Spectroscopy fast transient optical

The reduction of obtainable light-pulse durations down to subpicosecond pulses (halfwidth about 10 sec) allows fast transient phenomena which were not accessible before to be studied in the interaction of light with matter. One example is the extension of spin echoe-techniques, well known in nuclear-magnetic-resonance spectroscopy, to the photon echoes in the optical region. 

Samples were irradiated by a 10 ps single or 2 ns electron pulse from a 35 MeV linear accelerator for pulse radiolysis studies (17). The fast response optical detection systems of the pulse radiolysis system for absorption spectroscopy (18) is composed of a very fast response photodiode (R1328U, HTV.), a transient digitizer (R7912, Tektronix), a computer (PDP-11/34) and a display unit. The time resolution is about 70 ps which is determined by the rise time of the transient digitizer. 

Two distinct primary transients have been observed by optical spectroscopy in its pulse radiolysis [54]. One of these is not affected by O2 and has been attributed to an A-centered radical cation, A/ N-EDTA , directly bridged to the second EDTA nitrogen. Using strong reductants as probes, e.%. N,N,N N -tetramethylphenylenediamine, G(A/ A/ -EDTA) = 1.6 x 10 mol J has been obtained. Besides generating iV iV-EDTA, the OH radicals produce C-centered radicals by H-abstraction. These have reducing properties and are rapidly oxidized by tetranitromethane, giving rise to nitroform anion, G(NF ) = 4.2 x 10 mol J . The C-centered radicals react rapidly k = 7.6 x 10 dm mol s ) with O2, and subsequent fast 02 elimination. The Schiff bases thus formed hydrolyze to the final products. 

For fast reactions, the simplest kinetics experiment is to resolve the disappearance of the reactants, for example, by transient emission if the reaction can be photoinitiated and a reactant is luminescent. If there are multiple reaction channels available for reactant decay, the kinetics are described by a mono-exponential decay according to the sum of these rates of which PCET is only one. A more powerful experiment is to observe the disappearance of PCET reactants and growth of PCET products directly. In photoinitiated optical experiments, this means probing by transient absorption (TA) spectroscopy rather than transient emission. If PCET proceeds in a concerted fashion then concomitant mono-exponential disappearance of reactant and growth of product will be observed. If a stepwise mechanism operates, the growth of the products will be delayed (and fit by a bi-exponential function), however, this observation does not reveal the sequence in which the electron and proton were transferred. Moreover, in the limit where one of the steps is significantly faster then the other, the bi-exponential character of the kinetics trace will not be discernible, and the reaction may appear as if it were concerted. 

Recently, the femtosecond time-resolved spectroscopy has been developed and many interesting publications can now be found in the literature. On the other hand, reports on time-resolved vibrational spectroscopy on semiconductor nanostructures, especially on quantum wires and quantum dots, are rather rare until now. This is mainly caused by the poor signal-to-noise ratio in these systems as well as by the fast decay rates of the optical phonons, which afford very fast and sensitive detection systems. Because of these difficulties, the direct detection of the temporal evolution of Raman signals by Raman spectroscopy or CARS (coherent anti-Stokes Raman scattering) [266,268,271-273] is often not used, but indirect methods, in which the vibrational dynamics can be observed as a decaying modulation of the differential transmission in pump/probe experiments or of the transient four-wave mixing (TFWM) signal are used. 

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