Time-resolved optical spectroscopy

Bruckner V, Feller K-H and Grummt U-W 1990 Applications of Time-Resolved Optical Spectroscopy (New York Elsevier)... 

Cluster Fx was also identified via its EPR spectral features in the RCI photosystem from green sulfur bacteria 31, 32) and the cluster binding motif was subsequently found in the gene sequence 34 ) of the (single) subunit of the homodimeric reaction center core (for a review, see 54, 55)). Whereas the same sequence motif is present in the RCI from heliobacteria (50), no EPR evidence for the presence of an iron-sulfur cluster related to Fx has been obtained. There are, however, indications from time-resolved optical spectroscopy for the involvement of an Fx-type center in electron transfer through the heliobacterial RC 56). 

The first theoretical attempts in the field of time-resolved X-ray diffraction were entirely empirical. More precise theoretical work appeared only in the late 1990s and is due to Wilson et al. [13-16]. However, this theoretical work still remained preliminary. A really satisfactory approach must be statistical. In fact, macroscopic transport coefficients like diffusion constant or chemical rate constant break down at ultrashort time scales. Even the notion of a molecule becomes ambiguous at which interatomic distance can the atoms A and B of a molecule A-B be considered to be free Another element of consideration is that the electric field of the laser pump is strong, and that its interaction with matter is nonlinear. What is needed is thus a statistical theory reminiscent of those from time-resolved optical spectroscopy. A theory of this sort was elaborated by Bratos and co-workers and was published over the last few years [17-19]. 

In the previous Maxwelhan description of X-ray diffraction, the electron number density n(r, t) was considered to be a known function of r,t. In reality, this density is modulated by the laser excitation and is not known a priori. However, it can be determined using methods of statistical mechanics of nonlinear optical processes, similar to those used in time-resolved optical spectroscopy [4]. The laser-generated electric field can be expressed as E(r, t) = Eoo(0 exp(/(qQr ot)), where flo is the optical frequency and q the corresponding wavevector. The calculation can be sketched as follows. 

Measurements of the lifetimes of NIESST states by time-resolved MES [51] and of LIESST states by time-resolved optical spectroscopy [52] on the very same system (a single crystal of [FC , Mni , (bpy)3] (bpy = bipyridine)) gave similar results. This supports the suggestion that the mechanisms for LIESST and NIESST relaxation are very similar, at least for the low-energy regime. The NIESST effect was also studied in Co(ll) SCO compounds, viz. [ Co/Co(terpy)2]X2 H20 (X = [CIOJ, n = V2,X = Cl , n = 5), where terpy is the tridentate ligand terpyridine... 

M = (Mx,My,M ) is the dipole moment of the system. Moreover, the indices i, j designate the Cartesian components x, y, z of these vectors, ()q realizes an averaging over all possible realizations of the optical field E, and () realizes an averaging over the states of the nonperturbed liquid sample. Two three-time correlation functions are present in Eq. (4) the correlation function of E(t) and the correlation function of the variables/(q, t), M(t). Such objects are typical for statistical mechanisms of systems out of equilibrium, and they are well known in time-resolved optical spectroscopy [4]. The above expression for A5 (q, t) is an exact second-order perturbation theory result. 

In some systems, triplet BET can occur, as deduced from time-resolved optical spectroscopy, magnetic field effects, CIDNP, or optoacoustic calorimetry. Triplet BET is governed by energetic factors, which determine rates, and by the relative topologies of the potential surfaces of parent molecule, radical ions, and of accessible triplet or biradical states. Divergent topologies for different states may cause rearrangements. 

Abstract Time-resolved optical spectroscopies are now routinely used in condensed matter... 

In summary, these results constitute strong evidence for the two-step reaction sequence. They require that the deprotonation of the aminium radical cation be competitive on the CIDNP timescale i.e. surprisingly fast since it involves a carbon acid. The results delineate the fate of the amine derived intermediates with particular clarity, since they are observed directly for amine derived products. The conclusions based on the above CIDNP results were confirmed by time resolved optical spectroscopy in a variety of systems [179-182]. However, in essentially all these systems the reaction progress is monitored by following the complementary spectra of the acceptor derived radical intermediates, such as ketyl, semiquinone, stilbene, or thioindigo radical anions. 

