Transient laser spectroscopy

The kinetics of the oxidation of iodide by the oxidized state of c -[Ru (dcbpy)2-(NCS)2] sensitizer adsorbed on nanocrystalline Ti02 films was measured by transient laser spectroscopy [92]. Figure 16 shows the transient absorption kinetics recorded in propylene carbonate with various electrolytes added. In all cases, the recovery of the ground-state absorption of the dye, after the fast electron injection into the solid, does not follow a simple kinetic law. In the absence of any electrolyte (trace a), the time needed to reach half of the initial absorbance (/1/2) through back electron transfer is 2 ps. Total recovery of the initial absorption, however, requires several hundreds of microseconds to milliseconds. Traces b, c, and d were recorded after addition of a common concentration of 0.1 m of iodide in the form of tetra-butylammonium (TBA+), Li+, and Mg + salts, respectively. Addition of the electrolyte in all three cases led to a considerable acceleration of the dye regeneration with ti/2 < 200 ns and complete suppression of the slow kinetic tail. 

A suitable method for a detailed investigation of stimulated emission and competing excited state absorption processes is the technique of transient absorption spectroscopy. Figure 10-2 shows a scheme of this technique. A strong femtosecond laser pulse (pump) is focused onto the sample. A second ultrashort laser pulse (probe) then interrogates the transmission changes due to the photoexcita-lions created by the pump pulse. The signal is recorded as a function of time delay between the two pulses. Therefore the dynamics of excited state absorption as... 

The transient triplet carbene formed from irradiation of DABA in benzene can be observed to react with styrene. For this process laser spectroscopy reveals a bimolecular rate constant (ksty) equal to 1.2 x 107M-1s-1. The product of the reaction is the expected cyclopropane. This observation clearly supports the spectral assignment of the carbene made above. When deuteriated a-methylstyrene is substituted for styrene as a trap for the carbene, the cyclopropane that results is a 1 1 mixture of stereoisomers, (20) (Table 6). This finding indicates that BA is reacting in this sequence exclusively from its ground triplet state. 

Photolysis of DMDAF in benzene containing methyl alcohol gives the ether expected from the reaction of the singlet carbene. Monitoring this reaction by laser spectroscopy reveals that the detected transient reacts with the alcohol with a bimolecular rate constant very near the diffusion limits. In contrast, the transient reacts with triethylamine at least 100 times more slowly than it does with alcohol (Table 7). This behavior is inconsistent with identification of the transient as the cation or radical and points to its assignment as the singlet carbene. 

Figure 8.7. Delayed fluorescence and diffuse reflectance transient absorption spectroscopy on scattering substrates. Example terthicnyl on silica gel excited with = 354 nm (neodymium/yttrium-aluminum-garnet) (Nd/YAG) laser pulse of 10 nsec, 20 mj), recorded with a gated diode array spectrometer.

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... 

The UV-visible absorption and fluorescence spectra of NKX-2311 in solution are measured. The absorption and fluorescence maxima are located at 490 and 555 nm, respectively. The fluorescence spectrum is shown in Fig. 1. This figure also shows the transient absorption spectrum of NKX-2311 in deuterated methanol (CD3OD) solution measured by the nanosecond laser system. The spectrum is ascribed to the excited singlet state (dye ). The absorption spectrum of the oxidized form of NKX-2311 (dye+) was measured by nanosecond transient absorption spectroscopy after adding electron acceptor, 1,4-benzoquinone, in NKX-2311 solution of CD3OD. As shown in Fig. 1, there are two characteristic peaks at around 875 and 1010 nm. 

The dynamics of the interfacial electron-transfer between Dye 2 and TiOz were examined precisely by laser-induced ultrafast transient absorption spectroscopy. Durrant et al.38) employed subpicosecond transient absorption spectroscopy to study the rate of electron injection following optical excitation of Dye 2 adsorbed onto the surface of nanocrystalline Ti02 films. Detailed analysis indicates that the injection is at least biphasic, with ca. 50% occurring in <150 fsec (instrument response limited) and 50% in 1.2 0.2 psec. 

The primary steps of the photolysis of aqueous monuron and diuron were investigated by Canle et al. by means of transient absorption spectroscopy using an ArF laser (X = 193 nm) for excitation [89]. Under these conditions, photoionization occurred with a quantum yield of about 10%. Radical cations were detected after the laser pulse and found to deprotonate to yield neutral radicals [89]. 

Dr. Rohlfing s research interests include the experimental characterization of transient molecules relevant to combustion processes, linear and nonlinear laser spectroscopies, trace detection of pollutants, molecular beam and mass spectrometric studies of carbon and metal clusters, and vibrational relaxation dynamics. He is the author of approximately 50 peer-reviewed articles, holds membership in the American Chemical Society and the American Physical Society, and is a fellow of the American Association for the Advancement of Science. 

The P +-Q state can be observed by transient absorption spectroscopy because the absorption spectrum of the charge-separated state differs from that of the porphyrin first excited singlet state. Measurement of the transient absorption spectrum of a benzonitrile solution of 8 as a function of time following excitation with a 150 to 200-fs laser pulse at 590 nm allowed determination of the rate constant for decay of P" -Q back to the ground state (step 3 in Figure 3). The value obtained for kj, is 5.3 X 10" s- [61]. 

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