Tunneling sequential

Two types of in situ steam extraction systems, mobile and stationary, are available. The mobile system may have rotating cutter blades that release steam as they tunnel through the soil. This system treats small areas sequentially. The stationary system injects the steam into drilled wells, without disturbing the soil. 

In 2009 Schoonus et al. proposed a model to support experimental evidence for sequential tunneling in Alq3 junctions [59]. A schematic depiction of an Alq3 molecule in a junction between two ferromagnetic electrodes is shown in Fig. 15. The model assumes that, as the total Alqs layer thickness (d) increases, the total tunneling current (I) is the sum of a direct current (/direct) and a two-step current... 

The double proton transfer of [2,2 -Bipyridyl]-3,3 -diol is investigated by UV-visible pump-probe spectroscopy with 30 fs time resolution. We find characteristic wavepacket motions for both the concerted double proton transfer and the sequential proton transfer that occur in parallel. The coherent excitation of an optically inactive, antisymmetric bending vibration is observed demonstrating that the reactive process itself and not only the optical excitation drives the vibrational motions. We show by the absence of a deuterium isotope effect that the ESIPT dynamics is entirely determined by the skeletal modes and that it should not be described by tunneling of the proton. 

When the coupling to the leads is weak, electron-electron interaction results in Coulomb blockade, the sequential tunneling is described by the master equation method [169-176] and small cotunneling current in the blockaded regime can be calculated by the next-order perturbation theory [177-179], This theory was used successfully to describe electron tunneling via discrete... 

From coherent transport to sequential tunneling (basics)... 

D. A. Ryndyk, R. Gutierrez, B. Song, and G. Cuniberti (Hi) Sequential tunneling and the master equation... 

Let us come back to our favorite problem - transport through a quantum system. There is one case (called sequential tunneling), when the simple formulas discussed above can be applied even in the case of resonant tunneling... 

Assume that a noninteracting nanosystem is coupled weakly to a thermal bath (in addition to the leads). The effect of the thermal bath is to break phase coherence of the electron inside the system during some time Tph, called decoherence or phase-breaking time. rph is an important time-scale in the theory, it should be compared with the so-called tunneling time - the characteristic time for the electron to go from the nanosystem to the lead, which can be estimated as an inverse level-width function / 1. So that the criteria of sequential tunneling is... 

This system of equations solves the transport problem in the sequential tunneling regime. 

With these new assumptions, the theory of sequential tunneling is quite the same, as was considered in the previous section. The master equation is [172,180-182]... 

When the system is weakly coupled to the leads, the polaron representation (154), (162) is a convenient starting point. Here we consider how the sequential tunneling is modified by vibrons. 

Photophysical studies allow the measurement of rate constants for transmembrane electron transfer. The distributed kinetics observed in describing the decay of photovoltage across a BLM is consistent with a highly inhomogeneous disposition of donors and relays within the membrane [111, 112]. A typical value for the rate constant for electron transfer across a BLM (about 10 sec-1) would predict a spatial separation of about 10 A, whereas the thickness of the bilayer is usually about 40-50 A. This apparent discrepancy can be resolved if transmembrane electron transfer occurs by several sequential steps involving deeply buried redox sites, thus decreasing the operational tunnelling distance [113]. 

Substrate channeling is a process by which two or more sequential enzymes in a pathway interact to transfer a metabolite (or intermediate) from one enzyme to another without allowing free diffusion of the metabolite into bulk solvent. (Ovadi, 1991 Srere, 1987 Anderson, 1999). The substrate tunneling is one of fundamental process of regulating enzymatic processes in cells. Glycolysis, biosynthesis of nucleic acids, aminoacids, and fatty acids are found to be among these processes. 

The ions interpolated in tunnel structures, listed in Table I, are situated in continuous cavities enclosed by the fixed framework of the host (39). Although there is no diffraction evidence that the guest ions are ordered, dielectric absorption in the phase Baa,Ti8 a.Mga.016 favors a model with sequential ordering of barium ions and vacant positions in any one tunnel, but with no relationship to the sequence in any other tunnel (22). This seems to be true in some, at least, of the other nonstoichiometric phases falling into this group, and the application of newer techniques can be expected to amplify the rather limited knowledge of these substances. 

In summary, the experimental data confirm the models of carrier thermalization. Thermalization in extended states is very rapid and is completed in less than 10 s. Thermalization by tunneling between localized states becomes increasingly slow as the carriers move into the band tail and at high temperatures is overtaken by the multiple trapping mechanism of sequential thermal excitation and trapping. 

Protium/deuterium/tritium kinetic isotope effects are often used to support hydride transfer mechanisms over single electron transfer mechanisms. However, sequential electron/proton/electron transfer mechanisms can easily show isotope effects as well. Even though the rate limiting step in the overall two electron reduction of flavin or NADH may be the isotope independent endergonic electron tunneling to form a radical intermediate state, once formed, this radical state can return the electron to recreate the... 

The tetramer is also cyclic ", as illustrated in Fig. 5.24, and with a greater degree of symmetry than the trimer, belonging to the point group. The ring is sequential in the sense that each molecule acts as simultaneous donor and acceptor. This type of structure has been confirmed by analysis of the far-IR vibration-rotation tunneling spectrum . The correlated R(O- O) distances are all equal to 2.743 A, about 0.06 A shorter than in the trimer. (The ex-... 

---