Transient grating diffusive

A. Miller, Transient Grating Studies of Carrier Diffusion and Mobility in Semiconductors... 

The three-pulse experiments contain more information than two-pulse methods when the direction and timing of all three pulses is controlled. We have seen that this additional information cannot be interpreted within a Bloch picture. We will therefore outline in the following a more detailed theory, which includes spectral diffusion and which simultaneously explains the linear response (absorption spectrum) and the nonlinear response (four wave mixing, photon echo, transient grating, pump-probe) of vibrational transitions. 

Abstract. A recently developed new method to monitor reaction kinetics of intermolecular interaction is reviewed. This method is based on the measurement of the time-dependent diffusion coefficient using the pulsed-laser-induced transient grating technique. Using this method, conformation change, transient association, and transient dissociation on reactions are successfully detected. The principle and some applications to studies on changes in the intermolecular interactions of photosensor proteins (e.g., photoactive yellow protein, phototropins, AppA) in the time domain are described. In particular, unique features of this time-dependent diffusion coefficient method are discussed. 

Translational Diffusion of Transient Radicals Studied by the Transient Grating Method... 

Diffusion coefficients (D) of various radicals created by the photoinduced hydrogen abstraction reactions from alcohols (ethanol and 2-propanol) as well as those of the parent molecules are measured by using the transient grating (TG) method. Dependence of D on the viscosity, molecular size, and temperature are investigated, and the results are interpreted in terms of microscopic aggregation of the radicals with solvents or solutes. 

Frez C, Diebold GJ, Tran C et al. (2006) Determination of thermal diffusivities, thermal conductivities, and sound speeds of room temperature ionic liquids by the transient grating technique. J Chem Eng Data 51 1250-1255... 

TABLE 5. Themial Diffusivities (a). Sounds Speeds (c) and Damping Parameter (a) Determined by Transient Grating Technique Together with Some Reported Values. 

In addition to acoustic properties, the transient grating measurements also provide information on the thermal diffusivity of the ILs. In this study, various ILs with different anions and cations were used in order to elucidate the effect of the anon and the cation on their thermal diffiisivities and... 

Although, the transient grating measurements yield values for thermal diffusivity for each IL directly, the thermal conductivity must be found from a knowledge of the density and specific heat capacity through the relation a = Only a few values of density and heat capacity have... 

In summary, it has been demonstrated that the transient grating technique can be used to determine thennal physical properties (thermal diffusivity and thermal conductivity) of ILs. The technique has relatively high precision, namely relative errors are about 1%. It also has relatively high accuracy thermal conductivity values of ILs determined by this method agree, within 1% with values previously determined by other workers. More importantly. 

Comparable behavior has been observed by Payer et al. in a series of subpicosecond transient grating optical Kerr effect measurements on the reorientation of byphenyl molecules in neat biphenyl and n-heptane solutions [66,67]. They have shown that on the ultrafast timescale (r < 2 ps) the dynamics of the probe is controlled by librational motions having an inertial character, although diffusive reorientational relaxation of the whole molecule and internal torsional motions can also have a role. 

FlG. 14.1. The decay rate of the transient grating signal versus 92 (9 is the angle between the pump pulses) for anthracene crystals at 10 and 20 K (23). The magnitude of the slope is proportional to the diffusion constant of the excitations in the crystal. With increasing temperature, the diffusion constant decreases. The average diffusion constant obtained from these data is about 10 times larger than the value expected for incoherent exciton motion (25). 

In 1971, Eichler, et al of the I. Physikalisches Institut der Technischen Universitat Berlin, reported the first detection of a transient grating. They used a ruby laser to produce a thermal refractive index grating in dye-containing methanol. In 1972, they reported additional measurements, including the time decay of the grating due to thermal diffusion, on a time scale of 1 ms. 

In 1978, Salcedo, et al at Stanford University reported the measurement of singlet electronic excitation transport in a molecular crystal. The time scale of their measurement was 1 ns. Also in 1978, the first reports were published of the measurement of mass transport by transient grating techniques. Hervet, et al of the College de France reported the translational diffusion coefficient of the photochromic dye methyl red in an aligned liquid crystal, as a function of direction. The time scale was 0.1 s. Smith and McConnell at Stanford University used Fluorescence Redistribution After Pattern Photobleaching to measure the diffusion coefficient of dye-labeled phospholipid in oriented multibilayer films. The time scale was 100 s. 

One of the widely used methods of analysis of kinetic data is based on extraction of the distribution of relaxation times or, equivalently, enthalpic barrier heights. In this section, we show that this may be done easily by using the distribution function introduced by Raicu (1999 see Equation [1.16] above). To this end, we use the data reported by Walther and coworkers (Walther et al. 2005) from pump-probe as well as the transient phase grating measurements on trehalose-embedded MbCO. Their pump-probe data have been used without modification herein, while the phase grating data (also reproduced in Figure 1.12) have been corrected for thermal diffusion of the grating using the relaxation time reported above, r,, and Equation (1.25). 

Since the length scales associated with the thermal lens are on the order of 10 to 1000 times the grating constant, their characteristic time scale interferes with polymer diffusion within the grating. Such thermal lensing has been ignored in many FRS experiments with pulsed laser excitation [27,46] and requires a rather complicated treatment. A detailed discussion of transient heating and finite size effects for the measurement of thermal diffusivities of liquids can be found in Ref. [47]. 

The transport coefficients have been measured by the transient holographic grating technique of Thermal Diffusion Forced Rayleigh Scattering (TDFRS) that has already been described in more detail in previous works [85-87] and will only be briefly sketched in the following (Fig. 1). 

For transient wave mixings, the detailed calculations for the three-dimensional thermal grating buildup and temperature distribution and dissipation are obviously very complex, and are further complicated by the anisotropic thermal diffusion constants of the liquid crystals, as well as the enclosing glass slides. In the simplest case where the thermal grating is reducible to a one-dimensional problem [14] (e.g., the case of very small grating constant A compared to the cell thickness /), the thermal decay time constants for heat dissipation along and per-... 

---