Trapping-desorption mechanism

Fig. 9. Incidence energy dependence of the vibrational state population distribution resulting when NO(u = 12) is scattered from LiF(OOl) at a surface temperature of (a) 480 K, and (b) 290 K. Relaxation of large amplitude vibrational motion to phonons is weak compared to what is possible on metals. Increased relaxation at the lowest incidence energies and surface temperatures are indicators of a trapping/desorption mechanism for vibrational energy transfer. Angular and rotational population distributions support this conclusion. Estimations of the residence times suggest that coupling to phonons is significant when residence times are only as long as ps. (See Ref. 58.)...

The ejection of atoms or molecules from the surface of solid in response to primary electronic excitation is referred to as electronically stimulated desorption (ESD) or desorption induced by electronic transitions (DIET). Localization of electronic excitations at the surface of RGS induces DIET of atoms both in excited and in ground states, excimers and ions. Most authors (see e.g. Refs. [8,11,23,30] and references therein) discuss their results on DIET from RGS in terms of three different desorption mechanisms namely (i) M-STE-induced desorption of ground-state atoms (ii) "cavity-ejection" (CE) mechanism of desorption of excited atoms and excimers induced by exciton self-trapping at surface and (iii) "dissociative recombination" (DR) mechanism of desorption of excimers induced by dissociative recombination of trapped holes with electrons. 

Another view of the Si(lOO) etching mechanism has been proposed recently [28], Calculations have revealed that the most important step may actually be the escape of the bystander silicon atom, rather than SiBr2 desorption. In this way, the SiBr2 becomes trapped in a state that otherwise has a very short lifetime, pennitting many more desorption attempts. Prelimmary results suggest that indeed this vacancy-assisted desorption is the key step to etching Si(lOO) with Br2. 

It has been proposed that the precursor state [81, 82] for the adsorption-desorption reaction consists of weakly physisorbed CO. This can be CO sitting on an occupied site (COad-CO) or on an sterically unfavorable Pt site. According to Ertl [81], the desorption process occurs through a trapping mechanism on such sites if the surface is saturated by chemisorbed CO the desorption channel involves either a COad-CO potential well or a Pt-CO attractive well which is sterically weakened by the presence of pre-absorbed CO . 

Thermal desorption enables the exchange of solvent into a more environmentally friendly stream of gas at the stage where analytes are being released into a suitable trap (sorption tube, denuder, and passive dosimeter). Figure 19.8 illustrates the basic mechanisms of the thermal desorber. 

Second-order rate coefficient for the desorption of molecules by recombination of adatoms [see eqn. (14)]. constant of proportionality defined by eqn. (89) to give the rate of change of pressure in an atomisation system, rate coefficient for the production of atoms per unit are of surface at temperature T and gas pressure P2. rate coefficient for wall trapping of thermally excited molecules per unit area of filament, masses of atom (X) and molecule (X2), respectively, rate coefficient for the Rideal recombination mechanism [see equation (60)]. 

When a surfactant-water or surfactant-brine mixture is carefully contacted with oil in the absence of flow, bulk diffusion and, in some cases, adsorption-desorption or phase transformation kinetics dictate the way in which the equilibrium state is approached and the time required to reach it. Nonequilibrium behavior in such systems is of interest in connection with certain enhanced oil recovery processes where surfactant-brine mixtures are injected into underground formations to diplace globules of oil trapped in the porous rock structure. Indications exist that recovery efficiency can be affected by the extent of equilibration between phases and by the type of nonequilibrium phenomena which occur (J ). In detergency also, the rate and manner of oily soil removal by solubilization and "complexing" or "emulsification" mechanisms are controlled by diffusion and phase transformation kinetics (2-2). 

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