Incoherent transport process

The experiments just discussed made it clear that the motion of the hole on the series of As represents a different mechanism of transport than tunneling. Giese [13] and Bixon and Jortner [18] suggested that this mechanism is incoherent hopping of the hole between neighboring bases. This means that the hole wavefunction is Hmited to one base. The wavefunctions of the remaining electrons on that base would of course be distorted by the presence of the hole. Thus in this view of the transport process the base on which the hole sits could be called a molecular polaron, or a small polaron because it is limited to one site. 

In addition to measurements of how much and what type of chemical species are present, modification of the MR experiment allows us to identify the physical state of a given species (e.g., gas, liquid, gel, and solid) and to quantify temperature and any incoherent and/or coherent transport processes within the system. By integrating any of these measurements into an imaging experiment, we can spatially map these quantities or exploit the effect of these characteristics on the magnitude or frequency of the MR signal to preferentially observe sub-populations of spins within the system. In this latter application we are exploiting the so-called contrast mechanisms in the image acquisition. These concepts are illustrated in Sections II.B II.D. 

Up and spin-down states, is normally negligible compared to molecular selfdiffusion in liquids. However, with viscous systems with little motional averaging of dipolar coupling and with the aid of very strong field gradients, NMR diffusometry is able to detect this immaterial transport mechanism competing with molecular diffusion. Spin echoes can be attenuated on this basis just as with any other incoherent displacement process [181]. 

However it turned out that the structural, chemical and dynamical details are essential for complex descriptions of long-range proton transport. These parameters appear to be distinctly different for different families of compounds, preventing proton conduction processes from being described by a single model or concept as is the case for electron transfer reactions in solutions (described within Marcus theory [23]) or hydrogen diffusion in metals (incoherent phonon assisted tunneling [24]). 

The transport mechanism—coherent versus incoherent—can have a strong influence on the overall probability that a photon absorbed somewhere in the antenna will feed the reaction center with energy. Suppose the transfer is stochastic and occurs with a probability p per individual step. If there are n transfer steps, the overall probability that energy is fed into the reaction center is p" and can be rather low although p may be close to 1. On the other hand, if transport is coherent, the process of feeding the reaction center can be viewed as kind of an internal conversion process in a supermolecule comprising the various chromophores. As... 

The charge transport in organic crystals is conventionally described either as a coherent or an incoherent process with respect to the interaction strength of the charge carrier with lattice vibration. In the weak electron phonon interaction... 

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