Thermally activated, primary

H ig h-tem peratu re thermally activated primary reserve batteries... 

Tin High-temperature thermally activated primary batteries... 

The formalism for treating primary isotope effects on unimolecular processes follows analogously to the development above, once due account is taken of the difference in zero point energies on isotope substitution at the reaction site (which is reflected in an isotopic difference in the threshold energy Eo). For thermal activation the rate ratio in the high pressure limit is straightforwardly obtained from Equation 14.25. For H/D effects... 

Fig. 16. The configuration coordinate model of Fig. 15c on an expanded scale. Various relevant energies (see text) have been defined. Also illustrated are the three primary types of return to the electronic ground state (1) the direct transition from the upper minimum, (2) the tunneling (horizontal) transition to an excited vibrational state of the electronic ground state, and (3) the thermally activated transition to the intersection of the two parabolas.

In a thermal reaction R—>TS—>P, as shown in Figure 4.4, the transition state TS is reached through thermal activation, so that the general observation is that the rates of thermal reactions increase with temperature. The same is in fact true of many photochemical reactions when they are essentially adiabatic, for the primary photochemical process is then a thermally activated reaction of the excited reactant R. A non-adiabatic reaction such as R - (TS) —> P is in principle temperature independent and can be considered as a type of non-radiative transition from a state R to a state P of lower energy, for example in some reactions of isomerization (see section 4.4.2). 

A schematic energy-level diagram of Cr3+ and Tm3+ in YAG together with the luminescence and absorption spectra of Cr3+ are shown in fig. 18. Three primary Cr3+ - Tm3+ energy transfer pathways can be identified thermally activated energy transfer from the 4T2 state (4T2 ET), thermally activated energy transfer from the 2E anti-Stokes phonon sidebands (2E anti-Stokes ET), and temperature-independent energy transfer from the zero phonon and Stokes phonon sidebands of the 2E state (2E Stokes ET). 

Combined Primary and Secondary Unimolecular Isotope Effects in Thermal Activation Systems. Thermal unimolecular reactions offer a number of interesting combinations of primary and secondary inter-molecular effects and their variation with pressure. 

Experimental data on these primary processes are very limited. Reliable data do not appear to exist for the thermally activated decomposition of propane, or for the pressure dependence of the activated system that arises on association of methyl and ethyl radicals. The most consistent data are for the activation reaction, iso-CjH7 + H — CjHg, which is summarized in Table XV. At 25°C., the calculated value of 2 X 10T sec.-1 shows a discrepancy of about 7 compared to the observed average value of 3 X 10s sec.-1. Reliable data for the primary rate of decomposition of butane are absent. 

Fig. VC-3) [181]. Both the magnitude and the lifetime of transient PC increase by addition of a few percent C o to the pure polymer. Transient photoinduced absorption studies demonstrated rapid (sub-picosecond) photo-induced electron transfer from the polymer to C q thereby minimizing early time recombination and enhancing the quantum yield for mobile carrier generation [175,176]. If neutral excitons were the primary excitations, the observed charge transfer would be thermally activated this is not observed.

Unfortunately, however, conclusions based upon matrix studies may not be directly extrapolated to the photochemical behavior of compounds in room temperature fluid media. While allowing the observation of certain photoproducts, the very low temperatures and rigidity of matrices may concurrently prevent the formation of others. The dissociation of bulky ligands discussed above is an illustrative case. Primary photoprocesses that require much thermal activation are also obviously impeded in matrix media. 

Figure 4. Primary regions of surface composition during the thermal activation of a supported carbonyl complex.

To assess the consistency of this KMC model, a variety of materials-independent, materials-dependent, and geometrical parameters was investigated, and the ionic current calculated from the model was used as the primary metric. The materials-independent parameters included the oxygen partial pressure, system temperature, and the external applied potential. Of these parameters, the oxygen pressure had a weak influence on the current (Figure 4), unless its value falls below a threshold of approximately 0.05 atm. As the temperature increased (from 200 to 800°C), the current showed an exponential increase, owing to the thermally activated ion transport in YSZ. As the applied electric potential of the cell increased, a similar increase was found in the calculated ionic current. The materials-dependent parameters included the dopant level (i.e., Y2O3... 

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