Laser assisted reaction

Then, in Section 29.3, an emerging technology in the field of nano-patteming is discussed, a technique involving laser-assisted reactions of adsorbates on surfaces. Although this is still very much in its infancy, the potential in our miniaturized world is enormous. 

Laser-Assisted Thermonuclear Fusion. An application with great potential importance, but which will not reach complete fmition for many years, is laser-assisted thermonuclear fusion (117) (see Fusion energy). The concept iavolves focusiag a high power laser beam onto a mixture of deuterium [7782-39-0] and tritium [10028-17-8] gases. The mixture is heated to a temperature around 10 K (10 keV) (see Deuterium AMD tritium). At this temperature the thermonuclear fusion reaction... 

Direct laser-assisted myocardial revascularization (DMR) is an approved technique in the US, Europe, and parts of Asia to create numerous myocardial channels. This results in the induction of a massive inflammatory reaction, which in turn induces angiogenesis. The other FDA-approved pro-angiogenic therapy is the use of recombinant human platelet-derived growth factor (Regranex) for use in the treatment of diabetic neuropathic foot ulcer s. 

Easy availability of ultrafast high intensity lasers has fuelled the dream of their use as molecular scissors to cleave selected bonds (1-3). Theoretical approaches to laser assisted control of chemical reactions have kept pace and demonstrated remarkable success (4,5) with experimental results (6-9) buttressing the theoretical claims. The different tablished theoretical approaches to control have been reviewed recently (10). While the focus of these theoretical approaches has been on field design, the photodissociation yield has also been found to be extremely sensitive to the initial vibrational state from which photolysis is induced and results for (11), HI (12,13), HCl (14) and HOD (2,3,15,16) reveal a crucial role for the initial state of the system in product selectivity and enhancement. This critical dependence on initial vibrational state indicates that a suitably optimized linear superposition of the field free vibrational states may be another route to selective control of photodissociation. 

Figure 24. X-ray diffraction pattern (in the inset the 2D image) of the polyethylene sample recovered by the laser-assisted high-pressure reaction in the pure liquid monomer. The two measured sharp lines nicely fit the polymer diffraction pattern having a orthorhombic cell (Pnam) defined by the lattice parameters reported in the figure.

Figure 16. Examples of deposition shapes in pyrolytic-laser-assisted CVD (a) first-order surface reaction, no mass-transfer effects (b) first-order surface reaction, depletion effects and (c) Langmuir-Hinshelwood surface kinetics, no mass transfer effects. The ratio r/w is the radial position relative to the beam...

Moreover, the absorption spectra or the emission during the dissociation [03, (Imre et al. 1982), Nal (Foth et al. 1982)] give direct information on the intermediate states of reaction, and on the dynamics, not only at the turning point as for the laser-assisted collisions. 

That lasers have played a key role as promoters and as probes of chemical reactions is well known and extensively documented.1,4,7,62-72 In many of these applications the laser is employed as an intense, nearly monochromatic, light source whose characteristics ensure species selectivity, a well-characterized spectroscopy, and adequate intensity for multiphoton processes. Some possible applications, notably laser-assisted collisions 73,74 and transition-state spectroscopy,75,76 are yet in their infancy, but the extant studies already suggest considerable promise for influencing and probing chemical reactions. 

Laser-assisted CVD (LACVD) is a technique for localized processing. In pyrolytic LACVD the laser beam is used as a heating source. The chemical reactions take place only in the heated spot allowing for direct writing of TiN lines on substrates. Silvestre et al. [57, 58] have studied LACVD of TiN using TiCU, N2 and FI2 and a CO2 laser. They were able to deposit golden colored lines of TiN close to stoichiometry with laser powers of 400 to 700 W. 

The effect of non-resonant laser fields upon the cross-section for simple chemical reactions ( laser-assisted collisions ) is currently of theoretical interest, and... 

The last two theories described (chemical and thermodynamic) are intimately linked together because both of them induce a modification of the chemical composition at the surface. On the one hand, this modification can change the thermodynamic parameters (wettability) of the surface. On the other, changes in chemical composition influence the chemical adhesion established between the adherend and the adhesive layer. Numerous treatments are available for surface modification with coronas [8], plasmas [9, 10], lasers [11, 12], ion-assisted reactions [13], or coupling agents [14, 15]. All these treatments do not only change the chemical composition they can also affect the roughness, the orientation of macromolecular chains, and the mechanical behavior. 

Nanometric boron carbide particles can also be prepared by CVD. The reaction of boron trihalides with carbon or gaseous carbon-containing precursors using radiofrequency (RF) plasma [142, 143] or laser-assisted CVD [144] has been appUed. 

Short (or discontinuous) fibers are best prepared in a batch process, e.g., in a small cylindrical reaction chamber. The value of the technology, however, lies in its capability to facilitate the growth of continuous (potentially endless) fibers with a recently discovered automatic self-regulating growth mechanism [2], Finally, the diameter of the laser focus determines the diameter of fibers grown by laser assisted chemical vapor deposition, just as the diameter of the metal particles determines the diameter of the whiskers grown by metal catalyzed chemical vapor deposition. 

Laser assisted chemical reactions are defined as reactions in which the product yield is enhanced by exciting the transition state (i.e. the reactants are not excited). A classical example for a photon-mediated atom-diatom reaction is that of... 

Figure 1.4 Laser-assisted (endothermic) chemical reaction. Note that /rr pump excites neither reagents nor products...

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