Periodic assessments of the potential for exploiting lasers in the chemical industry are merited, as are continued explorations of photochemical systems which might be scaled to industrial operations. k brief review will be presented of photochemical processes in current use, so that benchmark levels of material costs, of levels of through-put, and of competing conventional sources of radiation, be available. For contrast, one should consider trends in the development of moderate to high-power lasers, to estimate efficiencies and relative costs which may be attained in the not too distance future. In particular, using optimistic design parameters, what can be forseen with respect to the construction and use of a free-electron laser system, continuously tunable in the vicinity of 3pm, with an output of 90 Kw? If a choice must be made between the use of excimer lasers for single-photon processing, or the use of IR lasers for multiphoton absorptions, how does one balance the advantages of high frequencies and high laser powers (and very short pulse durations), against the disadvantages of injecting too much energy per molecule and the possible further decomposition of the desired photofragments due to their own absorptions? In previous reviews attention was called to the types of reactions which are of industrial interest, and also may satisfy the limiting criteria imposed by laser costs. Has progress in the development of laser systems altered these assessments?

A pulsed laser pyrolysis method has been developed to study kinetic processes at high temperatures. A CO₂ laser is used to irradiate a 100 torr mixture of an infrared absorber (SF₆), bath gas (N₂), and reactants. Rapid heating to 700-1400 K occurs, followed by two-stage cooling. Unimolecular reactions are studied by competitive kinetics with a known standard, using mass-spectrometric or gas-chromatographic analysis. Bimolecular processes are examined using laser-induced fluorescence (LIF). The technique offers great advantages in reaching reactive temperatures in a fast and time-resolved manner, without the complications of hot surfaces. It is thus an ideal tool for analyzing and measuring some of the basic processes occurring in more complicated, real, hot systems. Our recent applications of the laser pyrolysis method in the areas of catalysis and combustion are summarized here. Several transition metal-carbonyl bond dissociation energies have been measured, and catalysis by the hot metal particulate products was observed. Since the use of LIF as a flame diagnotic requires some knowledge of the fluorescence quenching rates at high temperatures, the laser pyrolysis method was used to measure these rates for the important OH radical. Its reaction rate with acetylene was also measured, with implications for flame modeling and the mechanism of soot formation. Finally, this method can be used to ignite low concentrations of fuel and oxidant, and then study the time-resolved evolution of the flame chemistry by LIF and chemiluminescence observations.

This paper describes a technique for obtaining high resolution absorption sepctra of electronic transitions involving bound excited states via stepwise two photon induced dissociation with LIF detection of the photofragments. As an example, we present spectra of the A-LA"-X-LA' system of NCNO obtained both at room temperature, and with expansion-cooled samples.

Lasers have found a variety of applications within the research laboratories of the Standard Oil Company (Ohio). We report here the determination of the conformational energy of ethanol, the laser enhanced ionization detection of zinc, and the use of tunable infrared dye lasers to excite molecular vibrations.

The ability to rapidly heat samples using infrared laser radiation without the complicating effects of hot surfaces offers new opportunities for pyrolysis techniques in materials characterization and process control. By using pulsed radiation, timescales on the order of microseconds are achieved, restricting the chemistry primarily to initial reactions. The homogeneous nature of laser powered heating minimizes wall reactions and improves reproducibility by eliminating effects of surface contamination in the pyrolysis reactor. In Laser Powered Homogeneous Pyrolysis (LPHP), a pulsed CO₂ laser (10μm) is used to rapidly heat a gas mixture to be pyrolyzed. If the mixture does not absorb 10um radiation, a chemically inert sensitizer such as SF₆ or SiF₄ must be added to couple energy into the mixture. Temperatures up to 1200K can be reached, with reaction times ranging from lOpsec to lOmsec. Product analysis is by gas chromatography after a sufficient number of laser pulses to generate detectable amounts of products. Applications of LPHP to hydrocarbon mixture analysis will be presented, as well as potential applications to process control. The short reaction times in LPHP will be illustrated by methane and ethane pyrolysis, which also provide information on the details of the temperature profile during laser powered pyrolysis.

