R=rare gas atom). The negative electron affinity (Table 1) is a moving force of the cavity ("bubble") formation around A-STE in the bulk of crystal, and the desorption of atoms and excimers from the surface of solid Ne and Ar [11], Radiative "hot" transitions in desorbed excimers of Ar and Ne result in a W-band. 4-bands are emitted by A-STE (R ). 

Atomic cryocrystals which are widely used as inert matrices in the matrix isolated spectroscopy become non-inert after excitation of an electronic subsystem. Local elastic and inelastic lattice deformation around trapped electronic excitations, population of antibonding electronic states during relaxation of the molecular-like centers, and excitation of the Rydberg states of guest species are the moving force of Frenkel-pairs formation in the bulk and desorption of atoms and molecules from the surface of the condensed rare gases. Even a tiny probability of exciton or electron-hole pair creation in the multiphoton processes under, e.g., laser irradiation has to be taken into account as it may considerably alter the energy relaxation pathways. 

Fig. 2.13 Microscopic-level diagrams of the desorption of atoms and molecules via mobile and immobile transition states. If the transition state resembles the ground state,...

In order to discuss the factors which relate rates of desorption of atoms and molecules to concentrations of adsorbed species, we require a fairly detailed description of the adsorbed state. We do not possess the knowledge necessary for a truly quantitative account, but will adopt a simple model of the overlayer which will permit us to develop the discussion in a semi-quantitative way. In this manner, we will be able to discover the important ingredients of the problem and even at times to obtain results which are in fair agreement with experiment. However, we ought to recognise at the outset that the simplicity of our model will severely limit its value and, indeed, there will be occasions when we will find it difficult even to maintain self-consistency. 

Values at 1000 K of the pre-exponential factor A2 [eqn. (20)] for localised and mobile adsorption. The parameters for the localised state relate to tungsten, and p has been calculated assuming that the activation energy for surface diffusion (Em) is one-fifth the energy of desorption of atoms (— AUX i) [107] (K0)i 2 has been assigned the value 0.5 and Ns the value 1015 cm 2. 

The principle of microscopic reversibility enables us to say that, at equilibrium, the rates of adsorption and desorption of molecules are equal, and independently that the rates of adsorption and desorption of atoms are also equal. In the first case, we will not attempt to apply the discussion of Sect. 2.1, but will simply make use of the molecular and atomic sticking coefficients (s2 and s, respectively). This procedure is useful when s2 and Sj are constants, as in the case when the coverage is low and molecular and atomic adsorption are unactivated. The resulting kinetic description contains only the sticking coefficients as adjustable parameters. 

We have seen in Sect. 2.2.3(b) that, when the rate of adsorption of molecules becomes comparable with the rate of desorption of atoms, we expect failure of the half-order rate law and, eventually, first-order behaviour will be observed. All the metals investigated by Brennan and Fletcher [5, 7] exhibited this transition. Figure 11(a) illustrates the behaviour using the example of the H2— Pt system at 1750 K. According to eqn. (51), the limiting probability of atomisation is the molecular sticking coefficient, s2, values of which are listed in Table 5. 

Hickmott measured the rate of desorption of H2 molecules from tungsten in the temperature range 600—800 K, when the desorption of atoms was negligible, and demonstrated that it was second order with respect to the concentration of adatoms. This result confirms that the overlayer at these temperatures exists in the form of atoms. The experimental relation for the rate coefficient, d2, (cf. eqn. (20) and Table 1) is... 

Figure 13(a) shows a typical set of plots for the rate of desorption of atoms at different temperatures and pressures. Although straight lines have been drawn through the points, it should not be concluded that Fj depends linearly on the pressure in these reactions. On the contrary, as we have seen, Fj will depend on the rate of desorption of atoms as compared with the rate of adsorption of molecules. For all the conditions represented in Fig. 13(a), the rate of atomisation is only an extremely small fraction of the rate of adsorption of molecules, so the kinetics will be half-order with respect to P2. However, the range of pressure over which the measurements have been made is insufficient to reveal the /P dependence. The variation of d1 with temperature is shown in Fig. 13(b) and the equation of the least squares straight line gives the relation... 

F, rate of desorption of atoms from the experimental surface in a flow system. 

Molecules interact with the surfaces of solids in almost every environment in the universe. In addition to purely intellectual interest, we customarily justify studying these interactions on technological grounds, heterogeneous catalysis and the fabrication of microchips being the most frequently listed applications. However the field is much more broadly relevant the adsorption and desorption of atoms and molecules on the surfaces of dust grains is very important to molecule formation in the interstellar medium, reactions on the surfaces of ice crystals is important in atmospheric chemistry and reactions at surfaces determine the behaviour of medical implants in our bodies. 

Kidd, Lennon and Meech [26] also studied the related process of photodesorption, finding that the photodesorption cross sections for NO and SO2 also exhibited peaks near the small particle surface plasmon resonance for silver. For an earlier study that also presents evidence for surface plasmon induced desorption, in this case desorption of atoms from the metal nanoparticles (composed of sodium) themselves, see Ref [27]. The review article by Watanabe et al. [12] also discusses some more recent results on plasmon induced desorption. 

The theory of desorption of atoms and molecules by temperature-programmed desorption has been reviewed in references [61, 74-76, 124-126]. Discuss the assumptions made in deriving the first- and second-order desorption rates and their correlation to the temperature of the maximum desorption rates. How does the magnitude of the preexponential factor reflect the assumption of (a) a mobile adsorbate layer or (b) an immobile adsorbate layer ... 

Possibly, for a larger

[Pg.22]

Kinetic phenomena, such as rates of adsorption and desorption of atoms or molecules, measured as a function of pressure, temperature (substrate or gas), surface structure, etc., contain important information on the energetics of adsorption. Unambiguous relationships between kinetics and energetics are, however, at least in the field of adsorption, frequently not available. The simplest case is the rate of adsorption, rad, of atoms or simple molecules onto a uniform surface [64Hay] ... 

In the previous section, we considered one of the most basic gas-solid kinetic processes the simple adsorption or desorption of atoms to/from a surface under the assumption that the rate is limited by the impingement of atoms from the gas phase to the surface. In this section, we consider a more complex situation in which a gas species actively etches or corrodes a solid surface via a chemical reaction process, thereby continuously removing material from the surface over time. Consider, for example, the corrosion of a Ti metal surface with HCl acid vapor ... 

The accuracy on the activation values is calculated by several measurements. The two activation energies obtained for AI2O3 show that the recombination mechanism may be different for the two temperature ranges around 1200 K. In the thermal study, this difference is also encountered but around 1400 K. At high temperature, the desorption of atoms om the surface is faster than adsorption, so the overall activation energy goes from positive to negative values and therefore the recombination coefficient decreases as temperature increases. 

The lowering of file first layer melting temperature (in Ae monolayer phase) can be explained by the second layer promotion and partial desorption of atoms. Such mechanism of melting has been observed before in the case of Kr atoms adsorbed in a model of MCM-41 pores [12]. As a consequence, the in-layer density is lower, it fiicilitates the melting at lower temperature. However, when the second layer is present, the promotion of the first layer atoms into the second one starts to be energetically much more difficult. As a result, the first layer structure is stabilized by the interaction with the second one and the melting temperature of the first layer merges. 

---