Laser-driven reactions

S. Kawai, A. D. Bandrauk, C. Jaffe, T. Bartsch, J. Palacian, and T. Uzer, Transition state theory for laser-driven reactions, J. Chem. Phys. 126, 164306 (2007). 

A novel setup was developed to study laser-driven reactions in solid matrices (e.g., polymers) using time-resolved IR spectroscopy. The first experiments have provided one of the first examples of how ultrafast infrared spectroscopy may be used to examine laser-driven reactions in polymeric matrices. The photo chemically as well as the thermally initiated reaction of a model compound has been studied in a PMMA matrix. It is remarkable that both initial reactions happen on a time scale faster than our experimental limit of 20 ps. While the initial reaction products are probably the same, the... 

While conventional synthesis without a laser takes about 10-20 hours at a temperature of 425 K and demands high pressures of S2F10, the laser-driven reaction proceeds much more quickly, even at the lower temperature of 350 K [1390, 1391]. 

Haggerty JS, Cannon WR. Sinterable powders from laser driven reactions annual report. MIT Energy Laboratory, Cambridge, MA 1979. 

A. Hartford and J. H. Atencio, Inorg. Chem., 1980, 19, 3060. Carbon dioxide-laser-driven reactions of B2H6 and B2H6/B5H9 mixtures. 

Danforth, S.C. and Haggerty, J.S., Synthesis of Ceramic Powders by Laser Driven Reactions. Energy Laboratory Report, MIT-EL-81-003, (1980). 

Cannon, M.R., Danforth, S.C., Flint, J.H., Haggerty, J.S., and Marra, R.A., Sinterable Ceramic Powders from Laser-Driven Reactions I, Process Description and Modeling. J. Amer. Ceram. Soc. 65 [7] 324-3% (1982). 

Nuclear science in particular obtains from laser-driven electron sources a brand new input to perform interesting measurements in the context of many laboratories equipped with ultrashort powerful lasers. The ultrashort duration of these particle bunches represent a further attractive feature for these kinds of studies. In the following, we will focus on nuclear reaction induced by gamma radiation produced by bremsstrahlung of laser-produced electrons in suitable radiator targets. This way is usually mentioned as photo-activation and is particularly efficient for photons of energy close to the Giant Dipole Resonance of many nuclei. 

Recently, laser-driven photocathode accelerators and lasers coupled with compressed pulses have been able to produce electron pulses in the vicinity of 5-10 ps. This has enabled one to improve on the time resolution available from the original Hunt experiments but without the limitations of the multiple pulses. These short times have been used for measuring electron transfer reactions, electrons in... 

Recently, new techniques such as laser-driven photocathode accel-erators have increased the time resolution available for radiation-chemical studies. They have been of great use in studying fast electron-transfer processes, but are not the one-to-two orders of magnitude improvement that would be needed to explore some of the fundamental questions of electron-precursor reactions and initial distribution of radiolytically produced species. Newer techniques, such as the laser-wakefield accelerator, have the potential to answer these sorts of questions however, they have not reached their maximum potential. ... 

Although the constraints associated with using NF(a) or NCI (a) as energy carriers and transfer agents are challenging, pulsed lasers driven by these metastables have been demonstrated. In the following, I will review the development of these devices and related studies of elementary reaction kinetics. 

The generation of active coordinates for non-adiabatic dynamics is related with our interest in laser-driven control. The optimal control of photochemical reactions is based on shaped laser pulses designed to generate photoproducts selectively. 

Here Ef is the amplitude, t the duration, and co the frequency of the ith pulse. This scheme has been applied in Ref [46] to a generic two-dimensional HT model which incorporated a H-atom reaction coordinate as well as a low-frequency H-bond mode. In a subsequent work [47] the approach has been specified to a simple model of HT in thioacetylacetone. The Hamiltonian was tailored to the form of Eq. (4.1) based on the information available for the stationary points, that is, the energetics as well as the normal modes of vibration. From these data an effective two-dimensional potential was constructed including the H-atom coordinate as well as a coupled harmonic oscillator, which describes the 0-S H-bond motion. Although perhaps oversimplified, this model allowed the study of some principle aspects of laser-driven H-bond motion in an asymmetric low-barrier system. 

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... 

P. R. Buerki, and S. Leutwyler, Homogeneous nucleation of diamond powder by C02-laser-driven gas-phase reactions, J. Appl. Phys., 69(6) 3739-3744 (1991)... 

