Trapping-desorption

Figure A3.9.3. Time-of-flight spectra for Ar scattered from Pt(l 11) at a surface temperature of 100 K [10], Points in the upper plot are actual experimental data. Curve tinough points is a fit to a model in which the bimodal distribution is composed of a sharp, fast moving (lienee short flight time), direct-inelastic (DI) component and a broad, slower moving, trapping-desorption (TD) component. These components are shown...

Hurst J E, Becker C A, Cowin J P, Janda K C, Auerbach D J and Wharton LI 979 Observation of direct ineiastic scattering in the presence of trapping-desorption scattering Xe on Pt(111) Phys. Rev. Lett. 43 1175... 

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.)...

Fig. 5. Rotational temperatures ofNO desorbing from Pt(l 11). The data are representative of data published for (x) neat thermal desorption , ( +) thermal desorption in the presence of coadsorbed C0 ° (solid squares) and (solid triangles) trapping/desorption in molecular beam scattering, (open triangle) reaction limited desorption from NO-NHj complexes, (open circle) and (open square) NHj oxidation reactions. The solid line is for full accommodation. The dashed curve represents results for translational energy measurements in direct inelastic scattering ...

Alignment results reported for NO trapping/desorption from Pt(l 11). At high was positive implying a propensity for helicopter-type desorbing molecules. (Adapted from Re/ 29.)... 

At sufficiently low Ts 100 K, Ar residence times on the surface are sufficiently long that trapping-desorption measures desorption from a fully equilibrated Ar on the surface. In this case, measurements of Df(Ef, 6f, Ts) showed that (Ef) <2kBTs and D f(0f) is broader than cos 6f, i.e., somewhat different than the usual equilibrium assumptions [138]. However, these results are fully consistent with detailed balance and the , and 6t dependence observed for a(E, Qt, Ts) [32,138]. 

Modulated molecular beam studies show that while trapping-desorption is the dominant process at modest Et and high Ts, there is also some direct or rather indirect inelastic scattering as well [192,201]. The dominance of trapping is anticipated for... 

Figure 3.21. Rotational temperature 7 rot (defined as T in this chapter) in trapping-desorption scattering of NO from Pt(l 11) and covered Pt(lll) as a function of the surface temperature Ts. Open points are for Et = 80 meV and solid points are for Et = 220 meV. The straight line is for 7rot = Ts. From Ref. [206].

Trap desorption. The choice of the thermal desorption apparatus is critical in order to avoid contamination and to be able to work with aroma compounds in a wide range of retention indices. In all systems, problems can be encountered due to reactive compounds or cold spots within the analyzer. It is recommended that all transfer lines, valves, or surfaces in contact with the volatile compounds be made of an inert material such as fused-silica or deactivated glass-lined stainless steel. Even more ideal are systems that do not have long... 

Figure 1.2 Schematic of a thermal desorption process with tube desorption and normal/backflush trap desorption steps.

If the thermal desorption unit is able to provide inverted (back-flush) gas flow during trap desorption (Figure 1.2), the same technique described for tubes can also be used for the cold trap. A multi-bed trap can considerably extend the analytical window of the instruments. Commercial cold-traps packed with quartz beads, quartz wool and Tenax TA are reported to cover a substance range from C6 to C40. 

The operational conditions of the purge and trap must be compatible with the configuration of the GC system. A high carrier gas (desorb gas) flow rate can be used with a packed GC column. The trap desorption time is short at the high flow rate, producing a narrowband injection. The optimum flow is about 50 mL/min. Capillary columns are generally preferred over packed columns for better resolution, but these columns require lower flow rate. 

Figure 6 Schematic potential energy diagrams for the interaction between O2 and Ag(l 11). Four panels are shown. In (a), the three states into which O2 can adsorb at the surfaces are depicted as a function of a reaction coordinate. In (b), the two potentials leading to direct inelastic scattering are shown. In (c), a trajectory representing a one dimensional representation of transient trapping-desorption in the O2 state is shown. In (d), two path ways leading to dissociative chemisorption are shown. From Kleyn et al. [45],...

Figure 19 Normal energy dependence of the probabilities for physisorption trapping (closed symbols) and transient trapping-desorption (open symbols) for three different values of Oj. The full lines through the data points serve to guide the eye only. The molecular chemisorption probability is shown by the dashed and the dash-dotted line. Note the different Y-axes. From Raukema and Kleyn [152].

A variety of processes can occur in the interaction of 02 molecules and Ag(l 11). At first scattering from and trapping in the physisorption potential can occur. Secondly, scattering from the chemisorption (02 ) potential occurs, together with transient trapping-desorption. The chemisorption potential well is very shallow. From being transiently trapped the molecule can be captured in the molecular chemisorption well presumably surface imperfections are necessary to stabilize the molecular adsorbate in this case. From the molecular chemisorption well the molecule can proceed to dissociation. In this step ad atoms may be involved on Ag(l 11). Finally, there is a small probability for direct dissociative chemisorption of 02 at Ag(l 11). The formation of added Ag-O rows (fences) at the surface inhibit further sticking at the surface. 

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