Laser surface processing, definition

Several formal and informal intercomparisons of nitric acid measurement techniques have been carried out (43-46) these intercomparisons involve a multitude of techniques. The in situ measurement of this species has proven difficult because it very rapidly absorbs on any inlet surfaces and because it is involved in reversible solid-vapor equilibria with aerosol nitrate species. These equilibria can be disturbed by the sampling process these disturbances lead to negative or positive errors in the determination of the ambient vapor-phase concentration. The intercomparisons found differences of the order of a factor of 2 generally, and up to at least a factor of 5 at levels below 0.2 ppbv. These studies clearly indicate that the intercompared techniques do not allow the unequivocal determination of nitric acid in the atmosphere. A laser-photolysis, fragment-fluorescence method (47) and an active chemical ionization, mass spectrometric technique (48) were recently reported for this species. These approaches may provide more definite specificity for HN03. Challenges clearly remain in the measurement of this species. 

Thus, recent experimental evidence seems to support the idea that growing small particles have maximum chemical reactivities, and certain sized/shaped small particles may have the highest reactivities. What size and/or shape varies with the metal in question and the reaction in question This information strongly supports three ideas (1) structure sensitivity in chemical reactions on metal surfaces is very important, (2) more than one atom is necessary to carry out at least some bond breaking processes, and (3) defect sites on growing small particles are extremely reactive (see Fig. 9). It has also been possible by pulsed laser vaporization to produce many types of gas phase metal clusters. Particularly interesting have been reactivity studies of niobium clusters Nb where X = 5-20. A definite cluster size dependence on reactivity was observed. Exposure... 

Finally, we consider the interaction of photons with adsorbed molecules and, specifically, laser-induced desorption and laser-induced photoffagmentation processes. The potential practical applications of the former have resulted in considerable activity, but definitive generalizations on the dynamics are still not at hand. Photochemistry at surfaces is being actively explored (Dai and Ho, 1995), and the ability of the surface to align the absorbed molecules (Polanyi, 2001) allows reactions to occur with restricted geometries of approach as shown in Figure 12.9. 

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