Stimulated surface reactions

Spontaneous surface reactions vs. stimulated surface reactions (by polarization). 

In chemical vapor deposition (CVD) complex shaped surfaces can be coated with homogeneous layers, especially when carried out at low pressure (LPCVD, low pressure chemical vapor deposition) (review Ref. [410]). A gas reacts with the heated substrate surface to give a solid coating and gaseous by-products which have to be removed continously. Layer thicknesses created by chemical vapor deposition are usually in the order 5-10 pm.. In cases where it is necessary to keep the temperature low, a plasma can stimulate the surface reaction in plasma enhanced chemical vapor deposition (PECVD). 

The approach to time resolution of very fast processes described above has been used to study a variety of gas phase chemical reactions stimulated by pulses of either photons or electrons (37) While similar studies have not yet been performed for processes occurring at solid surfaces, it should be feasible to do so. Thus, for example, the dynamics of adsorption and reaction at a single crystal surface, could be studied using a chopped molecular beam as the source of reactants. Alternatively, a pulsed photon beam could be used to repeatedly stimulate a reaction between adsorbed species. 

There are four requirements for these experiments a magnetic substrate, adsorbed chiral molecules, a source of ionizing radiation, and a technique to monitor the reactions or products. If the products are emitted into the gas phase then mass spectroscopy may be employed. However, detection of neutral desorbing species is problematic, particularly if the species are a major component of the residual gas. Detection of ions may be employed and often such photon-stimulated ion desorption measurements can reveal a great deal about the surface reactions [112]. 

Emetine A. V., Kataeva G. V., Ryabchuk V. K. and Serpone N. (1999c), Photo-stimulated generation of defects and surface reactions on a series of wide bandgap metal-oxide solids , J. Phys.Chem. B. 103, 9190-9199. 

Apart from desorption, surface reaction with adsorbate can be stimulated by the laser irradiation. In this chapter we will demonstrate the formation of new surface species by the CO2 laser induced reaction of CDF3 with the surface of SIO2 (17,18). In order to elucidate the mechanism of the reaction especially to determine the surface species, ir spectroscopy was used. A systematic investigation was performed Including the determination of reaction yields as a function of the laser frequency, the laser intensity and the gas pressure as well as the reaction products, and the determination of the correlation between the excited species and the reaction path. 

A TEA CO2 laser (Lumonics 103-2) was used to stimulate the reaction. In order to determine the configuration and the electronic state of the surface adsorbed species, IR transmission spectra (NICOLET FT-IR 5DX) and X-ray photoelectron spectra (XPS HP 5950A) were observed. The binding energy of the emission peak in XPS was calibrated assuming that the Au 4f 7/2 of the evaporated Au film on the sample to be 84.0 eV. 

The observation of oscillations in heterogeneous catalytic reactions is an indication of the complexity of catalyst kinetics and makes considerable demands on the theories of the rates of surface processes. In experimental studies the observed fluctuations may be in catalyst temperature, surface species concentrations, or most commonly because of its accessibility, in the time variation of the concentrations of reactants and products in contact with the catalyst. It is now clear that spontaneous oscillations are primarily due to non-linearities associated with the rates of surface reactions as influenced by adsorbed reactants and products, and the large number of experimental studies of the last decade have stimulated a considerable amount of theoretical kinetic modelling to attempt to account for the wide range of oscillatory behaviour observed. 

PECVD is a technique commonly used in microfabrication to deposit layers of insulating materials and amorphous or polycrystalline silicon. The plasma is used to help stimulate a reaction on the substrate surface of two or more species from the gas phase. The plasma helps break down the parent molecules and allows the reaction to occur at a lower temperature than cOTiventional CVD. The major advantage of PECVD, in fact, is its capability of working at a lower temperature with respect to other conventional CVD systems. For example, while deposition temperatures of 700-900 °C are required for silicon deposition in CVD, a temperature range of 250-350 °C is sufficient in the PECVD systems. 

The book is divided into seven distinct parts and these are subdivided into individual chapters. In Parts I -3 we present the general principles that underpin the operation of lasers, the key properties of laser radiation, the main features of the various laser sources, and an overview of the most commonly used laser spectroscopic techniques, together with the instrumentation and methods for data acquisition. In Parts 4-6 we address the principles of unimolecular, bimolecular, cluster and surface reactions, which have been probed, stimulated or induced by laser radiation. In the final part, Part 7, we summarize a range of practical laser applications in industry, environmental studies, biology and medicine, many of which are already well established and in routine use. 

In experiments aiming at a search for emitters, which surface-ionize organic compounds more efficiently [20] (based on the understanding that the work func tion of the surface is greatly increased by the chemisorption of electronegative gases [21]), a considerable increase in alkali metal ion currents is always observed, when chlorine (or fluorine) is introduced into a mass spectrometer with a thermal ionization source. This phenomenon is well interpreted as the so-called stimulated surface ionization [22] in which the exothermic reaction on the surface is responsible for the ionization. This is another interesting example of RSI. 

With infrared lasers vibrational motions of adsorbed molecules or atom groups in the adsorption potential at the surface can be excited. The surface mobility of these excited vibrational modes is much higher than the mobility of molecules in the ground state. If the vibrational energy reaches the adsorption potential barrier, these atom groups can move nearly freely across the surface. They may collide with other molecules or they may be desorbed. This may allow a selective control of surface chemical reactions by selective excitation of surface potential vibrations. This has been demonstrated by DJIDJOEV et al. [14.10] who studied the stimulation of surface reactions between hydroxyl groups OH and amino groups NH2 on a silica surface by irradiation with a CO2 laser. 

Very fast energy dissipation after vibrational excitation in the electronic ground state, however, has been inferred from picosecond relaxation measurements on large molecules in the gas phase. Moreover the view has been adopted that in solids even at low temperature rapid energy dissipation prevents multiphoton excitation and vibrational ladder climbing in matrix isolated molecules. On the other hand, in recent experiments surface reactions, such as desorption, evaporation and molecular decomposition, stimulated by vibrational multi-quantum excitation with resonant laser infrared have been observed at moderate threshold laser intensity and with high frequency selectivity. 

Many of the by-products of microbial metaboHsm, including organic acids and hydrogen sulfide, are corrosive. These materials can concentrate in the biofilm, causing accelerated metal attack. Corrosion tends to be self-limiting due to the buildup of corrosion reaction products. However, microbes can absorb some of these materials in their metaboHsm, thereby removing them from the anodic or cathodic site. The removal of reaction products, termed depolari tion stimulates further corrosion. Figure 10 shows a typical result of microbial corrosion. The surface exhibits scattered areas of localized corrosion, unrelated to flow pattern. The corrosion appears to spread in a somewhat circular pattern from the site of initial colonization. 

Dismption of the endothehal surface of blood vessels expose coUagen fibers and connective tissue. These provide surfaces that promote platelet adherence, platelet release reaction, and subsequent platelet aggregation. Substances Hberated from the platelets stimulate further platelet aggregation, eg, adenosine diphosphate maintain vasoconstriction, eg, serotonin and participate in blood coagulation, eg, platelet Factors III and IV. In addition, the release reaction modifies platelet membranes in a manner that renders phosphoHpid available for coagulation. The thrombin [9002-04-4] elaborated by the coagulation mechanism is a potent agent in the induction of the platelet release reaction. 

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