Laser initiation

A unique problem arises when reducing the fissile isotope The amount of that can be reduced is limited by its critical mass. In these cases, where the charge must be kept relatively small, calcium becomes the preferred reductant, and iodine is often used as a reaction booster. This method was introduced by Baker in 1946 (54). Researchers at Los Alamos National Laboratory have recently introduced a laser-initiated modification to this reduction process that offers several advantages (55). A carbon dioxide laser is used to initiate the reaction between UF and calcium metal. This new method does not requite induction heating in a closed bomb, nor does it utilize iodine as a booster. This promising technology has been demonstrated on a 200 g scale. 

A qualitatively different approach to probing multiple pathways is to interrogate the reaction intermediates directly, while they are following different pathways on the PES, using femtosecond time-resolved pump-probe spectroscopy [19]. In this case, the pump laser initiates the reaction, while the probe laser measures absorption, excites fluorescence, induces ionization, or creates some other observable that selectively probes each reaction pathway. For example, the ion states produced upon photoionization of a neutral species depend on the Franck-Condon overlap between the nuclear configuration of the neutral and the various ion states available. Photoelectron spectroscopy is a sensitive probe of the structural differences between neutrals and cations. If the structure and energetics of the ion states are well determined and sufficiently diverse in... 

Park, J., and M. C. Lin, Laser-Initiated NO Reduction by NH, Total Rate Constant and Product Branching Ratio Measurements for the NH2 + NO Reaction, . /. Phys. Chem. A, 101, 5 f3 (f997a). 

A laser-initiated pyrotechnic system (Zr-Amm perchlorate) for spacecraft application has been shown to be superior to conventional electroexpl initiation (Ref 22). Primary high expls such as LA and LSt can be initiated directly. Q-switched-mode lasers can directly deton PETN, RDX, and Tetryl in a properly designed device and can be useful where microsecond simultaneity is important (Ref 6)... 

Laser-initiated Radical Production. Although there are different physical mechanisms involved in laser chemistry, we are concerned here with the photodissociation, i.e., the breaking of molecular bonds directly by UV photons. The laser emission is used to produce electronically excited molecules which split into reactive radicals, with the highest possible quantum yield. Since the substrate usually behaves as a poor photoinitiator, an additional molecule must be introduced in order to enhance the radical production, much in the same way as in conventional photoinitiated reactions. In this work,... 

It is important to note that values are of the same order of magnitude whether conventional UV or laser radiation were used (Table I). This result is in contrast with the recent work of Sadhir et al. (7) on the photopolymerization of maleic anhydride and styrene, induced by the 363.8 nm emission of an argon ion laser. That work showed to be the laser initiation 1000 times more energy efficient than the UV-induced polymerization. This discrepancy may arise from two factors (i) the lower light-intensity used in the laser irradiation that should favor chain propagation and (ii) differences in the way of comparing the... 

Aberration, spherical, 7 Acetaldehyde, in Novolak resins, 346 p-Acetamidostyrene, preparation, 286 Acetophenone derivatives, 457 p-Acetoxystyrene, preparation, 271 Acrylate systems, UV-laser initiation, 222 Acrylic backbones, pendant groups, 277-80... 

B. Product State Distributions from Laser-Initiated Bimolecular... 

Stereolithography is simple in concept and it provides great economies for the design lab as well as for the modeling process. It also provides previously unrecognized challenges for the polymer photochemist, for it is entirely a laser-initiated technology, and the polymerization reactions take place to depths below a finitely thin surface layer. 

A final alternative for metal patterning is the use of lasers to selectively and accurately etch small metal areas. Lasers are routinely used to trim thin-film resistors (126) they may also be used to repair defects or etch conductor lines by direct ablation in an inert atmosphere (127,129) or by laser-initiated etching in a reactive gas or liquid (128). 

The Mechanism of Initiation of Condensed Explosives by Laser Initiation , FizGorVzr 4, 475-80 (1969) 190) W.R. Woodruff, Radia-... 

V.J. Menichelli L.C. Yang, Direct Laser Initiation of Insensitive Explosives , Proc 7th Symp Expls Pyrots, 111-2, Franklin Inst Res Labs, Phila (1971), 1-8 207) L.C. Yang ... 

Various further experiments with laser radiation as an ignition and initiation source have been conducted since the seventies. For example, for the ignition of propellants, various pyrotechnic mixtures and metastable intermolecular composites (MICs), the effects of the laser wavelength, gas pressure or dopants on the initiation thresholds, the development of laser initiated detonators, as well as a... 

Concluding, based on these aspects, the laser initiation of explosives by laser diode radiation seems to be a very attractive and initiation method of the future. However, due to the limited power of laser diodes, the explosives might be thermally ignited and undergo a DDT, in contrast to the shock initiation which results by high-power solid-state laser radiation. The operating mode of laser diode initiators can be compared to that of electric hot-wires but with faster function times as has been demonstrated in the literature. 

The time to measure spectra of this quality under high-pressure conditions has been about I min. The absolute time scale of the experiment depends on the method of initiation. In thermally initiated (spontaneous) polymerizations reaction time can be several hours or even days. In contrast, in excimer laser-initiated free radical polymerizations application of a few laser pulses each of about 20 ns duration can induce changes between subsequent spectra as on this figure. 

Jacobs JWM (1988) Laser initiated metal deposition on semiconductors from aqueous solutions. Thesis, University of Eindhoven (Holland)... 

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