The pulse radiolysis of trimethyl-(TM-)cyclopropenylium cation, TMCP+ examined by time resolved optical spectroscopy, provided evidence for the eventual formation of hexamethylbenzene cation, HMB +. Apparently TMCP+ and its one-... 

The current detailed understanding of photo-induced electron transfer processes has been advanced dramatically by the development of modern spectroscopic methods. For example, the application of time-resolved optical spectroscopy has developed from modest beginnings (flash-phyotolysis with millisecond resolution) [108,109] to the current state of the art, where laser spectroscopy with nanosecond resolution [110-113] must be considered routine, and where picosecond [114-116] or even femtosecond resolution [117] is no longer uncommon. Other spectroscopic techniques that have been applied to the study of electron transfer processes include time-resolved Raman spectroscopy [118], (time resolved) electron spin... 

Applications of Time-Resolved Optical Spectroscopy by V. Bruckner, K.-H. Feller... 

Time-Resolved Optical Spectroscopy as a Probe of Local Polymer Motions... 

The relationship between the structure of a polymer chain and it dynamics has long been a focus for work in polymer science. It is on the local level that the dynamics of a polymer chain are most directly linked to the monomer structure. The techniques of time-resolved optical spectroscopy provide a uniquely detailed picture of local segmental motions. This is accomplished through the direct observation of the time dependence of the orientation autocorrelation function of a bond in the polymer chain. Optical techniques include fluorescence anisotropy decay experiments (J ) and transient absorption measurements(7 ). A common feature of these methods is the use of polymer chains with chromophore labels attached. The transition dipole of the attached chromophore defines the vector whose reorientation is observed in the experiment. A common labeling scheme is to bond the chromophore into the polymer chain such that the transition dipole is rigidly affixed either para 1 lei (1-7) or perpendicular(8,9) to the chain backbone. 

WALDOW ETAL. Time-Resolved Optical Spectroscopy... 

During the year a number of specialized monographs of relevance to photophysics have appeared. Two of general interest deal with applications of time resolved optical spectroscopy and luminescence techniques in chemical and biochemical analysis. ... 

V. Bruckner, K.-H.Feller, and U.-W.Grummt, Applications of Time-Resolved Optical Spectroscopy, Elsevier Science Publishers, Amsterdam, 1990. Luminescence Techniques in Chemical and Biochemical Analyses, ed. 

An atomic resolution model of the plant lightharvesting complex LHC-II has been published (Kiihlbrandt, 1994 Ktihlbrandt et al., 1994 Hunter et al., 1994b). Two xanthophyll molecules are located at the eenter of the complex and were identified as lutein based on the fact that this pigment is the most abundant. They apparently have a structural role in addition to their triplet quenching ability (Plumley and Schmidt, 1987 Paulsen et al., 1990 Heinze et al., 1997). The main xanthophylls are lutein, neoxanthin, and violaxanthin (Siefermann-Harms, 1985, 1990a). Time-resolved optical spectroscopy showed that at least two spectroscopically distinct xanthophylls participate in triplet quenching, apparently lutein and violaxanthin (Peterman et al., 1995, 1997). 

The growth of thallium clusters has been also observed by time-resolved optical spectroscopy. The coalescence steps are comparable with those of silver and the final plasmon band of Tl is located at 300 nm. ... 

However, time-resolved optical spectroscopy is perhaps the premier method for learning about the dynamics of a complex system, especially on nanosecond or picosecond time scales. Some DNA dynamics data from NMR spectroscopy are presented in Table 4.3. Time-resolved emission decays, time-resolved fluorescence anisotropy, and time-resolved Stokes shifts measurements of probe molecules in DNA have been described (and see below) and fast components in the time decays assigned to various DNA motions. The dynamics as a function of sequence are incompletely mapped and provide an exciting area for future investigations. 

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