Thermal grating

This temperature rise can be detected directly (laser calorimetry and optical calorimetry), or indirectly by measuring the change in either the refractive index (thermal lensing, beam deflection or refraction and thermal grating) or the volume (photo- or optoacoustic methods). This review will focus primarily on photoacoustic methods because they have been the most widely used to obtain thermodynamic and kinetic information about reactive intermediates. Other calorimetric methods are discussed in more detail in a recent review.7... 

Several techniques are used to follow the photoreactions their intermediates and the reaction products. Fluorescence, phosphorescence, U V or visible spectra, or chromatography can be used to follow the lifetime of the excited state. Recently, the thermal grating method was used to study various processes involved in photoreactions [2], Stabilizers of a certain type of intermediate (singlet or triplet) or the opposite (quencher) were used to determine which kind of intermediate is active. 

After the thermal grating signal, the signal rose again and finally it decayed to the baseline. This rise-decay component depended on q2 (Fig. 8.6a) and this q2 dependence is a clear indication that these components represent the diffusion processes. On the basis of considerations similar to the previous PYP case, it was concluded that this rise-decay feature of the diffusion signal... 

Acoustic (Thermal) Grating-Volume Grating. Since the acoustic signal can originate from the thermal effect as well as the volume change, the separation of these origins is not trivial. Several methods for the separation will be discussed in later sections (Section III.B-G). 

Matrix selection The efficiency of the thermal grating is proportional to (dn/dT)2. By using a matrix with a small dn/dT such as water or a solid matrix, one can easily obtain the population grating signal. 

This technique was applied to study the photophysical processes involved in the excited triplet states [87-89], After the photoexcitation of a photo-chemically stable organic molecule in solution under air saturated condition, generally a strong thermal grating signal, which decays to the baseline with the thermal diffusion time, was observed. Initially, the signal rise can be analyzed by... 

The relaxation processes of Br2 from the photoexcited state in carbon tetrachloride (CC14) was investigated by the diffusive component of the thermal grating [95]. A long-lived component (18 ns) and a fast-rising component were observed in the TG signal. The slower dynamics was assigned to the decay from the lowest excited state A ( n2u). From the amplitude of these components, the quantum yield of the A state formation was estimated to be 0.50 + 0.08. 

The thermal grating method was applied to a free ion yield measurement after a photoinduced electron-transfer reaction [98], The principle was similar to the measurement of (j)isc by the diffusive component of the Dens.G. The free ion yields in the photoinduced electron transfer from various donors to 9,10-dicyanoanthracene were determined from the ratio of the fast and slow rising intensities. The later one came from the released energy by the recombination reaction of the free ions and the former one came from the other processes (energy diagram Fig. 11). The quantum yield decreased from 0.5 to about 0 by changing the donor from biphenyl to N,N-dimethylaniline [different ox(Z))]. The determined [Pg.290]

The quantum yield of the photodissociation was estimated by using the excitation dependence of the diffraction efficiency from the anharmonic thermal grating (section III.F.l) created by a saturated absorption [116,117], The rate constant of the nongeminate recombination of photo-dissociated iodine in solution was extracted from the temporal profile of the TG signal. 

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