Thermal relaxations

Lipa J A, Swanson D R, Nissen J A, Chui TCP and Israelsson U E 1996 Heat capacity and thermal relaxation of bulk helium very near the lambda point Phys. Rev. Lett. 76 944-7... 

Excited-State Relaxation. A further photophysical topic of intense interest is pathways for thermal relaxation of excited states in condensed phases. According to the Franck-Condon principle, photoexcitation occurs with no concurrent relaxation of atomic positions in space, either of the photoexcited chromophore or of the solvating medium. Subsequent to excitation, but typically on the picosecond time scale, atomic positions change to a new equihbrium position, sometimes termed the (28)- Relaxation of the solvating medium is often more dramatic than that of the chromophore... 

Grover, R., and Urtiew, P.A. (1974), Thermal Relaxation at Interfaces Following Shock Compression, J. Appl. Phys. 45, 146-152. 

T. Irisawa, M. Uwaha, Y. Saito. Scaling laws in thermal relaxation of fractal aggregates. Europhys Lett 50 139, 1985. 

Using the known thermal conductivity data [46] of the wafers, their internal thermal relaxation time was estimated to be less than 1 ms, i.e. much shorter than C/G. Such estimate was confirmed by the fact that, within the experimental errors, a single discharge time constant r was always observed. 

Using (5.14) and the determined value of aabs, we can estimate 8 if ab is known, and vice versa. Two examples of thermal bistability data, fit to a calculated tuning curve based on (5.12), are shown below. Figure 5.7 is for the bare sphere, and Fig. 5.8 is for the PDDA-coated sphere. In the figures, the laser scans slowly across a TM-polarized WGM dip (taking several thermal relaxation times to scan Av), first down in frequency, then reversing at the vertical dashed line, and scanning back up in frequency across the same mode. The continuous smooth lines are the theoretical fits. 

The traps are a sink for the electronic excitations of the donors. The donors are considered as the only source for electronic excitation of the traps. The traps, T, can relax by emitting a photon with a rate constant kFT or by thermal relaxation k f , which can be expressed as follows ... 

Transient bleaching of low spin species followed by thermal relaxation t s 46-192 ns at 255-205 K... 

Little is known about the fluorescence of the chla spectral forms. It was recently suggested, on the basis of gaussian curve analysis combined with band calculations, that each of the spectral forms of PSII antenna has a separate emission, with Stokes shifts between 2nm and 3nm [133]. These values are much smaller than those for chla in non-polar solvents (6-8 nm). This is due to the narrow band widths of the spectral forms, as the shift is determined by the absorption band width for thermally relaxed excited states [157]. The fluorescence rate constants are expected to be rather similar for the different forms as their gaussian band widths are similar [71], It is thought that the fluorescence yields are also probably rather similar as the emission of the sj tral forms is closely approximated by a Boltzmann distribution at room temperature for both LHCII and total PSII antenna [71, 133]. 

In a subsequent communication, Elliott and coworkers found that uniaxially oriented membranes swollen with ethanol/water mixtures could relax back to an almost isotropic state. In contrast, morphological relaxation was not observed for membranes swollen in water alone. While this relaxation behavior was attributed to the plasticization effect of ethanol on the fluorocarbon matrix of Nafion, no evidence of interaction between ethanol and the fluorocarbon backbone is presented. In light of the previous thermal relaxation studies of Moore and co-workers, an alternative explanation for this solvent induced relaxation may be that ethanol is more effective than water in weakening the electrostatic interactions and mobilizing the side chain elements. Clearly, a more detailed analysis of this phenomenon involving a dynamic mechanical and/ or spectroscopic analysis is needed to gain a detailed molecular level understanding of this relaxation process. 

If the thermal relaxation time Ti is sufficiently short, this time may determine the width of the resonance line. This effect is referred to as lifetime broadening. The Uncertainty Principle expressed in terms of energy and time is as follows... 

The Pound-Knight type of spectrometer has the disadvantage that the dispersion mode is not observed. The dispersion mode is important for the study of solids, since it does not saturate as readily as the resonance absorption in most solids 46), and in some cases (long thermal relaxation time) is the only mode that gives a measurable signal. On the other hand, high temperatures (up to 600°) are more conveniently attained with the Pound-Knight type spectrometer the conventional crossed-coil versions have been somewhat limited (up to 300°) in this respect. 

---