Time-resolved electronic absorption spectroscopy, limitations

Time resolved electronic absorption and emission spectroscopy has been extensively used for solution phase kinetic studies of fast chemical processes initiated by ionizing radiation. A wealth of information on rate parameters and reaction mechanisms on a variety of chemical reactions has been obtained by this technique. As valuable as these techniques are, they have limitations. In particular, the electronic spectra in solution are often broad and featureless and offer little structural information. As a consequence, the identification of a reaction intermediate is based on chemical intuition and not on its spectral characteristics. Moreover, when more than one transient is present in the system with overlapping electronic absorption, the kinetic monitoring of the individual concentration becomes difficult. Vibra-... 

Transient terahertz spectroscopy Time-resolved terahertz (THz) spectroscopy (TRTS) has been used to measure the transient photoconductivity of injected electrons in dye-sensitised titanium oxide with subpicosecond time resolution (Beard et al, 2002 Turner et al, 2002). Terahertz probes cover the far-infrared (10-600 cm or 0.3-20 THz) region of the spectrum and measure frequency-dependent photoconductivity. The sample is excited by an ultrafast optical pulse to initiate electron injection and subsequently probed with a THz pulse. In many THz detection schemes, the time-dependent electric field 6 f) of the THz probe pulse is measured by free-space electro-optic sampling (Beard et al, 2002). Both the amplitude and the phase of the electric field can be determined, from which the complex conductivity of the injected electrons can be obtained. Fitting the complex conductivity allows the determination of carrier concentration and mobility. The time evolution of these quantities can be determined by varying the delay time between the optical pump and THz probe pulses. The advantage of this technique is that it provides detailed information on the dynamics of the injected electrons in the semiconductor and complements the time-resolved fluorescence and transient absorption techniques, which often focus on the dynamics of the adsorbates. A similar technique, time-resolved microwave conductivity, has been used to study injection kinetics in dye-sensitised nanocrystalline thin films (Fessenden and Kamat, 1995). However, its time resolution is limited to longer than 1 ns. 

Mass spectra of short-lived, unstable episelenides were obtained by taking advantage of a mass spectrometric technique developed for the time-resolved detection of transient intermediates in flash-photolyzed systems <66JA4277>. Detection does not depend on the electronic absorption characteristics of the transient, and, in combination with kinetic absorption spectroscopy, the technique assumes great flexibility. The apparatus consists, essentially, of a photolysis cell attached to a small leak into the ion source of an Atlas CH4 mass spectrometer. Selected mass peaks can be studied with a response time of a few milliseconds, and thereafter at times limited by bleeding of the photolyzed mixture into the ion chamber. Typical photolytic flash energies were 480 calories, passed into a reaction volume of 5 ml. 

The D1/D2/cytochrome b-559 complex contains 4 chlorophylLa molecules, 2 pheophytin-a molecules, 1 cytochrome b-559 and some p-carotene it contains no plastoquinone. In the absence of the secondary acceptors (quinone), electron transfer within this complex Is limited to the formation of the primary radical pair P680+Pheophytin". Absorption spectroscopy of this preparation has indicated the presence of a component decaying with a lifetime of 32-36 ns, corresponding to the lifetime of the primary radical pair (Danielius et al., 1987 Takahashi et al., 1988). More recently, time-resolved fluorescence studies (MImuro et al., 1988 Seibert et al., 1988) have shown that this complex exhibits a lifetime of 25-35 nanoseconds this has also been attributed to charge recombination of the primary radical pair, however, the fluorescence from this component was observed to be less than 2% of the total light emitted. 

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