The alkaline earth metals form a host of unique monovalent free radicals. Most of these molecules can be formed by the laser-driven chemical reactions of metal vapors with a wide variety of organic and inorganic molecules. This photochemical production of new molecules has led to an extensive gas-phase inorganic chemistry and spectroscopy of alkaline earth derivatives. In recent years, the Broida oven source has been displaced by the pulsed molecular beam spectrometer. The chemical dynamics and photochemistry of these new molecules are still at a very early stage of investigation. 

In a series of laser-driven photochemical reactions of N2F4 (seee.g., N2F4 + NO, pp. 372/3), it was claimed that an (unstabilized ) mixture of 50Torr N2F4 and lOOTorr H2 reacted upon irradiation with a single 10" s, 50 Watt CO2 laser pulse (10.6 fxm) in a chemiluminescent reaction [8] N2F4 + 2H2 N2 + 4HF... 

Contents Tunable and High Energy UV-Visible Lasers.—Tunable IR Laser Systems.—Isotope Separation and Laser Driven Chemical Reactions. -Nonlinear Excitation of Molecules.—Laser Photokinetics. Atmospheric Photochemistry and Diagnostics. Photobiology. Spectroscopic Applications of Tunable Lasers. 

Laser beams can also be used to drive chemical reactions in a technique known as laser-assisted chemical etching. Much higher local etch rates can be achieved with this method than are possible with RIE. Similarly, a laser beam can supply energy to drive a deposition the method is called laser-driven deposition. An example is depositing W onto specific areas of a substrate from tungsten hexacarbonyl [W(CO)J. Laser-assisted processes can be used to create interesting three-dimensional structures. However, the processes are intrinsically serial, not parallel, meaning that a beam must be scanned across the areas to be processed spot by spot instead of simultaneously. 

Laser-induced roughening of the polymer surfaces can be observed below, near and above the ablation threshold. Below the threshold, it can be initiated by photolytically driven reactions, for example, by photo-oxidation or photobleaching. Near the ablation threshold, the change of surface topography plays an important role, for... 

Si N powders were produced via laser-driven gas phase reactions. Details of the process have been reported elsewhere, and therefore are only... 

Perhaps the best example of bond-specific chemistry driven by absorption of laser light has been the set of reactions involving heavy water [14940-63-7], HOD ... 

Chemical advances frequently are driven by technology. The discovery that atoms have inner structure was an outgrowth of the technology for working with radioactive materials. In Chapter 2 we describe a famous experiment in which the structure of atoms was studied by bombarding a thin gold foil with subatomic particles. A contemporary example is the use of lasers to study the details of chemical reactions. We introduce these ideas in Chapters 7 and 8. 

The explosive phenomena produced by contact of liquefied gases with water were studied. Chlorodifluoromethane produced explosions when the liquid-water temperature differential exceeded 92°C, and propene did so at differentials of 96-109°C. Liquid propane did, but ethylene did not, produce explosions under the conditions studied [1], The previous literature on superheated vapour explosions has been critically reviewed, and new experimental work shows the phenomenon to be more widespread than had been thought previously. The explosions may be quite violent, and mixtures of liquefied gases may produce overpressures above 7 bar [2], Alternative explanations involve detonation driven by phase changes [3,4] and do not involve chemical reactions. Explosive phase transitions from superheated liquid to vapour have also been induced in chlorodifluoromethane by 1.0 J pulsed ruby laser irradiation. Metastable superheated states (of 25°C) achieved lasted some 50 ms, the expected detonation pressure being 4-5 bar [5], See LIQUEFIED NATURAL GAS, SUPERHEATED LIQUIDS, VAPOUR EXPLOSIONS... 

The observations of complex dynamics associated with electron-stimulated desorption or desorption driven by resonant excitation to repulsive electronic states are not unexpected. Their similarity to the dynamics observed in the visible and near-infrared LID illustrate the need for a closer investigation of the physical relaxation mechanisms of low energy electron/hole pairs in metals. When the time frame for reaction has been compressed to that of the 10 s laser pulse, many thermal processes will not effectively compete with the effects of transient low energy electrons or nonthermal phonons. It is these relaxation channels which might both play an important role in the physical or chemical processes driven by laser irradiation of surfaces, and provide dramatic insight into subtle details of molecule-surface dynamics. 

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