Population grating

Compared to the studies using the population grating (Section II.C), studies on the excited-state dynamics from the heating effect have been rather limited. A probable reason is that the time resolution of the signal is determined not only by the pulse width but also by the acoustic transit time, Tac = A/v, as described in Section II.B.l. 

The population grating appears by the spatial modulation of the refractive index (or absorbance) due to the presence of molecules (or states) that have different optical properties. The refractive index is related to the molecular polarizability (a) by the Lorenz Loren tz relation [27],... 

The polarization selection was used for the differentiation of the direction of the transition dipole moment. This technique was also demonstrated to be very useful to separate spectrally overlapped transient signals from different chromophores in a complex reaction. A careful analysis of the population grating data was carried out to determine rotational anisotropy, two-photon absorption state symmetry, and the relative orientation of the donor and acceptor. 

Therefore, the thermal contribution can be suppressed by the cross-polarization condition (x(t3li2) or it can be maximized by the parallel polarization. The contribution of the population grating can be estimated by changing the polarization of the probe and the signal under the parallel polarization condition as long as r 0. However, if /i, and fij are not correlated, r — 0,... 

In studying photochemical or photophysical processes by the TG method, one may preferably detect the thermal or population grating selectively and at least one of them should be dominant. Several strategies have been used for this purpose. 

Time scale for observation The response of the acoustic effect is limited by the acoustic propagation between the fringe distance and that time constant is rac = A/v. Therefore, in 0 < t rac, the density contribution is small and fast response of the population grating can be easily observed (although the Temp.G signal could contribute to the signal even in this time scale). In principle, even rather slow dynamics of the population, which is not disturbed by the thermal... 

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. 

Strong absorption band near the probe wavelength As shown in Eq. (26), the population grating signal becomes significant when the probe wavelength is close to the absorption band. 

While the wavelength of the excitation light should be resonant to the energy level of the excited state, the wavelength of the probe light is best removed from any absorption bands for the thermal detection. Otherwise, the population grating will contribute to the signal considerably. 

The shorter limit of the S, lifetime was further narrowed by using the Temp.G or Pop.G. For that purpose, NB was dissolved in water and the temperature was lowered to around 4°C. Since at 4°C, the density of water becomes maximum (dp/dT = 0), the contribution of Dens.G can be suppressed (Section IV). In this way, the S, lifetime was found to be shorter than 20 ps. These triplet energy and lifetime values are consistent with the speculated values obtained previously by using the energy-transfer method [83]. Recently, the lifetime of the S, state was directly determined by the population grating method in a variety of solvents [85], The lifetime was found to be 10-3 ps depending on the solvent. 

The observed TG signal in the picosecond-to-nanosecond time scale was analyzed in terms of a combination of the thermal effect (including AH), volume grating, and population grating components. 

If the reaction takes place in an aqueous solution, the temperature variation method can be used. The thermal expansion coefficient of water strongly depends on the temperature, and in particular, it vanishes at 4°C. Therefore, the signal observed at this temperature should come from the volume effect (volume grating). Of course, it also includes the population grating or other effects except for the thermal effect, and these contributions should be subtracted. When organic solvents are the media, a series of solvents or binary mixtures of solvents, which do not affect the other reaction parameters (such as the quantum yield, the volume change, the... 

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