Time-resolved absorption difference spectroscopy

While there can be no doubt about the role ofBg as a bridge for triplet energy transfer a number of open questions remained and have been addressed in more recent work. Using the temperature dependence of time-resolved absorption difference spectroscopy, Frank et al. (1996b) have determined the energy barriers of B with respect to P 7o in wild-type, 13 -hydroxy-[Zn]-bacteriochlorophyll a- and [3-vinyl]-13 -hydroxy-bacteriochlorophyll a -exchanged (and spheroidene-reconstituted) RCs as 140 100,380 ... 

On the basis of these accurate calculations, it is shown that whereas the lowest singlet state can be assigned to a pure XLCT state in [Ru(I)(Me)(CO)2(Me-DAB)], its character is mainly MLCT in [Ru(Cl)(Me)(CO)2(Me-DAB)] in agreement with the time-resolved absorption/emission, IR, and resonance Raman experiments. This remarkable difference has profound consequences not only on the spectroscopy but also on the photoreactivity of this class of molecules used as sensitizers and/or catalysts in photochemical reductions. 

We have performed picosecond time resolved absorption spectroscopy for organic dyes in alcoholic solution and have shown the following results. The recXral shape of the difference spectrum before and after the excitation is expressed as the superposition of the absorption and fluorescence spectra detected under steady state condition when the solvent relaxation time is sufficiently short compared with the time resolution of the experimental equipment and the excited state lifetime. On the other hand, the spectrum in the viscous solvent at low temperature shows slightly sharp in initial and broadens its shape with time. 

Although different synthetic approaches have been used, the experimental techniques employed to verify the properties of the systems are very often similar, and generally include steady-state and time-resolved absorption and luminescence spectroscopy. 

Figure 3.4 depicts the transient Raman spectra of the all-trans and cis isomers of retinal recorded at 20 ns after excitation [7]. Consistent with the above results of time-resolved absorption spectroscopy, the 7-cis, 9-cis, and 11-cis isomers give rise to the same Ta Raman spectra as that of the all-trans isomer. However, the difference Raman spectrum of 13-cis minus all-tranf exhibits another spectral pattern ascribable to the 13-cis... 

The reactions between tyrosine and radiolyticaily generated OH radicals have been investigated by Solar et al [42], in neutral N20-saturated aqueous solution. They employed a combination of pulse radiolysis and time-resolved absorption spectroscopy techniques, and were able to determine the relative yields and reaction rates for addition to different ring sites as well as for subsequent water elimination from the ortho position and for direct abstraction of the phenolic hydrogen. Figure 2 summarizes their results. As seen, the predominant addition site is at the ortho position, and the reaction rates for OH addition or direct H-atom abstraction are 10 -10 times faster than the water elimination fi om the ortfm adduct. 

The essentials of a quantum mechanical treatment of the interaction of electromagnetic radiation with molecules is described in the first chapter, and the second one deals with supramolecular photochemistry, with particular emphasis on energy and electron transfer with a description of the Marcus theory. The following chapters are devoted to the different photochemical and photophysical techniques spectrophotometry and spectrofluorimetry, actinometry, absorption and luminescence techniques with polarized light excitation, time-resolved absorption and luminescence spectroscopy, down to femtosecond resolution. Each... 

Two and Three Component Photoinitiating Systems Based on Coumarin Derivatives. AUonas, X. Fouassier, J.P. Kaji, M. Miyasaka, M. Hidaka, T. Departement de Photochimie Genetale, UMR No. 7525, Ecole Nationale Superieure de Chimie, Mulhouse, Fr. Polymer (2001), 42 (18), 7627-7634 (Eng.). Several coumarin or ketocoumarin/additives combinations (bisimidazole deriv., mercapto-benzoxazole, titanocene, oxime ester) can initiate quite efficiently radical polymn. The interactions between the excited states of coumarins or aketocoumarin and various additives have also been studied by laser absorption spectroscopy, time resolved fluorescence and photocond. the redox potentials of these different compds. have been measured and allowed the calcn. of free enthalpy variations for a possible electron transfer reaction. The whole results explain the interaction mechanism and show that the coumarins are able to form radicals through an electron transfer reaction with the different additives whereas the ketocoumarin leads to an energy transfer with bisimidazole and to an hydrogen abstraction with the benzoxazole deriv. 

A qualitatively different approach to probing multiple pathways is to interrogate the reaction intermediates directly, while they are following different pathways on the PES, using femtosecond time-resolved pump-probe spectroscopy [19]. In this case, the pump laser initiates the reaction, while the probe laser measures absorption, excites fluorescence, induces ionization, or creates some other observable that selectively probes each reaction pathway. For example, the ion states produced upon photoionization of a neutral species depend on the Franck-Condon overlap between the nuclear configuration of the neutral and the various ion states available. Photoelectron spectroscopy is a sensitive probe of the structural differences between neutrals and cations. If the structure and energetics of the ion states are well determined and sufficiently diverse in... 

Structure, then the time-resolved photoelectron spectra [20, 21] could reveal signatures of two different intermediate structures, representing two different pathways on the PES. Transient absorption spectroscopy and other femtosecond time-resolved techniques may also be applicable to this problem. 

The dynamics of photoinduced charge separation, kcs, and charge recombination, kcr (Fig. 2a), have been studied in several families of hairpins containing an Sa linker and a single G C base pair by means of femtosecond time-resolved transient absorption spectroscopy [27, 28]. Both the singlet state and anion radical of Sa have strong transient absorption centered at 575 nm. The difference in the independently determined band shapes for Sa ... 

Conclusions. Time-resolved CO laser absorption spectroscopy can provide information useful in characterizing the primary photochemical channels in gas-phase transition metal carbonyls. We have found that product vibrational energy distributions indicate that W(CO)g and Cr(CO>6 dissociate via different... 

In the case of NOS 12, different kinetics were observed at 436 nm. In cyclohexane, there was a rapid rise, with a lifetime of 6.6 psec followed by a decay with a 100-psec lifetime. In 1-butanol, there was a rapid rise (lifetime=4.3 psec), a decay (43-psec lifetime), and a second longer decay within a 1.4-nsec lifetime. These findings were confirmed by picosecond time-resolved resonance Raman spectroscopy. In these Raman studies in cyclohexane, a single rate constant was observed, whereas in 1-butanol, three spectral components grew with different time constants. The data were said to be consistent with the photo-formation of two or three isomers trans about the central methine bond however, other transient species could be responsible for the observed kinetics because the absorption envelope obviously shifts and this would affect the resonance Raman bands. 

Direct detection of DPC is made by time-resolved EPR spectroscopy. In this method, DPC is first generated by photolysis of 30 in a hydrocarbon matrix at 16 K and is excited by a 465-nm laser, which corresponds to a T-T absorption of the To state of DPC. The transient triplet spectrum of the species having a decay rate of 160 ns at 16 K is assigned to the EPR spectrum of DPC. The ZFS parameters are determined by computer simulation to be D = 0.201 m and E = 0.0085 cm The D values observed by different methods are essentially identical. 

Time-resolved Photoacoustic Spectroscopy. In photoacoustic spectroscopy (PAS) the heat evolved by the absorption of light in the sample is transformed into sound waves which are detected by a microphone. In steady-state spectroscopy the light is continuous, but it is also possible to use a pulsed laser and to observe the change in the intensity of the sound signal with time. In this respect time-resolved PAS is somewhat similar to thermal lensing, but both techniques have different limitations and advantages. 

Most recently the TICT model has been verified by transient absorption spectroscopy,234 that is, by time-resolved studies of vertical transitions which start from different points on the horizontal reaction pathway. 

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