Rate constant wavelength dependence

Again the radiative association kinetics described above allow a direct comparison for some realistic values of k and k. For most chemically activated systems at the threshold for unimolecular dissociation, the observed radiative rate constants are of the order of 10-100 s and hence are much below the values expected for k of about 10 s . Therefore, the first limit is most likely to be valid, with the interesting conclusion that the observed unimolecular dissociation rate constant will depend only on the photon density and the absorption cross section (rate constant) at a given wavelength. 

In systems such as the 2- and 6-hydroxypteridines, sudden addition of an alkaline solution to a neutral buffer, or of a neutral solution to an alkaline buffer, displaces the equilibrium between hydrated and anhydrous species (because the anions are less hydrated than the neutral molecules). By measuring the time-dependent change of optical density at a selected wavelength, a first-order rate constant, obs5 can be obtained. This constant is a composite one, and to see its relationship to other quantities some discussion is necessary. 

R26 are shown. From Fig. 4, we can see that the dynamics of the initial ET depends on the probing wavelengths. This phenomenon is usually called the wavelength-dependent kinetics. Obviously, the one-step model cannot account for this wavelength-dependent kinetics phenomenon because in this case only one rate constant is involved. In order to understand the origin of wavelength-dependent kinetics, the detailed mechanism of the initial ET in RCs should be constructed. 

Most commonly absorption or fluorescence spectroscopy is used for detection of the changes in the concentration of G or HG. The monitoring wavelength is chosen so that the difference between the molar absorptivities, in case of absorption, or emission quantum yields, in the case of fluorescence detection, between G and HG is maximized. The amplitude of the relaxation process depends on the difference in the molar absorptivities or fluorescence quantum yields, but the observed rate constants are the same at all observation wavelengths when the kinetics are first- or pseudo-first order (Fig. 3). 

The photoablation behaviour of a number of polymers has been described with the aid of the moving interface model. The kinetics of ablation is characterized by the rate constant k and a laser beam attenuation by the desorbing products is quantified by the screening coefficient 6. The polymer structure strongly influences the ablation parameters and some general trends are inferred. The deposition rates and yields of the ablation products can also be precisely measured with the quartz crystal microbalance. The yields usually depend on fluence, wavelength, polymer structure and background pressure. 

When the same reaction was carried out in the presence of L Ni2 +, the formation of Craq002+ in reaction 44 was followed by an absorbance increase at 360 nm, the wavelength of maximum absorption for L Ni i +. The rate of formation of L Ni i + and its yield exhibited little dependence on the initial concentration of L,Ni2 +. showing that the formation of oxidizing intermediates is rate determining. This observation places a limit on the rate constant for the scavenging reaction with UNi2-1- at > 108 M-1 s 1. 

This is consistent with the relative ease of dissociation of poly(styryl)lithium in the presence of bases and also with the concentration dependence of enthalpy versus R plot (Fig. 4) with a break observed at an R value of ca. 1.0. These calorimetric results are also in agreement with the stoichiometric dependencies observed by Helary and Fontanille 92> from their kinetic and spectroscopic studies. For example, they reported that the UV wavelength of maximum absorption of poly(styryl)lithium shifted upon additions of TMEDA until an R value of 1.0 and then was constant. They also observed that TMEDA additions increased or decreased the rate of polymerization, depending on [PSLi] the increase or decrease in the rate leveled off at an R value of ca. 1.0. All... 

In conclusion then, it seems evident that better laboratory rate constants are required for some reactions, and particularly photochemical data on quantum yields, channels, and cross-sections, and wavelength dependencies of such species as NO3, HNO4, HOCl, CINO2, while global measurements of trace species are equally vital for the understanding of this complex chemistry. 

Thus, upon irradiation of an aqueous reineckate solution with visible radiation between 316 and 750 nm the release of one or more thiocyanate ions is observed with a quantum yield of O = 0.3. The photogenerated thiocyanate can easily be quantified spectrophotometrically at 450 nm as its Fe (SCN)3 complex. This actinometer is quite sensitive to visible light and covers a broad wavelength range. However, the quantum yield does show a wavelength dependence (27), which also varies with the temperature and the pH of the solution. Moreover, because the same thiocyanate release is also observed as a result of the thermal degradation processes (the rate constants depend on the pH of the solution), appropriate control... 

The pressure dependence of kg/ku at X = 2288 A. can be accounted for by the observation that at this wavelength small amounts of disulfides are formed in the ethane and propane reactions, which were not taken into consideration evaluating the rate constant values. The disulfide can arise (1) from the secondary photolysis of the mercaptan product when the COS pressure is low, and 2) by cracking of hot RSH molecules at low total pressures. From Table III there appears to be a twofold variation in the value of h/kn for the ethane reaction, in going from low COS and total pressure to high COS and total pressure. 

The quantum yield of (Pzp)3-Pzc-P -C6o is wavelength-dependent. Based on light absorbed by the free base porphyrin the yield is unity, due to the very large rate constant for photoinduced electron transfer, k. The quantum yield based on excitation of the zinc porphyrins in the antennas is 0.70. In this process, the lightgathering power of the system is increased tremendously at many wavelengths, as four zinc porphyrin moieties feed excitation energy to the reaction center. 

The quantum yields of fluorescence and phosphorescence, 4>f and d>p, may be determined experimentally by means of a fluorescent standard such as a rhodamine B solution whose independent of the exciting wavelength within a wide range. Lifetimes rp and rp are also experimentally accessible through time-resolved fluorescence measurements (phase method or single-photon counting) or by measuring the time dependence of phosphorescence. (Cf. Rabek, 1982.) In Table 5.2 the observable quantities and their relationship to rate constants are collected. 

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