Time dependent absorption

Figure 1. Time-dependent absorption spectra obtained after laser excitation (248 nm, 20 ns) of benzophenone (frans-2,3-diphenylaziridin-l -yljimine (134d) in oxygen-saturated TFE.64 Inset influence of the laser dose on the yields of stilbene (monophotonic) and diphenylmethyl cation (biphotonic).

In the laser photolysis experiments the aromatic compound (4-10" M) and the nucleophile (0 04 M ) in acetonitrile-water (1 1) were irradiated with the frequency doubled pulse (100 mj, 6 ns, 347 nm) of a ruby laser. Only time-dependent absorption changes were measured (double pulsed xenon flash lamp with 10 /is continuous output as light source) absorption spectra were constructed from these measurements at 12 or 25 nm intervals. 

To test the first hypothesis, solutions of 3,5-dinitroanisole and hydroxide ions were flashed and the absorption spectra at different time intervals after excitation were compared. The absorption ( max 400-410 nm) that remains after all time-dependent absorptions have decayed can be shown to be due to 3,5-dinitrophenolate anion, the photosubstitution product of 3,5-dinitroanisole with hydroxide ion. When the absorption band of the 550-570 nm species is subtracted from the spectrum of the solution immediately after the flash, there remains an absorption at 400-410 nm, which can also be ascribed to 3,5-dinitrophenolate anion. The quantity of this photoproduct does not increase during the decay of the 550-570 nm species. Therefore the 550-570 nm species cannot be intermediate in the aromatic photosubstitution reaction of 3,5-dinitroanisole with hydroxide ion to yield 3,5-dinitrophenolate. Repetition of the experiment with a variety of nucleophiles on this and other aromatic compounds yielded invariably the same result nucleophilic aromatic photosubstitution is, in all cases studied, completed within the flash duration (about 20jLts) of our classical flash apparatus. 

Experimental details for the cross-section measurements were presented in the literature. Briefly, after the irradiation by electron beam pulse for a few nanoseconds, the time-dependent absorption for the atomic line transition Rg Rg -i-/zv was measured to observe the time-dependent population of the excited rare gas atoms Rg. The population of excited Rg was determined using an absorption law for the atomic lines, where the broadening of the absorption profile due to the thermal Doppler effect and due to the attractive interatomic potentials was reasonably taken into consideration. The time-dependent optical emission from energy transfer products, such as ... 

A reconstruction of the measured time-dependent absorption spectra of the hydrogen-bonding complexes of HPTA with DMSO in DCM is shown in Fig. 3 as the superposition of the absorption and the gain bands of the photoacid. 

Fig. 1. A. Noise level expressed in milli optical density, obtained after 1 minute of data acquisition. B. Time dependent absorption change of the keto group of the primary donor of the bacterial reaction center, at 1685 cm 1 and 1715 cm 1 upon excitation at 600 nm, noise level 30 pOD, measured in the Lissajous scanner. The solid line through the data points is a fit with = 3.8 ps, t2 = 16 ps, t3 = 4 ns and t5 = oc. The time scale is linear up to 3 ps and logarithmic thereafter.

Time-dependent absorption using a change-point model with orwithout lag time (Higaki etal.,2001)... 

Bersohn, R. and Zewail, A.H. (1988). Time dependent absorption of fragments during dissociation, Ber. Bunsenges. Phys. Chem. 92, 373-378. 

The transient T-T absorption in the gas phase has been measured recently for aromatic molecules such as naphthalene (119,211) and anthracene (80,81) using flash kinetic spectroscopy and tandem laser pulse absorption techniques. Particularly, the later technique (211) provides time-dependent absorption spectra of the "isolated" unrelaxed triplet molecules because of its capability for rapid monochromatic excitation and detection. It will certainly provide a wealth of Important kinetic and spectroscopic information about the evolution and decay of triplet states. Direct observation of the formation of transient hot ground-state (Sq) molecules through an internal conversion process has also been achieved with laser excitation and laser... 

Rg. 12. Time dependent absorption of reaction intermediates A to D observed upon UV-photopolymerization of TS-6 crystals at 270 K. AOD is the change in optical density of the sample (from Ref. )... 

Fig. 14. Time dependent absorption of the reaction-intermediate B (trimer) observed upon UV-polymerization of TS-6 under hydrostatic pressure (a 1 bar, b 0.5 kbar, c 1.0 kbar, d 2,2 kbar). Excitation was by a single flash of excimer laser operated at 308 nm (from Ref...

The photochemically induced reaction between Bt2 and CI2 has been investigated by Christie et al. by photodissociating Br2 at 4300 A. Kinetic data were obtained by measuring the time-dependent absorption of Br2 at a longer wavelength. Temperatures ranged from 20 to 60 C. 

Paul and Dalby have investigated the reactions of CN radicals produced in the flash photolysis of C2N2 and CNCl. The kinetics were inferred from the time-dependent absorption by CN radicals. The absorbed intensity was related to concentration by means of a numerically evaluated theoretical expression. This expression was calibrated using an experimental measurement of the integrated absorption coefficient. 

Silverman s studies on mechanism based MAO inactivation have provided overwhelming support for the role of electron transfer in the MAO catalyzed dealkylation of amines. It must be mentioned however that spectroscopic attempts for detecting the radical ion intermediates have hitherto been unsuccessful. Yasanobu and coworkers could not find EPR spectral evidence for radical intermediates in MAO-catalyzed oxidation of benzylamine [205]. Miller et al. failed to observe the flavin semiquinone or an amine-flavin adduct in rapid-scan-stopped flow spectroscopy [206]. The only time-dependent absorption change observed in this study was the bleaching of the oxidized flavin. Furthermore, no influence of a magnetic field up to 6500 G was observed on the rate of MAO B reduction. The reaction rates of systems with kinetically significant radical pair intermediates are known to be altered... 

Finally, forward ISRS excitation can be followed by measurement of time-dependent absorption or Raman spectra, second harmonic generation efficiency, or any other optical property that may be affected by vibrational distortion. Preliminary time-resolved absorption spectroscopy of nonequilibrium, vibrationally distorted species is discussed further in the next section [46]. 

The second absorption model examined a time-dependent absorption rate constant. In this model, intranasal cocaine was dosed into a single compartment at time 0 and then again 15 s later. Absorption occurred directly from the dosing compartment into the central compartment via the rate constant kai, which was allowed to vary with time. A variety of time-dependent functions for kai could be examined, but a simple one was... 

The time-dependent absorption dynamics of photogenerated electrons in water is a basic study for the understanding of the whole range of charge transfer effects. Missmer and Simon have compared various models for this system using published data for this fs time range process. 

The depletion and recovery of the ground-state e absorption spectrum are monitored through time-dependent absorption or transmission measurements of the photobleached system. 

We have pinpointed the essential processes contributing to the slow component in the time-dependent absorption signal as orientational relaxation of the liquid during electron solvation. But what other evidence is there to describe the fast, unresolved but significant IR absorption that is ascribed to localization To what extent does the local liquid structure and density continue to play a quantitative role at earlier times Is the electron self-trapped, as in a polaron model, during localization It is to these... 

According to Jana et al., ° the catalytic reaction was carried out in a standard quartz cuvette of 1 cm path length, 2.5 mg of sohd catalyst was taken along with an aqueous solution of o-NBA (0.1 mM), and the volume of the solution was made up to 2mL with water. Next, 0.2 mL of 0.1 M aqueous NaBH4 solution was added to the reaction mixture, and time-dependent absorption spectra were recorded using a UV-visible spectrophotometer at 27 2 °C. 

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