Transient absorption techniques

Transient absorption techniques now have a venerable history. The development of flash kinetic spectroscopy was the work of Norrish and Porter (62). This technique typically employed a flash lamp to produce... 

The recombination kinetics of the charge carriers have been studied in detail by the groups of Gratzel, Serpone and Colombo [4e, 5, 6]. Since recombina-tion of electrons and holes is monitored by transient absorption techniques most of the observed decay is due to reaction (7.15). 

The triplet state decay kinetics of benzophenone has been monitored in fluorinated surfactants (sodium perfluorooctanoate, SPFO) where the surfactant does not quench the triplet40>. In this case, the excited benzophenone cannot react with the surfactant and the excited molecule escapes into the aqueous phase. When increasing amounts of SDS is added to solution, the observed triplet lifetime (by nanosecond transient absorption techniques) decreases indicating that hydrogen abstraction is occurring from the SDS (Fig. 17). 

In 1990, Osuka, Maruyama, Mataga, and coworkers examined porphyrin dyad 4 and other molecules closely related to dyad 3 with different aromatic linkers joining the macrocycles [19], When the zinc porphyrin of 4 was excited in dimethylforma-mide solution, Pzn-PFe(m) was produced. It decayed with a time constant of 52 ps, and the Pzn -PFe ii) state was observed by transient absorption techniques. The charge-shifted state decayed in 1.6 ns. By studying photoinduced electron transfer in the entire series of molecules with different linkages, the dependence of rate constant on the separation of the porphyrins was determined. A value for f in Eq. 2 of 0.4A- was obtained. 

Conclusive evidence of this elusive species awaits systematic studies using transient absorption techniques. Further, the characteristic excimer emission has not been observed in the solid phase, where intermolecular distances are such as to minimize the Interaction and because achievement of the necessary orientation is inhibited in the frozen samples. 

Time-resolved UV/vis absorption spectroscopy has been initiated by Norrish and Porter who developed flash photolysis in the late 1940s, opening the way to the detection of transient chemical species with time resolution of a few microseconds [30, 31]. The present state of art transient absorption techniques allow detection of chemical intermediates with less than 10 fs resolution. The techniques used depend on the explored time scale but the principle, which is illustrated in Fig. 7.14, is the same. 

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. 

The transient absorption method utilized in the experiments reported here is the transient holographic grating technique(7,10). In the transient grating experiment, a pair of polarized excitation pulses is used to create the anisotropic distribution of excited state transition dipoles. The motions of the polymer backbone are monitored by a probe pulse which enters the sample at some chosen time interval after the excitation pulses and probes the orientational distribution of the transition dipoles at that time. By changing the time delay between the excitation and probe pulses, the orientation autocorrelation function of a transition dipole rigidly associated with a backbone bond can be determined. In the present context, the major advantage of the transient grating measurement in relation to typical fluorescence measurements is the fast time resolution (- 50 psec in these experiments). In transient absorption techniques the time resolution is limited by laser pulse widths and not by the speed of electronic detectors. Fast time resolution is necessary for the experiments reported here because of the sub-nanosecond time scales for local motions in very flexible polymers such as polyisoprene. 

Electron transfer is one of the fundamental chemical reactions in solution. So far, the transient absorption technique has been widely used for the studies of the mechanism, kinetics, and the quantum yield of the free ion formation (< >ion). However, for the determination of extinction coefficients of the ions should be known, but this is usually difficult to obtain. Of course, if the absorption bands of the ions are not located at an experimentally accessible wavelength, the study will be impossible from the transient absorption study. On the other hand, by monitoring the thermal energy, these properties can be measured and the applicability should be quite wide. 

Bachilo (1995) recently used transient absorption techniques to detect a weak T, T transition at 8100 cm in jS-carotene. This places T ( B or A ) at almost the same energy as S,. Assuming (as appears to be the case for shorto polyenes (Allan et al. 1984)) that the lowest triplet state is and that there... 

The transition from S to the lowest excited singlet state of carotenoids, S, is electric dipole forbidden (Kohler, 1991), and is not observed in the usual single-photon absorption experiment. S, is readily populated by relaxation from Sj. The lifetime of the S state, as measured by transient absorption techniques, is on the order of 10-40 ps for most carotenoids (Wasielewski and Kispert, 1986 Trautman et al 1990). The state decays almost totally via in ternal conversion. Fluorescence from S, is virtually undetectable (quantum yield <10" ), and intersystem crossing to the triplet state is not observed. Because the forbidden nature of the S, S, transition... 

Triplet-excited xanthyl cations have also been shown to undergo electron transfer processes with aromatic donors [12,13]. The reactivity of the triplet state 9-phenylxanthyl cation, its p-fluoro analog, and the 9-phenylthioxanthyl cation was examined using transient absorption techniques. Irradiation of the 9-phenylxanthyl cation 1 in the presence of biphenyl resulted in an enhanced rate constant for decay of triplet 1, with concomitant production of transients corresponding to the 9-phenylxanthyl radical at 340 nm and the biphenyl radical cation at 670 nm (Fig. 10). Rate constants for triplet decay or radical growth were measured as a function of added quencher concentration for a variety of aromatic donors. Plots of the observed first-order rate constant versus the... 

Although transient absorption techniques are a sensitive probe of electron population dynamics in several systems, there are numerous situations where these techniques are not directly applicable. For example, type-II core/shell nanocrystals exhibit absorption spectra with indistinct features due to the occurrence of a number of accidentally degenerate transitions. Transient absorption methods also fail to shed much light on VB dynamics in most II-VI materials, presumably due to a higher density of state in the VB. Transient absorption is also ill-suited for probing dynamics in indirect gap semiconductor nanocrystals such as silicon where the band edge absorption is weak and unstructured. These problems make it necessary to supplement such studies with other alternate probes of carrier dynamics. 

The PSII RC complex was isolated from market spinach by the Nanba Satoh procedure [2]. This material was subsequently stabilized by adding a polyethylene glycol (PEG) precipitation step [3,6] and removing O2 from the sample material with an enzymatic 02 scrubbing system [6]. Femtosecond transient absorption techniques and equipment have been described elsewhere [1,7]. 

The characterisation of a DSSC device or the study of partial processes that occurs at such cells uses a series of optical and electrochemical techniques, either stationary or time-resolved. The studies cover a wide range of timescales, accompanying the wide time span of phenomena occurring in a DSSC (from fs/ps for electron injection to ms for electron transport). Optical transient absorption techniques (see Chaps. 8, 14, 15) are used in combination with transient electrical measurements to follow the appearance and disappearance of chemical species and charges on a DSSC [27]. 

Measurements by any of these techniques can be in the stationary state or time-resolved. The former is simple, inexpensive to set up and focuses on the photocurrent measured at the electrode. With the latter, photophysical processes of the excited state are probed using the luminescence of the dye and formation of photoproducts monitored by time-resolved transient absorption techniques. Both have their own merits and often a complete picture can be obtained only using results from both these approaches. 

The study of the primary process of photosynthesis has been performed using transient absorption techniques. In this case, the reaction is monitored by measuring the absorption of the probe by the reactant, the product, or... 

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