Energy exchange mechanism

In addition to energy-exchange mechanisms between the pool and its surroundings, the following parameters are also known to influence hazard zones ... 

A perfect crystal equilibrates very rapidly, following a perturbation from equilibrium, by a collisional energy exchange mechanism requiring of the order of 10 sec. Liquids and defective crystals, on the other hand, equilibrate slowly because of the need for the overall structure to change (to a new distribution of coordination numbers, and a new concentration of defects, respectively). At least at the lower temperatures, the latter processes require the system to explore phase space through an activated diffusion process (see below) for which the time requirements are 10 sec and are also strong functions of temperature. 

In conclusion, then, the dynamics by which a highly excited local vibration relaxes its energy into other modes in the molecule is well understood. The energy exchange mechanism is dominated by the role of nonlinear resonances. 

C3.4.6 EXCHANGE MECHANISM OF ENERGY TRANSFER IN FORBIDDEN TRANSITIONS... 

At low energies the abstraction process dominates and at higher energies the exchange mechanism becomes more important. The cross-sections for the two processes crossing at 10 eV. The END calculations yield absolute cross-sections that show the same trend as the experimentally determined relative cross-sections for the two processes. The theory predicts that a substantial fraction of the abstraction product NHjD, which are excited above the dissociation threshold for an N—H bond actually dissociates to NH2D" + H or NH3 during the almost 50-ps travel from the collision chamber to the detector, and thus affects the measured relative cross-sections of the two processes. 

The sarcolemmal Na/K pump plays an imp>ortant, although indirect role in the regulation of cellular calcium homeostasis. The transmembrane Na gradient is maintained by the activity of the Na/K pump and the thermodynamic energy of this gradient in turn drives the Na/Ca exchange mechanism (Sheu and Fozzard, 1982 Barry and Bridge, 1993). Thus, the intracellular Ca concentration is closely related to intracellular Na and the activity of the Na/K pump (Bers and Ellis, 1982). 

Schreckenbach, G., 1999, The 57Fe NMR Shielding in Ferrocene Revisited. A Density-Functional Study of Orbital Energies, Shielding Mechanisms, and the Influence of the Exchange-Correlation Functional , J. Chem. Phys., 110, 11936. 

Ivey, H. J., and D. J. Morris, 1962, On the Relevance of the Vapor Liquid Exchange Mechanism for Subcooled Boiling Heat Transfer at Higher Pressure, Rep. AEEW-R-137, UK Atomic Energy Authority, Winfrith, England. (2)... 

The triplet-triplet energy transfer, which is doubly forbidden by a resonance mechanism, is allowed by an exchange mechanism ... 

When energy transfer takes place via an exchange mechanism, the system obeys Wigner s spin rule.(32) If and xs are the initial spin quantum numbers... 

In addition, it can be shown for the concentration range of the 4,4 -BPDC used, assuming each molecule occupies a spherical volume, the average radius of this volume is about 108 a. This calculation predicts, on the average, the probability of an excited DMT molecule having a 4,4 -BPDC molecule within the required 15 A for energy transfer to occur by the exchange mechanism, which would be spin allowed, is small. 

The conclusion from the monomer solvent studies is that, in nearly equal molar solutions, DMT and 4,4 -BPDC compete for absorption of the 298 nm radiation. However, the results also show that, even in equal concentrations, the DMT emission, when excited by 298 nm light, is several times as intense as the 4,4 -BPDC emission at 472 nm. It must be emphasized that these studies do not preclude the existence of energy transfer from excited DMT to 4,4 -BPDC. From the volume calculation used above, it can be shown that a concentration of v 0.1 M 4,4 -BPDC is needed to assume an occupied volume with radius of 15 8, the required distance for the exchange mechanism. 

Such a process can naturally be expected to play a certain part in the mechanism of directed energy transport in biological systems, in particular, in the transfer of absorbed energy from the antenna chlorophyll molecules to the reactive center in the photosynthetic system of plants. In Ref. [30], energy exchange between molecules of the photosynthetic pigments chlorophyll a and pheophytin a was studied experimentally with pigments introduced into the polar matrix. 

The rate of energy transfer at a very short donor-acceptor separation R by the exchange mechanism has been given by Dexter (1953) as follows ... 

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