Energy-dissipating processes

The results obtained in this work indicate that both the structure of the stabilizer and the nature of the surrounding environment are important factors in determining the efficiency of the energy dissipation processes in these derivatives. Molecular structures in which the planar form is favoured and where the intramolecular hydrogen bond is protected from interactions with the medium by the incorpoation of bulky substituent groups, should exhibit highest photostability and impart improved photoprotection to polymer substrates. 

The lability of the thiol bonds, and the energy dissipation processes resulting from their motion, will have very short time constants, and would likely be stochastic in nature. Even at steady-state, bond fluctuations may have an effect. For this reason, we now turn our attention to stochastic coupling. 

In the following treatment, only the S2 - Sq fluorescence and S2 V/- S2 internal conversion were included as energy dissipating processes from the S2 state. Namely, the other processes such as intersystem crossing and chemical reactions have not been taken into account. If the concentration of S,... 

An estimate of the electron-ion recombination rate constant in high-mobility systems based on an empirical model of energy dissipation processes was provided by Warman [38]. He related the rate constant to the field dependence of the electron mobility, and proposed... 

Figure 1 illustrates different modes of electron transfer between electrolyte states and carriers in the bands at the semiconductor surface. If the overlap between the electrolyte levels and the semiconductor bands is insufficient to allow direct, isoenergetic electron transfer, then an inelastic, energy-dissipating process mustnbe used to explain experimentally observed electron transfer. Duke has argued that a complete theory for electron transfer includes terms that allow direct, inelastic processes. The probability of such processes, however, has not been treated quantitatively. 

Little information has been published on the question of how filler network structure actually affects the energy dissipation process during dynamic strain cycles. The NJ-model focuses on modeling of carbon black network structure and examination of the energy dissipation process in junction points between filler aggregates. This model was further developed to describe the strain amplification phenomenon to provide a filler network interpretation for modulus increase with increasing filler content. 

Condensation of water vapor leads to heat gain by a leaf. In particular, water condensation is the reverse of the energy-dissipating process of water evaporation, so the heat gain per unit amount of water condensed is the heat of vaporization of water at the temperature of the leaf, //vap. Because the condensation is on the leaf surface, the diffusion is across the air boundary layers of thickness Sbl that are present on each side of a leaf. To describe the rate of heat gain per unit area accompanying the water vapor condensation... 

As has been discussed above, molecular clusters produced in a supersonic expansion are preferred model systems to study solvation-mediated photoreactions from a molecular point of view. Under such conditions, intramolecular electron transfer reactions in D-A molecules, traditionally observed in solutions, are amenable to a detailed spectroscopic study. One should note, however, the difference between the possible energy dissipation processes in jet-cooled clusters and in solution. Since molecular clusters are produced in the gas phase under collision-free conditions, they are free of perturbations from many-body interactions or macro-molecular structures inherent for molecules in the condensed phase. In addition, they are frozen out in their minimum energy conformations which may differ from those relevant at room temperature. Another important aspect of the condensed phase is its role as a heat bath. Thus, excess energy in a molecule may be dissipated to the bulk on a picosecond time-scale. On the other hand, in a cluster excess energy may only be dissipated to a restricted number of oscillators and the cluster may fragment by losing solvent molecules. 

The high value of J.Tm parameter is the consequence of a high energy, dissipative process in such a material. 

Any subsequent thermal chemical reactions or physical energy dissipation processes are of a secondary nature. Specific examples for both primary and secondary photochemical reactions will be given in later chapters. 

According to more recent theories, the toughness of high impact polystyrene is caused by flow and energy dissipation processes in the continuous polystyrene phase. The rubber particles act as initiating elements. Considerable differences in the thermal expansion coefficients and in the moduli of the polystyrene phase on the one hand and of the rubber particles on the other lead to an inhomogeneous stress distribution in impact polystyrene. Stress maxima create zones of lower density, called crazes (3), in which the polystyrene molecules are extended parallel to the direction of stress. Macroscopi-cally craze formation appears as whitening the flow processes result in irreversible deformation (cold flow). 

The following are specific aspects of phosphors we need to discuss before we can delve further into energy dissipation processes. 

Time Scale for Energy Dissipation Processes in Phosphors... 

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