Optical relaxation

Figure 9.4. Arrhenius plot of the optical relaxation time of RbBr Ag+. 60 and 90° transitions are measured for electric directions [100] and [110], respectively. (From Kapphan and Luty [1983].)...

The behaviour of the polarized reflectivity and optical conductivity spectra of new quasi-two-dimensional organic conductor p -(BEDO-TTF)5[CsHg(SCN)4]2 versus temperature for E L and E1. L are quite different. For E . L, the temperature changes of R(ro) and ct(co) are due to the decrease of the optical relaxation constant of the free carriers as expected for a metal. For E L at temperatures below 200 K, the energy gaps in the ct(co) spectra at about 4000 cm 1 and at frequencies below 700 cm 1 appear simultaneously with the two new bands of ag vibrations of the BEDO-TTF molecule activated by EMV coupling. This suggests a dimerization of the BEDO-TTF molecules in the stacks, which leads to a metal-semiconductor transition.. In the direction perpendicular to L, the studied salt shows metallic properties due to a very favourable overlap of the BEDO-TTF molecular orbitals. 

Isotropic polymeric systems as well as particulate systems might also show time-dependent moduli after cessation of flow. As long as the shear does not induce structure growth, the moduli always increase with time after flow. An increase of the moduli upon cessation of flow has also been reported for thermotropic PLCs (18) as well as for lyotropic solutions of hydroxy propyl cellulose in water (19) and in acetic add (20). The possibility of changing in either direction seems to be characteristic for mesomorphic materials. A fundamental theory for describing complex moduli does not exist for such materials. The present results, combined with the information about optical relaxation mentioned above, could be explained on the basis of reorientation of domains or defects. The different domains orient differently, even randomly, at rest whereas flow causes an overall orientation. Depending on the molecular interaction the flow could then cause an increase or decrease in moduli as recently suggested by Larson (21). 

EqCOS Sit and Qj the optical frequency of the molecule excited. T, and Tj are the longitudinal and transverse optical relaxation constants also... 

One of the great advantages of photochemical hole-burning over coherent optical techniques in obtaining information on optical relaxation is that low-temperature line-shift measurements can be done as shown by Voelker et al. in a study of porphin in n-octane. We have therefore tried to develop another technique that would permit low-temperature line-shift measurements in photochemically stable systems. Preliminary temperature-modulation experiments were done whereby the absorption spectra for different temperatures were subtracted. Figure 21 shows a modulation... 

It seems useful to summarize all optical relaxation data that have been obtained so far. In the preceding sections it was shown that for the temperature dependent part of 7 the following phenomenological expression fitted all presently available data T (7 )= 7 (oo)exp(AE/kT ). For... 

I32 and T3. are the optical relaxation rates (radiative and radiationless) of the upper level into the lower ones. and IF,2 are the spin-relaxation rates among the hyperfine levels in the ground state. We note here that our calculations extend the work of Schenzle, Grossman, and Brewer on a three-level system and that our results, for infinite relaxation times, for a two-pulse sequence are in agreement with theirs. 

As is evident from this figure, the calculated values (solid line) coincide very well with the experimental ones (circles) over the entire region of time covered. The values of parameters are also sited on the same figure. The change in the optical relaxation curve will be discussed in terms of the change in the parameters, hereafter. 

Fig. 10. Optical relaxation time t plotted logarithmically a-gainst shearing stress a for L2, L3, and L4.

Sasaki, K., and Maier, J. (2000). Re-analysis of defect equilibria and transport parameters in Y203-stabilized Zr02 using EPR and optical relaxation. Solid State Ionics 134 303-321. Birkby, I., and Stevens, R. (1996). Applications of zirconia ceramics. Key. Eng. Mater. 122-124 327-332. 

Fig. 11 Disturbance of an optical relaxation measurement by shock-waves, demonstrated from the relaxation signal of Tropaeolin 0 in H O (pH 11.5, temperature 298 K), occuring in our measuring cell, after the voluntary creation of a temperature gradient of 25 %.

VoUcer, S. Spectral hole-burning in crystalline and amorphous organic solids. Optical relaxation processes at low temperatures. In Funfschilling, J. (ed.) Relaxation Processes in Molecular Excited States, pp. 113-242. Kluwer Academic Publ, Dordrecht (1989)... 

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