Laser microprobe mass analysis (LMMA) is a relatively new microanalytical technique used in the characterization of materials elemental and molecular composition. The LMMA technique is based on performing a mass and intensity analysis of the ionic components formed during high power density laser irradiation of a materials surface. The micro-analytical characteristic of this materials analysis technique results from utilizing a finely focused (%1.0pm diameter) laser pulse to initiate the vaporization and ionization of the materials constituents. The combination of this finely focused, high power density (variable between %108 to 1012 w/cm2) laser pulse with time-of-flight (TOF) mass analysis techniques provides a complete mass spectrum of the ionization produced during each laser shot and results in elemental detection sensitivities which are in the ppm (atomic) range. This paper presents a discussion of the principles of laser microprobe mass analysis along with a series of applications of the technique in the microanalytical characterization of such materials as metals, insulators, and semiconductor devices.

Thermal elimination of hydrogen chloride from 1,2-dichloroethane and 1,1,1-chlorodifluoroethane is a main industrial route to some important monomer compounds. Two reaction paths are possible - unimolecular four-center-elimination and a radical chain mechanism. The latter is started by the endothermic C-Cl bond rupture, followed by the chain consisting of an abstraction reaction and a monomolecular decomposition step. Inducing the chain by photo-initiation offers the advantage that the monomolecular process becomes the rate determining step leading to a lower activation energy for the total reaction. This allows lower process temperatures, therefore decreasing energy expense and avoiding the high temperature formation of byproducts. In our investigation we used as photolysis source several lines of an UV-Exciplex-Laser which provide short pulses of monochromatic UV-radiation with high energy. It was possible to study the effects of initial radical concentration on the quantum yield 0 as function of the temperature. Long reaction chains and a decrease of 0 inversely proportional to the square root of irradiation intensity were observed. The formation of the monomers was detected by a time-resolved UV-absorption technique. Experimental results were analysed in order to yield information about the temperature and pressure dependence of the rate determining reaction. By application of our data in computer simulations wP were able to extrapolate the reaction behaviour to various technical conditions.

Ultraviolet laser initiated chain reactions are described. Preliminary laser photochemistry economics, laser power, and stability requirements for large scale chemical synthesis employing chain reactions is reported. A two step laser scheme for converting methane to ethylene with chlorine as an intermediate is proposed. Photochlorination of methane has been studied at 248 nm and 308 nm laser wavelengths as a function of pulse energy and temperature. KrF laser (248 nm) excitation gives a factor of 3 higher quantum yield than XeCl laser (308 nm) excitation. At 200°C using KrF laser, auantum yield of 27,000 was observed.

The theoretical and experimental considerations involved in laser-generated biradical trapping with molecular oxygen are discussed. This method has been applied in the elucidation of the mechanism of the photodegradation of Vitamin K via oxygen trapping of a preoxe-tane biradical. The trapping of biradicals derived from azoalkanes has been applied to the syntheses of pine beetle pheromone mimics and prostaglandin endoperoxide analogues.

Consideration of viable commercial ventures involving laser chemistry must take into account not only the cost structure of the laser production step but must also consider all other steps in the venture through to provisions of goods or services to clients. Recent work on the two-stage infrared laser isotope separation of carbon-13 and the selective wavelength synthesis of vitamin-D is reviewed. The processes, which may be viewed as surrogates for direct carbon dioxide and excimer laser driven synthetic schemes, are discussed in terms of their technical description and the business opportunity they present.

We have investigated the advantages of using laser-initiation for the synthesis of cumenehydroperoxide and t-butylhydroperoxide. Laser-initiation significantly improves the oxidation rates of cumene in the liquid phase and iso-butane in the vapor phase (using HBr promoters) with moderate photoefficiencies (418 and 490 respectively). The primary effect of laser-initiation is to reduce the induction period of the reaction. For the oxidation of cumene the beneficial effect of laser initiation is strongly dependent on laser wavelength, alternately enhancing (at 351 nm) or inhibiting (at 249 nm) the oxidation rate. For isobutane oxidation, laser-initiation also minimizes the HBr depletion rate relative to oxidation rate.

