Laser reaction initiation

The most notable feature of these intrazeolite photooxygenations (Fig. 30) is that the oxygen CT band experiences a dramatic bathochromic shift in comparison to solution. This was detected initially by recording the product growth as a function of irradiation wavelength (laser reaction excitation spectrum)98,110 and was later verified by direct observation using diffuse reflectance UV-Vis spectroscopy.111 For example, 2,3-dimethyl-2-butene CT-absorbance is shifted to lower energy by more than 300 nm... 

A report considers the reactions of 1-butoxy and 1-pentoxy radicals with oxygen (eqs 1 and 2) and of their isomerizations by 1,5-H-shift (eqs 3 and 4) using direct and time-resolved monitoring of the formation of NO2 and HO radicals in the laser flash-initiated oxidation of 1-butyl and 1-pentyl radicals. ... 

We describe beamline ID09B at the European Synchrotron Radiation Facility (ESRF), a laboratory for optical pump and x-ray probe experiments to 100-picosecond resolution. The x-ray source is a narrow-band undulator, which can produce up to 1 x 1010 photons in one pulse. The 3% bandwidth of the undulator is sufficiently monochromatic for most diffraction experiments in liquids. A Ti sapphire femtosecond laser is used for reaction initiation. The laser mns at 896 Hz and the wavelength is tunable between 290-1160 nm. The doubled (400 nm) and tripled wavelength (267 nm) are also available. The x-ray repetition frequency from the synchrotron is reduced to 896 Hz by a chopper. The time delay can be varied from 0 ps to 1 ms, which makes it possible to follow structural processes occurring in a wide range of time scales in one experiment. 

Perhaps it is most useful to define a chemical laser simply as one in which the population inversion is produced by a chemical reaction. This includes cases in which energetic precursors may be supplied externally or produced by external energy input, and in fact all known chemical lasers are initiated or driven by some external energy input. We exclude lasers in which the inversion is produced directly in the primary process of photodissociation. Possible chemical laser systems and mechanisms have been reviewed282-286. We will discuss here a few particular cases. 

The pathway and kinetics of electron transfer in photochemically activated reaction centers of chloroplasts and photosynthetic microorganisms have been largely solved thanks to ultrafast lasers. The initial steps of light-activated electron transfer do not involve the breaking and making of chemical bonds unlike the great majority of chemical and biochemical reactions. To study the latter type of reactions. 

Transition metal carbonyl is important in laser chemistry as sources of metal atoms or as precursors for chemical vapor deposition (CVD) [104]. Ni(CO)4 shows a typical photofragmentation reaction initiated by the XeCl laser (308 nm, 4.03 eV), and the knowledge on the mechanism is valuable for the design and control of the laser-induced CVD. The SAC-Cl method was applied to the excitation spectrum and the potential energy curves relevant to the photofragmentation reaction [105]. 

The mechanisms of ion formation in MALDI-MS are still not fully imder-stood. Two main possibilities exist ions may either be "pre-formed" in the solid state and simply be liberated upon laser irradiation, or they may be formed by ion-molecule reactions initiated by the laser pulse. ... 

The oxidation of alkanes, alkenes and simple aromatics at 293 K under NOx rich tropospheric conditions has been studied using laser pulse initiation combined with cw laser long path absorption/LIF for the detection of OH and NO2. In the case of aliphatic hydrocarbons the absolute yield and the kinetics of the formation of these products have been found to be sensitive indicators for the reaction behaviour of the oxy radicals RO. In combination with mechanistic simulations rate constants for individual reactions as well as branching ratios have been derived, which permit the evaluation of the compound specific NO/NO2 conversion factors (NOCON - factors) for the first oxidation steps. In the case of benzene and toluene oxidation the results indicate that reaction of the primary formed X cyclohexa-dienyl radical (X = Cl, OH) with O2 is the dominant pathway, although the rate coefficients were found to be lower than 2 x 10" cmVs. 

Another promising design employs high-power lasers to initiate the fusion reaction. In test mns, a number of laser beams transfer energy to a small fuel pellet, heating it and causing it to implode, that is, to collapse inward from all sides and compress... 

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