Laser‐powered homogeneous pyrolysis

Abstract

The large absorption cross sections for infrared radiation possessed by a few rather stable molecular species, their relatively fast vibration-translation relaxation times, and the availability of large laser fluxes at their characteristic frequencies have been effectively exploited to generate high bath temperatures for gas-phase reactions under strictly homogeneous conditions. Controlled temperatures from 500 to 1500°K have been developed with ii modest COs laser. Rise times for heating are estimated to be in the millisecond range, with reaction times = 10 seconds. Operating pressures [reagents, radiation absorber (generally SFe,), and diluent] range from 10 to 100 torr. Thus laser-powered homogeneous pyrolysis (LPHP) is complementary to the single-pulse shock-tube technique [rise time -10-8 sec; reaction time sec] for obtaining product distributions in gas-phase pyrolvses, and for measuring relative rates in the absence of hot walls. The latter may (and generally do) introduce catalytic effects. Further, LPHP is very simple and ideally suited for exploratory investigations of small amounts of reagent ( mole); also, sample cells may be heated externally to augment the vapor pressure of slightly volatile reagents. Conventional analytical procedures (gas chromatography, mass spectrometry) were used to follow the course of pyrolysis.The primary chemical kinetic parameters, the temperature and density of the reactants, have both spatial and temporal distributions. Computer programs were developed for estimating T ( r , x ; t ) , given the radial intensity distributon in the incident beam, the absorption coefficient of the SF6 (as a function of density and temperature) for the irradiating line, as well as the mean heat capacity and thermal conductivity of the gas mixture. However, the computed profiles did not prove useful since the effects of convection, which could not be included in the calculations, perturb the initial distribution significantly within a few milliseconds. Indeed, it proved practical to enhance convection so as to homogenize the cell composition. When the incident radiation was chopped, frequencies ranging from 100 Hz to 1 Hz gave the same product distributions. A practical procedure is to use a "chemical thermometer" to establish a mean effective temperature for a given experimental configuration. Examples showing how to measure relative rate constants for molecular conversions, and the use of LPHP for qualitative determinations of product distributions as a function of temperature are presented. Finally, a number of hightemperature conversions are described to illustrate the utility of LPHP for flash thermolysis.

Perhydridosilicone films produced by IR laser-induced chemical vapour deposition from disiloxane

“…16,17 The DSO decomposition by the TEA CO 2 laser in the presence of SF 6 apparently takes place through primary absorption in SF 6 as a photosensitized process, 18 since the absorptivity of SF 6 at 934-952 cm À1 is significantly higher than that of DSO (Fig. 1).…”

“…sely excited atmospheric (TEA) CO 2 laser (Plovdiv University) and a cw CO 2 laser (Synrad, model E48-1-28 W), the first operating at the P (12) or P (18) line of the 00°1→10°0 transition (951.2 or 946.0 cm À1 , respectively) with repetition frequency of 1 Hz, and the second operating in the range 934-952 cm À1 . The wavelengths of the IR radiation were checked with a model 16-A spectrum analyser (Optical Engineering Co., Santa Rosa, CA).…”

Infrared multiphoton isomerization and fragmentation reactions of organic molecules

“…-* --* -I- (6) emission rising with the laser pulse is observed in the bending mode region of C02 (650 cm1) whereas weak emission with a slow rise time is observed in the asymmetric stretch region (2340 cm1) (Fig. 7).…”

Reactions of methyl radicals with isobutane at temperatures between 800 and 950 kelvin

“…The best-suited technique for studying the reaction of methyl radicals with hydrocarbons at temperatures below 1000 K is the laser-powered homogeneous pyrolysis, pioneered and further developed by Shaub and Bauer [1]. We present an application of this method for the investigation of the methyl radical induced pyrolysis of isobutane.…”