We present the experimental results of three laser initiated free-radical reactions. As one might expect, each reaction exhibits a wavelength dependence, but the mechanisms for each are not intuitively obvious. In all three examples, photochemical induction followed by thermal propagation results in higher conversions with fewer by-products when compared to strictly thermal synthetic routes.

This paper describes Battelle's ongoing research efforts to evaluate a novel, and as yet undeveloped, technology which has the potential to revolutionize the casting industry. The key to this new technology, which may be referred to as "Photochemical Machining", or PCM, is the computer-assisted fabrication of a plastic pattern directly from design specifications by the selective, three-dimensional modification of a plastic material which is exposed simultaneously to two different laser beams which are focused onto a single point within the volume of the material. The research conducted during the past year has been successful in developing a theoretical framework for relating optical and materials properties to the performance of a PCM system. While the photosensitive materials developed to date in the course of this research do not yet perform well enough for a commercial PCM system, laboratory experiments have confirmed the formation of isolated three-dimensional polymeric shapes within a volume of monomer solution by this technique. It can be shown that the fabrication of relatively small (<10 cm on a side) three-dimensional solid objects having highly precise shapes and surface finish can, in prin-ciple, be achieved in a time of only hours or days using the PCM technique given improved materials. This technology could have a major impact on the metal casting industry if indeed it can he developed successfully.

Much of the early work in laser-driven powder synthesis has employed as reactants small molecules (especially hydrides) which have a limited range of potential reaction products. We have been interested in powder synthesis reactions beginning with larger molecular species, in order to access a wider range of reactivity and material compositions, and have found that the broader range of gaseous and solid products obtained provides insight into the details of the laser-driven synthesis process. We have dealt mainly with organosilicon compounds, using a cw CO2 laser for material synthesis and a pulsed CO₂ laser to probe the reaction kinetics at short times. The cw reactions are carried out on a flowing gas stream at 145-4000 W/cm² and an exposure time of 20 ± 10 msec. The pulsed experiments are done in a static cell at low pressure using laser pulse widths of 0.5 psec and intensities of 1 MW/cm². The pulsed experiments provide data on the initial bond-breaking steps which occurs the reactant molecules heat up. We find that, even at ultimate reaction temperatures of 1500 K in cw experiments, the kinetics of the first reaction steps are important to product formation. We also find that quenching in the cw experiments is sufficiently rapid to allow back-calculation of the pyrolysis temperature from the composition of the light hydrocarbon products. The implications of these results for laser driven powder synthesis are discussed in terms of a general mechanism for reaction initiation, powder growth, and reaction quenching.

A process for synthesizing Si, Si₃N₄ and SiC powders from laser heated gas phase reactants has been developed and modelled. The superior process control and the inherent process attributes achievable with laser heating permits powders having nearly ideal characteristics to be produced. Projected manufacturing costs for this process indicate that resulting powders should be lower in cost than conventional powders.

A technique which employs high-intensity, pulsed lasers to bond metallic foils to different metal substrates is reported. Laser-generated, large amplitude thermal and pressure waves heat and compress a coating /substrate combination and mix the two materials both by plastic or hydrodynamic motions, and by diffusion at rates determined by the elevated temperatures. The degree of mixing, the rate of cooling, and thus the depth, homogeneity and metallurgical structure of the bond depend on the laser intensity, fluence, and material properties. Optical microscopy and Robinson backscatter electron imaging of cross-sections of several such coatings indicate that they are unusually well-bonded, suggesting good resistance to abrasion and corrosion, and that they exhibit unique distributions of elements.

