Lasers transient absorption experiments

Figure 1 depicts the time resolved spectrum generated by photolysis of 30mtorr of Fe(C0>5 with a KrF excimer laser. As with all transient absorption experiments a major potential problem involves assigning the observed absorptions to specific species. For the... 

M solutions of 6HQ (Kodak) in distilled, deionized water containing distilled perchloric acid (Aldrich 99.999%) or potassium hydroxide pellets (Aldrich 99.99%) were circulating in a flow cell (2.5 mm optical path length). The subpicosecond transient absorption experiment has been already described [5]. The polarizations of the laser pump (266 nm) and continuum probe pulses were set at the magic angle (overall time resolution 300 fs). 

As has been shown by time-resolved flash photolysis measurements in colloidal titanium dioxide suspensions trapping is a very fast process. Rothenberger et al. performed picosecond and nanosecond transient absorption experiments on titanium dioxide and observed that the electron trapping time was faster than 30 ps, the time resolution of their laser system [4e]. The trapping time for holes was estimated to be < 250 ns. In a recent picosecond study by Serpone et al. on titanium dioxide colloids solutions of varying diameters it was observed that the spectra of trapped electrons as well as of trapped holes are fully developed after a laser... 

The formation of the ketene derived from 12 has been reported to be complete within 10 ns following laser photolysis in aqueous solution [110b]. For a microsecond TRIR study of 12, see S. Oishi, Y. Watanabe, Y. Kuriyama, Chem. Lett. 1994, 2187. Picosecond transient absorption experiments have indicated that the carbene derived from 13 (R=H) has a lifetime of 20 ps in methanol however, the ketene growth rate could not be determined due to overlap with carbene absorption J.J.M, Vleggaar, A. H. Huizer, P. A. Kraakman, W. P. M. Nijssen, R. J. Visser, C. A. G. O., Varma, J. Am. Chem. Soc. 1994, 116, 11754. See also Ref. 110a. 

In the experiment, ZnCcP is produced by laser photolysis, then transient absorption spectroscopy follows the formation (k ) and decay (/Ceb) of ZnCcP + in wild-type and mutant crystals. Kang and Crane have studied the effects of interface mutations on electron transfer rates in single crystals using complexes between a zinc-substituted cytochrome c peroxidase (ZnCcP) and site-directed mutants of yeast cytochrome c (yCc).The mutants replaced the... 

Only three experiments involving transient absorption were found, probably because the populations required for a good signal to noise ratio require more powerful and therefore more expensive lasers. One of these used transient absorption spectroscopy to study ozone formation (26), making this an... 

Table II. Laser Experiments - Fluorescence and Transient Absorption...

In order to study the influence of electron concentration on the observed dynamics, we performed experiments with different laser power densities. As an illustration, the transient absorption signals recorded at 715 nm in ethylene glycol upon photoionisation of the solvent at 263 nm with three different laser power densities are presented in Fig.3. As expected for a two-photon ionization process, the signal intensity increases roughly with the square of the power density. However, the recorded decay kinetics does not depend on the 263 nm laser power density since the normalised transient signals are identical (Cf. Fig.3 inset). That result indicates that the same phenomena occur whatever the power density and consequently that the solvation dynamics are independent of the electron concentration in our experimental conditions i.e. we are still within the independent pair approximation as opposed to our previous work on hydrated electron [8]. 

The experimental configuration of the pump-probe experiment is similar to Ref. [5]. A home built non-collinear optical parametric amplifier (nc-OPA) was used as a pump, providing Fourier-transform-limited 30 fs pulses, which could be spectrally tuned between 480-560 nm. In all experiments white-light generated in a sapphire crystal using part of the fundamental laser (800 nm), was used as probe light. In the pump-probe experiments the pump was tuned to the S2 0-0 band for carotenoids with n>l 1. In the case of M9, it was not possible to tune the nc-OPA to its 0-0 transition, and hence another nc-OPA tuned to 900 nm was frequency doubled and used for excitation. In addition to conventional transient absorption pump-probe measurements, we introduce pump-deplete-probe spectroscopy, which is sensitive to the function of an absorbing state within the deactivation network. In this technique, we... 

Nanosecond laser Flash Photolysis experiments were performed with 355 and 532 nm laser pulses from a Brilland-Quantel Nd YAG system (5 ns pulse width) in a front face (VIS) and side face (NIR) geometry using a pulsed 450 W XBO lamp as white light source. Similarly to the femtosecond transient absorption setup, a two beam arrangement was used. However, the pump and probe pulses were generated separately, namely the pump pulse stemming from the Nd YAG laser and the probe from the XBO lamp. A schematic representation of the setup is given below in Fig. 7.3. 0.5 cm quartz cuvettes were used for all measurements. 

The potential of laser flash photolysis in the study of carbene reactions with heteroatoms has come to be recognized in recent years. A number of kinetic studies using this technique have been carried out with carbene precursors in nitrile solvents.122-127 An absorption band at 470 nm was observed in the laser flash photolysis of diazofluorene (246) in inert solvents. This band was assigned to triplet fluorenylidene (247). In acetonitrile, however, a second band was also detected at 400 nm and whose buildup is concurrent with the decay at 470 nm.122 Laser flash experiments in other nitrile solvents (i.e., benzonitrile and pivalonitrile) also produced a transient absorption band which is very similar to that observed in acetonitrile. The band at 400 nm was assigned to an intermediate nitrile ylide (248). This absorption could be quenched on addition of an electron-deficient olefin providing good support for its... 

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