Energy transfer weak coupling

Figure 1. Potential energy as a function of reaction coordinate for a self-exchange reaction. AE, energy barrier for thermal electron transfer (weak coupling) AE2, energy of an intervalence transition which is possible for the system.

In many instances tire adiabatic ET rate expression overestimates tire rate by a considerable amount. In some circumstances simply fonning tire tire activated state geometry in tire encounter complex does not lead to ET. This situation arises when tire donor and acceptor groups are very weakly coupled electronically, and tire reaction is said to be nonadiabatic. As tire geometry of tire system fluctuates, tire species do not move on tire lowest potential energy surface from reactants to products. That is, fluctuations into activated complex geometries can occur millions of times prior to a productive electron transfer event. 

The coupling remains relatively weak even at tire otlier extreme of distance, where tire energy transfer occurs witliin a single piece of matter and tire donor and acceptor are separated by, let us say, only a few nanometres. 

Fig. 9. Incidence energy dependence of the vibrational state population distribution resulting when NO(u = 12) is scattered from LiF(OOl) at a surface temperature of (a) 480 K, and (b) 290 K. Relaxation of large amplitude vibrational motion to phonons is weak compared to what is possible on metals. Increased relaxation at the lowest incidence energies and surface temperatures are indicators of a trapping/desorption mechanism for vibrational energy transfer. Angular and rotational population distributions support this conclusion. Estimations of the residence times suggest that coupling to phonons is significant when residence times are only as long as ps. (See Ref. 58.)...

Fig. 4.12. Energy level scheme of donor and acceptor molecules showing the coupled transitions in the case where vibrational relaxation is faster than energy transfer (very weak coupling) and illustration of the integral overlap between the emission spectrum of the donor and the absorption of the acceptor.

Dexter s formulation of exchange energy transfer (very weak coupling) In contrast to the inverse sixth power dependence on distance for the dipole-dipole mechanism, an exponential dependence is to be expected from the exchange mechanism. The rate constant for transfer can be written as... 

Resonance energy transfer between the aromatic amino acids proceeds by very weak coupling between the donor and acceptor.151,52) Very weak coupling implies that the interaction between the donor and acceptor wave functions is small enough so as not to perturb measurably the individual molecular spectra. This transfer process, which is distinct from the trivial process of absorption of an emitted photon, involves radiationless deexcitation of an excited-state donor molecule with concomitant excitation of a ground-... 

Singlet excitonic energy transfer between chls is most commonly discussed in terms of the two limiting cases of very strong and very weak electronic coupling (J) between donor (D) and acceptor (A) transition dipoles [159-161]. J(cm" ) may be calculated by the expression given by Pearlstein [162]... 

The fluorescence excitation polarization of the monomer is almost 1/7 regardless of the excitation wavelength. A value of 1/7 is typical when both the absorption and the emission oscillators are degenerate and polarized in the same plane. Since the dimer is regarded as a weakly coupled, three-dimensional, double-oscillator, energy transfer between the dimer partners will randomize the excitation between the two porphyrin planes oriented in a tilt angle. In fact, the observed polarization of the dimer is less than 1/7. 

For vibrationally relaxed excited state in which initial and final states are weakly coupled through a dipole-dipole interaction, the rate constant for energy transfer is given by... 

Given the density of Chi in PSI, it is immediately obvious that any detailed description of energy transfer in this system will need to handle both weakly coupled (Forster-type) and strongly coupled (excitonic) systems self consistently. It is also evident that discerning rate limiting steps and optimizing principles may not be straightforward. 

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