Reactions induced time scale

Detailed studies on the decomposition of organic peroxides are of fundamental interest and of high importance in polymerization reactions. The time-scales of intermediate radical formation and of their subsequent decomposition determine process parameters such as the initiator efficiency in radical polymerizations. An improved understanding of the mechanism and dynamics of photo-induced fragmentation is achieved by systematic investigations in which quantum-chemical calculations are carried out in conjunction with highly time-resolved experiments. 

Femtosecond lasers represent the state-of-the-art in laser teclmology. These lasers can have pulse widths of the order of 100 fm s. This is the same time scale as many processes that occur on surfaces, such as desorption or diffusion. Thus, femtosecond lasers can be used to directly measure surface dynamics tlirough teclmiques such as two-photon photoemission [85]. Femtochemistry occurs when the laser imparts energy over an extremely short time period so as to directly induce a surface chemical reaction [86]. 

This is no longer the case when (iii) motion along the reaction patir occurs on a time scale comparable to other relaxation times of the solute or the solvent, i.e. the system is partially non-relaxed. In this situation dynamic effects have to be taken into account explicitly, such as solvent-assisted intramolecular vibrational energy redistribution (IVR) in the solute, solvent-induced electronic surface hopping, dephasing, solute-solvent energy transfer, dynamic caging, rotational relaxation, or solvent dielectric and momentum relaxation. 

Calculations within tire framework of a reaction coordinate degrees of freedom coupled to a batli of oscillators (solvent) suggest tliat coherent oscillations in the electronic-state populations of an electron-transfer reaction in a polar solvent can be induced by subjecting tire system to a sequence of monocliromatic laser pulses on tire picosecond time scale. The ability to tailor electron transfer by such light fields is an ongoing area of interest [511 (figure C3.2.14). 

Thus cooling-warming cycles, suitably induced, allowed temporal resolution of the reaction, step by step. The absorption, optical rotary dispersion, and electron spin resonance (ESR) spectra of pure compounds I and II were then recorded and found to be similar to those obtained under fast-reaction conditions (Douzou et al., 1970 Douzou and Leter-rier, 1970). Additional interesting observations were made possible by the low-temperature technique for instance, instead of making the time scale of reactions feasible exp>erimentally by increasing the substrate concentrations as in fast techniques, reactions at low temperatures could be performed with stoichiometric concentrations of enzyme. Such con-... 

Finally, solute radical ions can be generated by light-induced, one-photon or multiphoton ionization of their parent compounds (Chaps. 5 and 16). This approach is particularly useful in the ultrafast studies of short-lived, unstable radical ions that aim to unravel their solvation, recombination, reaction, and vibrational relaxation dynamics of the primary charges (see, e.g., Chap. 10). Whereas the time scale of radiolytic production of secondary ions is always limited by the rate with which the primary species reacts with the dispersed parent molecules, light-induced charge separation can occur in <100 fsec. There are many studies on photoionization of solute molecules in liquid solutions we do not intend to review these works. 

B. Kohler 1 would like to ask two questions to Prof. Zewail. First, in your investigation of the electron transfer reaction in a benzene- complex, the sample trajectory calculations you showed appear to suggest that the charge transfer step may induce vibrationally coherent motion in h-. Have you tested this possibility experimentally My second question concerns your intriguing results on a tautomerization reaction in a model base-pair system. In many of the barrierless chemical reactions you have studied, you have been able to show that an initial coherence created in the reactant molecules is often observable in the products. In the case of the 7-azaindole dimer system your measurements indicate that reaction proceeds quite slowly on the time scale of vibrational motions (such as the N—H stretch) that are coupled to the reaction coordinate. What role do you think coherent motion might play in reactions such as this one that have a barrier ... 

The aim of this book is to provide a concise introductory overview of the various light-induced processes in physics, chemistry, biology, as well as, in medicine and industry. It is largely based on the course of photophysics and photochemistry given at the University of Fribourg, and every effort has been made to keep it up to date with the latest developments in this field, in particular the probing of the fastest light-induced reactions on picosecond and femtosecond time scales. 

Irradiation of a CO-adduct of cytochrome oxidase induces the transfer of a CO molecule, originally bound to an iron(II) center, to copper(I) within 1 ps [108]. Photoinitiated dissociation of CO actually occurs in less than 100 fs, probably on the time scale comparable with a vibrational period of the Fe-CO stretch ( 64 fs). Since the involved reaction centers, Fe(II) and Cu(I), are very close, the mechanism in which the Cu-CO bond begins to form as the Fe-CO bond breaks, seems to be a plausible description of the CO transfer. A similar mechanism is believed to hold for the 02 transfer in biological processes in which cytochrome oxidase participates. 

These azobenzene LCs display the liquid crystalline phase only when the azobenzene moiety is in the trans form, and no liquid crystalline phase at any temperature when the azobenzene moiety is in the cis form. In these azobenzene LC system, it was predicted that phase transition should be induced on essentially the same time-scale as the photochemical reaction of the photoresponsive moiety in each mesogen, if the photochemical reactions of a large number of mesogens were induced simultaneously by the use of a short laser pulse (Figure 7).1391 On the basis of such a new concept, the photoresponse of azobenzene LCs with the laser pulse was examined, and it was found that the N to I phase transition was induced in 200 xsJ39 40 This fast response, on the microsecond timescale, had been demonstrated for the first time in NLCs. From the viewpoint of application of LCs to photonic devices, such a fast response is quite encouraging. 

The usual calorimetric methods cannot be used for the measurement of reaction enthalpies and entropies of photo-induced e.t. reactions because of their slow response time. This is also a problem in most thermal e.t. processes which follow the light-induced step, such as charge recombination through diffusional encounter of ions in conditions of laser flash photolysis, the kinetics of such processes occur in the ps time scale. 

By using transient absorption spectroscopic techniques, time-resolved measurements of photo-induced interfacial ET with time-constants shorter than 100 fsec have become possible [51,52,58-60]. When ultrashort laser pulses are used in studying PIET, vibrational coherences (or vibrational wave packet) can often be observed and have indeed been observed in a number of dye-sensitized solar cell systems. This type of quantum beat has also been observed in ultrafast PIET in photosynthetic reaction center [22], It should be noted that when the PIET takes place in the time scale shorter than 100 fsec, vibrational relaxation between the system and the heat bath is slower than PIET this is the so-called vibrationally non-relaxed ET case, and it will be treated in this section. 

Pulsed laser-Raman spectroscopy is an attractive candidate for chemical diagnostics of reactions of explosives which take place on a sub-microsecond time scale. Inverse Raman (IRS) or stimulated Raman loss (.1, ) and Raman Induced Kerr Effect (2) Spectroscopies (RIKES) are particularly attractive for singlepulse work on such reactions in condensed phases for the following reasons (1) simplicity of operation, only beam overlap is required (2) no non-resonant interference with the spontaneous spectrum (3) for IRS and some variations of RIKES, the intensity is linear in concentration, pump power, and cross-secti on. 

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