Laser-induced decomposition

In another study several simple silenes RR Si=CH2 (R, R = Me, Vinyl etc.) were formed by laser-powered pyrolysis and were found to form linear polymers, in contrast to the usual behavior of silenes which yield cyclodimers when formed by conventional thermolysis techniques16. Reactions of the silenes in the presence of several monomers such as vinyl acetate, allyl methyl ether and methyl acrylate were also studied. Laser-induced decomposition of silacyclobutane and 1,3-disilacyclobutane gave rise to silenes and other oxygen-sensitive deposits17,18. 

Laser-induced decomposition of SCBs has been reported as an efficient route for the gas-phase deposition of thin films of Si-C-H and Si-G materials <1993JCF411, 1994JOM(466)29, 1990JOM(391)275>. 

N. L. Garland, H. D. Ladouceur, A.P. Baronavski, H.H. Nelson, Laser-Induced Decomposition of Energetic Materials JANNAF 35th Combustion Subcom 17th Propulsion Syst. Hazards Sub. Tucson, AZ, 1998 pp 161-166. 

Laser-induced decomposition of mixtures of polychlorinated biphenyls in the liquid phase has been investigated, employing radiation from three different excimer lasers (XeCl at 308 nm, KrF at 248 nm and ArF at 193 nm)475. The mixtures can be quantitatively and efficiently destroyed by means of UV radiation at 248 nm. A single-photon dissociation process, which leads to both HC1 elimination and biphenyl bond rupture, is induced by the KrF laser radiation. 

For the case of laser induced decomposition reactions, production rate of D is given by... 

In order to get more insight into the factors that determine thermal stability, which is crucial for the safe manipulation of (fluoromethyl)silanes, we have investigated the conventional thermal [24] and laser-induced decomposition [25, 26] of the parent silanes F H3 CSiH3. These compounds are stable gases at ambient temperature, and they may be conveniently studied by IR and multinuclear NMR spectroscopy. Therefore these appear to be particularly suited for gas phase investigations. 

It was supposed that, because of a rapid intramolecular energy pooling, infrared laser multiphoton decomposition at low laser fluence usually proceeds via the energetically most favourable reaction channel [26], From the material balance of gaseous decomposition products (Fig. 2) it was inferred that there are common primary steps for both the thermal and the explosive laser-induced decomposition of (fluoromethyl)silanes. 

The ruby laser-induced decomposition of 1,4-dloxane has been reported (199) to lead to ethylene, carbon monoxide, and hydrogen... 

The continuous-wave COj laser-induced decomposition of hexafluoropropanone, sensitized with SF, gave COF 3, perfluorinated hydrocarbons, carbon monoxide, and trifluoroethanoyl fluoride [1641] ... 

Laser-induced decomposition of dimethyl(2,4-pentanedionato)goldm, 73, has been used426,427 recently for selective formation of high quality and high purity thin gold metallic films426 -428. An understanding of the mechanism of the thermal and photochemical decomposition of 73 is thus of scientific and practical interest. 

Fig. 37 Schematic diagram of laser-induced decomposition and ablation dynamics of the photosensitive triazene polymer film (a) before excimer laser irradiation, (b) slight expansion of the film and darkening of the irradiated surface at the beginning of the excimer laser pulse, (c) initiation of etching of the film and ejection of gaseous fragments decomposed from the polymer, (d) completion of the etching and expansion of ejected plume, and (e) after the ablation. REPRINTED WITH PERMISSION OF [Ref. 125], COPYRIGHT (1997) American Chemical Society...

Therefore we decided to gather new data about the laser-induced decomposition of polyimide and contrast them with results from pyrolysis using the same experimental technique, i.e., diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy. 

Irradiation at the Threshold of Ablation 40 mj cm 2. To identify possible intermediates of the laser-induced decomposition of Kapton the threshold flu-ence of ablation was chosen. At this fluence ablation is just starting and changes in the material should be slow, giving the best chance to monitor intermediates. 

In the previous chapter polyimide was analyzed after UV laser irradiation using DRIFT spectroscopy. Various intermediates and products of the laser-induced decomposition could be identified. Experiments with the same material were performed to test whether it is possible to distinguish between UV laser-induced decomposition and thermally induced decomposition, i.e., pyrolysis. 

The aim of this work is to investigate the reaction scheme of the thermally induced decomposition of Kapton in air and compare it to the UV laser-induced decomposition. This should help to decide whether the laser-induced decomposition (ablation) of Kapton is comparable to pyrolysis. Using DRIFT spectroscopy the changes in the concentration of different functional groups of the polymer are monitored during the thermal decomposition process. This information is used to develop a kinetic reaction scheme and to calculate kinetic parameters. 

The experimental setup consists of a gas dosing system and the DRIFT spectroscopy apparatus. For the pyrolysis experiments KBr was selected as matrix, different to the laser-induced decomposition experiments [141], where SiC was used. KBr was chosen because the emissivity did not increase drastically, as in the case of SiC, where it interfered with the measurements. The Kapton-KBr mixtures are placed in the sample holder of the DRIFT cell and packed using a pressure of 1 MPa as described elsewhere [288, 306]. The sample is heated in an inert gas atmosphere to the desired temperature using a heating rate of 10 K min-1. The spectrum of the Kapton-KBr mixture at a given temperature is collected and used as background spectrum. The following experiments were carried out. 

If the thermally induced decomposition follows the same pathway as the UV laser-induced decomposition, then it could be possible to select the polymers for LPTs according to similar principles as for UV laser ablation. The most important design features for polymers designed for UV laser ablation are an exothermic decomposition into gaseous products well-defined primary decomposition sites in the polymer main chain and a high absorptivity at the irradiation wavelengths. In future experiments these assumptions will be tested. 

Buerki and Leutwylerl l also reported the homogeneous nucleation of spherical diamond powder by C02-laser-driven gas-phase reactions. Cubic and hexagonal diamond powders of average diameters 6-120 nm (maximum 300 nm) were obtained by laser-induced decomposition of C2H4... 

Comprehensive studies have been carried out on the MPD of hexafluoroacetone as a function of laser fluence, frequency and substrate pressure, and the influence of collisional effects on the formation of CF2 in the MPD of CF2CFCI and CFjHCl has been experimentally measured and theoretically modelled. Fluence dependences of the HF laser-induced decomposition of 2,2,2-trifluoroethanol, and the 9.4 pm CO2 laser MPD of C2F3C1 have been reported, and triethylphosphite joins the increasing list of molecules dissociated by CO2 laser radiation. ... 

There have been several recent papers on i.r. laser-induced decomposition of boron trichloride in the presence of other substances, e.g. phosgene and hydrogen sulphide. ... 

Transversely excited atmospheric (TEA) CO2 laser-induced decomposition of divinylsilane in the gas phase yields unsaturated C2-C4 hydrocarbons, benzene and vinylsilane, and it represents a convenient process for chemical vapor deposition of thin solid films composed of silicon carbide or polycarbosilane [16]. 

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