Time-resolved optical absorption measurements

Fig. 1.21 Schematic depiction of a set-up for time-resolved optical absorption measurements.

Fig. 1.22 Schematic depiction of a set-up for time-resolved optical absorption measurements in the femtosecond time domain. SHG second harmonic generation crystal PD photodiode OMA optical multichannel analyzer. Adapted from Lanzani et al. [68] with permission from Wiley-VCH.

Polyamide 2) was irradiated with a single 20 ns flash (530 nm) in N,N dimethylacetamide. Kinetics of cis to trans isomerization of backbone azobenzene residues was followed by time-resolved optical absorption measurement and a subsequent conformational change of the total polymer chain was followed by time resolved light scattering measurement. Before each laser experiment, the polymer was initially converted to a compact conformation by continuous ultraviolet irradiation and then the unfolding process was followed by the laser flash photolysis method. 

In many synthetically useful radical chain reactions, hydrogen donors are used to trap adduct radicals. Absolute rate constants for the reaction of the resulting hydrogen donor radicals with alkenes have been measured by laser flash photolysis techniques and time-resolved optical absorption spectroscopy for detection of reactant and adduct radicals Addition rates to acrylonitrile and 1,3-pentadienes differ by no more than one order of magnitude, the difference being most sizable for the most nucleophilic radical (Table 8). The reaction is much slower, however, if substituents are present at the terminal diene carbon atoms. This is a general phenomenon known from addition reactions to alkenes, with rate reductions of ca lOO observed at ambient temperature for the introduction of methyl groups at the attacked alkene carbon atom . This steric retardation of the addition process either completely inhibits the chain reaction or leads to the formation of rmwanted products. 

Sucheta et al. [28] published the experimental proofs of the theoretical mechanism of cytochrome c oxidase functioning proposed originally be Kharkats and Volkov [15,17 19,24]. Using time-resolved optical absorption difference spectra and singular value decomposition analysis, Sucheta et al. [28] found the presence of peroxy and ferryl intermediates at room temperature during reduction of oxygen by cytochrome c oxidase and measured the rate constants. 

In ultrafast, time-resolved infrared absorption measurements by the pump-probe method, the sample is first excited by an ultrashort pump pulse, and then irradiated by an ultrashort infrared pulse (probe pulse) after a certain delay time from the excitation by the pump pulse. The delay time of the probe pulse from the pump pulse is usually changed by the difference in the optical path lengths of the pump and probe pulses (a delay time of 1 ps arises from a path difference of about 0.3 mm). When the infrared spectrum of a molecule in an excited electronic state is measured, pulses in the ultraviolet to visible region are used for the pump purpose, and pulses in the infrared region are used for the probe purpose. When a vibrationally excited molecule is the target of such a measurement, pulses in the infrared region are used for both the pump and probe purposes. The transient (or time-resolved) infrared absorption spectra by this method are usually measured as the difference in absorption intensities for the probe pulses between the measurements with the pump pulses and those without the pump pulses. 

S. J. Madsen, B. C. Wilson, M. S. Patterson, Y. D. Park, S. L. Jacques, and Y. Hefetz. Experimental tests of a simple diffusion model for the estimation of scattering and absorption coefficients of turbid media from time-resolved diffuse reflectance measurements. Applied Optics, 31 3509-3517, 1992. 

The absorption lines of the low temperature photoreaction products in TS-6 monomer crystals are summarized in the diagram of Fig. 7. The correlation of the A, B, C,. .. photoproduct series to diradical DR intermediates and of the b, c, d,... photoproducts to asymmetric carbene AC intermediates is based on the ESR experiments discussed below. The correlation of the y, 8,6,... series to stable oligomers SO is based on their thermal and optical stability. The correlation of dimer, trimer, tetramer,... molecules follows from the chemical reaction sequences observed in the time resolved optical and ESR measurements as well as from the widths of the one-dimensional potential wells used in the simple electron gas theory , which already has proved successful in its application to dye molecules. Following Exarhos et al. the explicit dependence is given by... 

Henrici et al. [504] carried out a shock tube study of the CO + F2O reaction in mixtures heavily diluted with argon at higher temperatures. They obtained data on overall CO2, O2 and COF2 production from single pulse experiments, and they also made time-resolved optical measurements of the rate of formation of CO2 and depletion of F2O by studying the emission at 4.3 pm and the absorption at 2200 A, respectively. The major path for the decomposition of F2O was assumed to be by reactions (xcii)—(xciv)... 

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. 

The following is an excellent example where a special technique, in this case time-resolved resonance Raman spectroscopy added significant information which simple optical absorption measurements could not provide. The system of interest is aniline and its radicals generated upon oxidation (e.g., by N3 ). The latter are the radical cation and its deprotonated form existing in equilibrium 12 with a pK of 7.04.29... 

Rao reported measurement of third-order optical non-linearity in the nanosecond and picosecond domains for phosphorus tetratolyl porphyrins bearing two hydroxyl groups in apical position [89]. Strong nonlinear absorption was found at both 532 nm and 600 nm. The high value of nonlinearity for nanosecond pulses is attributed to higher exited singlet and triplet states. Time resolved studies indicate an ultra-fast temporal evolution of the nonlinearity in this compound. 

Winter, Underhill, and co-workers have published extensively on the cubic NLO properties of complexes of DT and related ligands,411 22 particularly those containing formally Ni11 centers. For example, time-resolved 1,064 nm DFWM was used to obtain resonantly enhanced values for group 10 complexes such as (157).411 15 The smaller of (157) compared with (156) is largely due to resonance effects since the absorption maximum of (157) is somewhat removed from the laser fundamental. However, figures of merit derived from measurements of 2 and linear and two-photon absorption (TPA) coefficients show that low optical losses render complexes such as (157) superior to (156)413 for potential all-optical switching applications.411 14... 

In a pulse-radiolysis optical-absorption method, the value of kje, where 8 is a molar absorption coefficient, is measured by the time-resolved measurement of the optical absorption of solvated electrons, and then the kr value is determined by the observed value of kr/s and the value of s known separately. 

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