In a country which the OECD ranks 23rd of 24 nations in the value of technology-intensive exports over imports, which "has been an industrial museum and our factories are working mdoels of the age of chisels, spanners and hammers" according to its Minister for Science and Technology and which now, according to the same man has a "detailed Science and Technology Policy, the best of any political party in the English speaking world" there are indications of change. Over the past seven years there have been grants of $135M to encourage major industrial research and development projects by companies and a further $20M has been contracted for 18 projects seen to have potential benefit for Australia. Our laser photochemistry project has been funded under the latter scheme. A brief review will be given of Government support of industrial chemistry projects in Australia. The nature of the industry-academic interaction required in our contract will be compared with the normal approach by academics to industrial research.

It has been demonstrated that continuous wave infrared CO₂ laser radiation can be util-ized to rapidly produce active catalysts from inert precursors. The activity and selectiv-ity of Ca0 produced from Ca(OH)₂ for the isomerization of 1-butene to cis- and trans-2-but-ene is discussed. Variation of the laser irradiation time produces catalytic activity and selectivity qualitatively similar to that resulting from conventional calcination at different temperatures. Pulsed infrared laser-induced reactions at catalytic surfaces are also discussed with emphasis on the dehydrobromination of 2-bromopropane and ethylene elimination from glycine ethyl ester hydrochloride at BaSO₄ , A1PO₄, and similar surfaces. Correlations are made of the extent of reaction with various experimental parameters including nature of the catalyst, laser frequency, laser fluence, number of laser pulses, and reagent-catalyst ratio.

A CO₂ laser pyrolysis technique has been used to prepare high surface area (-120 m2/gm) Fe_x Si_y C_z materials. These materials have been tested as catalysts for the reaction of hydrogen with carbon monoxide to form hydrocarbons. The reaction was studied as a function of temperature (250°C - 350°C) and feed-gas composition (H2/CO = 3:1, 1:1, 1:2). The Fex Siyz C materials show very high selectivity for the formation of low molecular weight olefins, with over 50% of the product being C₂-C₄ olef ins. They are moderately active and retain their activity and selectivity over long periods of time. In addition, the formation of undesirable CO₂ by-product is substantially reduced relative to other reported catalysts. The observed product distributions are, in general, consistent with the Schulz-Flory polymerization mechanism.

The pulsed laser excitation of iron pentacarbonyl in solutions of 1-pentene photoinitiates a highly active catalytic process for isomerization of the olefin. This process is observed in situ by rapid scanning FTIR spectroscopy, allowing subsecond acquisition of spectra. These are deconvoluted into discrete spectral components which are assigned molecular formulas. Specific activities have been obtained for two catalytically significant complexes from a correlation of catalytic activity with compositional changes. A similar interpretation of multipulse and cw experiments allowed development of a comprehensive cycle of thermal and photochemical interconversions among components.

An apparatus for the gas phase infrared spectroscopic detection of coordinatively un-saturated metal carbonyls is described. Coordinatively unsaturated species are produced by UV photolysis. Infrared spectra of coordinatively unsaturated iron carbonyls are reported for the carbonyl stretch region. Rate constants for the reaction of Fe(C0)₃ and Fe(C0)₄ with CO are also reported. The photophysics of Fe(C0)_x formation is discussed.

The gas phase hydrogenation of ethylene is homogeneously photocatalyzed by pulsed excimer laser irradiation of iron pentacarbonyl in situ. The experiments, performed under mild conditions of temperature (10-70°C) and total pressure (1-2.4 atm), result in quantum yields (defined as the number of product molecules formed per single photon absorbed) as high as 170. This efficiency per photon is much greater than unity in every measurement. The laser initiates catalytic activity with temporally distinct bursts of light, which allows most of the catalyst to revert to an inactive species before the next pulse enters the system. The catalyst generated by the laser light is thermally active, so that our method separates the photochemical initiation from the actual thermal catalysis, providing an opportunity for a detailed study of the elementary reactions involved in the catalytic system itself. Varying the pressures of Fe(C0)₅, ethylene, and hydrogen gives relative rates of steps within the cycle, while changing the time between successive laser pulses yields the average lifetime of the catalyst. Controlled temperature experiments reveal activation energies for more than one step in the catalyzed hydrogenation system.