Tunneling radiative

AM of the coupling arises, for example, for radiative-decay modulation due to atomic motion through a high-Q cavity or a photonic crystal [68,69], or for atomic tunneling in optical lattices with time-varying lattice acceleration [59,70], Let the coupling be turned on and off periodically, for the time and tq -, respectively,... 

The results of experimental research have also stimulated the appearance of theoretical papers devoted to the analysis of an elementary act of electron tunneling reactions in terms of the theory of non-radiative electron transitions in condensed media. It has been shown that this theory allows one to explain virtually all the known experimental data on electron tunneling reactions. 

In the previous sections we considered the processes of non-radiative electron tunneling. Along with them, processes are possible when, in the course of one elementary act, the electron tunnels from a donor to an acceptor and simultaneously radiates a quantum of light [7]. Using the... 

Fig. 6.17 Tunnelling and saddle point ionization in Li. (a) Experimental map of the energy levels of Li m = 1 states in a static field. The horizontal peaks arise from ions collected after laser excitation. Energy is measured relative to the one-electron ionization limit. Disappearance of a level with increasing field indicates that the ionization rates exceed 3 x 105 s 1. The dotted line is the classical ionization limit given by Eqs. (6.35) and (6.36). One state has been emphasized by shading, (b) Energy levels for H (n = 18-20, m = 1) according to fourth order perturbation theory. Levels from nearby terms are omitted for clarity. Symbols used to denote the ionization rate are defined in the key. The tick mark indicates the field where the ionization rate equals the spontaneous radiative rate, (c) Experimental map as in (a) except that the collection method is sensitive only to states whose ionization rate exceeds 3 x 105 s-1. At high fields, the levels broaden into the continuum in agreement with tunnelling theory for H (from ref. 32).

We now consider hydrogen transfer reactions between the excited impurity molecules and the neighboring host molecules in crystals. Prass et al. [1988, 1989] and Steidl et al. [1988] studied the abstraction of an hydrogen atom from fluorene by an impurity acridine molecule in its lowest triplet state. The fluorene molecule is oriented in a favorable position for the transfer (Figure 6.18). The radical pair thus formed is deactivated by the reverse transition. H atom abstraction by acridine molecules competes with the radiative deactivation (phosphorescence) of the 3T state, and the temperature dependence of transfer rate constant is inferred from the kinetic measurements in the range 33-143 K. Below 72 K, k(T) is described by Eq. (2.30) with n = 1, while at T>70K the Arrhenius law holds with the apparent activation energy of 0.33 kcal/mol (120 cm-1). The value of a corresponds to the thermal excitation of the symmetric vibration that is observed in the Raman spectrum of the host crystal. The shift in its frequency after deuteration shows that this is a libration i.e., the tunneling is enhanced by hindered molecular rotation in crystal. 

PESTM experiments are based on the fact that the tunneling process can produce electronically excited surface species that subsequently relax by radiative decay. This process is analogous to common bulk electroluminescence experiments and can be used to distinguish between chemically different surface species and/or species present in different environments. Gimzewski and coworkers and Alvarado and coworkers, both of the IBM Research Division, were the first to demonstrate PESTM experiments.150 151... 

A transition between states for which R > has low probability and consequently a long radiative lifetime. This recombination mechanism is called radiative tunneling and occurs when there are localized states at both band edges, as in an amorphous semiconductor or a compensated crystal. It is the dominant radiative recombination mechanism in a-Si H. 

Non-radiative transitions invariably involve the conversion of excitation energy into phonons. Thermalization involves many inelastic transitions between states in the band or band tails. Three mechanisms of thermalization apply to a-Si H. Carriers in extended states lose energy by the emission of single phonons as they scatter from one state to another. Transitions between localized states occur either by direct tunneling or by the multiple trapping mechanism in which the carrier is excited to the mobility edge and recaptured by a different tail state. 

Thermalization in the band tail at low temperature occurs by tunneling between localized states. The low temperature only permits transitions to states of lower energy. The transition probability to a neighboring site contains the same overlap factor as for the radiative transitions. 

Radiative tunneling is the only recombination mechanism which can explain the decay data satisfactorily. The electron and hole are localized at different sites separated by a distance R, and the recombination time is, from Eq. (8.5)... 

At time t, all pairs for which R< R. have recombined, while the more distant pairs remain. From the shape of the distribution in Fig. 8.16, about 20 % of the pairs remain after 1 s, reducing to about 5 % after 1000 s, and this is consistent with the LESR data. The measurable range of the recombination over which the radiative tunneling mechanism applies is therefore more than 10 orders of magnitude, from 10" to 10 s. 

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