Species grating

The temporal evolution of the species grating component is determined by the chemical reaction and protein diffusion processes. When there is no chemical reaction in the detection time window, and the molecular diffusion coefficient (D) is time-independent, the temporal profile of the species grating signal can be calculated by the molecular diffusion equation. The Fourier component at a wavenumber of q of the concentration profile decays with a rate constant Dq2 for both the reactant and the product. Hence, the time development of the TG signal can be expressed by [15-19,23]... 

As mentioned in Section III.B, it is essential to consider the volume effect for a quantitative analysis of the thermal effect. This volume effect should be particularly important for the dissociation (or combination) reaction because one molecule breaks into two or more species (or vise versa for combination), and this process creates (or decreases) an additional void volume around each species. In order to measure the volume effect experimentally, either the temperature or solvent dependence method was used, but it contains some difficulties (Section III.B). Another method was proposed to overcome the limitations of the traditional method [119,120]. This method was based on a time-resolved measurement in a longer time scale that is, separation of the thermal effect and the other contributions due to the chemical species (species grating) is achieved based on the difference of the diffusion constants of the thermal energy and chemical species. Next, the volume effect is considered in more detail. 

The time evolution of the species grating intensity depends on the diffusion process of the species and concentration change due to chemical reactions. If a reaction... 

Figure 14. Diffusive TG signal after photodissociation of DPCP (solid line) and the fitted lines by two exponential decays (dotted line). The strong signal represents the thermal grating signal and the two exponential decays are due to the species grating. The inlet is the plot of the decay rate constant versus q2. The diffusion constant of each species can be determined from the slopes [120].

Since the decay rate constant of the fastest component, kf, was Dthq2, it should be attributed to the thermal grating signal. The intermediate and slow components should originate from the chemical species. Considering the chemical species involved in the reaction, one may expect that the time profile of the species grating is expressed by... 

As described above, classical infrared spectroscopy using grating spectrometers and gas cells provided some valuable infonnation in the early days of cluster spectroscopy, but is of limited scope. However, tire advent of tunable infrared lasers in tire 1980s opened up tire field and made rotationally resolved infrared spectra accessible for a wide range of species. As for microwave spectroscopy, tunable infrared laser spectroscopy has been applied botli in gas cells and in molecular beams. In a gas cell, tire increased sensitivity of laser spectroscopy makes it possible to work at much lower pressures, so tliat strong monomer absorjDtions are less troublesome. 

We would like to thank Dr. Andrea Chow and Mr. Richard Hamlin for their contributions in the characterization of the polymer species and for discussions concerning the implications of these results. We gratefully acknowledge support for this research from the Strategic Defense Sciences Office through Office of Naval Research Contracts N00014-84-C-0392 and N00014-85-C-0668. 

Photorefractivity is a property exhibited by some materials in which the redistribution in space of photogenerated charges will induce a nonuniform electric space-charge field which can, in turn, affect the refractive index of the material. In a new material the active species is a highly efficient cyclopalladated molecule97,98 shown in Figure 5. The palladium-bonded azobenzene molecule is conformationally locked, and gratings derived from cis—trans isomerizations can be safely excluded. 

No discussion has been devoted to the recent use of Fourier transform spectrometers rather than dispersion instruments. The ease with which the spectral data can be manipulated and background subtracted make the FT methods particularly useful for studies of surface species, particularly during catalytic reaction. Recently there has been a surge of interest in the coupling of computer subtraction techniques to conventional grating instruments. For many IR surface studies, where only limited frequency range is required, this... 

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