Spontaneous recombination times

The spontaneous recombination times observed by Ozgiir et al. [152] for RF-sputtered ZnO thin films are comparable to other values reported in the literature. Guo et al. [119] reported 30 ps room-temperature excitonic recombination times for ZnO thin films grown on Si by OMVPE. Koida et al. [176] measured recombination... 

The constant Tj is known as the spontaneous carrier lifetime and may be interpreted as the average time for which a carrier lives in the conduction band before spontaneously recombining via any one non-radiative, radiative or Auger recombination processes. The spontaneous carrier lifetime is typically on the order of 1 ns. 

We use a7r/2 — vr — vr/2 pulse sequence to coherently divide, deflect and finally recombine an atomic wavepacket. The first vr/2 pulse excites an atom initially in the l,p) state into a coherent superposition of states l,p) and 2,p + hkeff). If state 2) is stable against spontaneous decay, the two parts of the wavepacket will drift apart by a distance hkT/m in time T. Each partial wavepacket is redirected by a vr pulse which induces the transitions... 

Although the rates of spontaneous mutation are low, they can be greatly increased by mutagenic chemicals (Chapter 27) or by irradiation. It is perfectly practical to measure the rates of both forward and back mutation. When this was done, it was found that certain chemicals, e.g., acridine dyes, induce mutations that undergo reverse mutation at a very much lower frequency than normal. It was eventually shown that these mutations resulted either from deletions of one or more nucleotides from the chain or from insertions of extra nucleotides. Deletion and insertion mutations often result from errors during genetic recombination and repair at times when the DNA chain is broken. 

While such a device has yet to be constructed, Debreczeny and co-workers have synthesized and studied a linear D-A, -A2 triad suitable for implementation in such a device.11641 In this system, compound 6, a 4-aminonaphthalene monoimide (AN I) electron donor is excited selectively with 400 nm laser pulses. Electron transfer from the excited state of ANI to Ai, naphthalene-1,8 4,5-diimide (NI), occurs across a 2,5-dimethylphenyl bridge with x = 420 ps and a quantum yield of 0.95. The dynamics of charge separation and recombination in these systems have been well characterized.11651 Spontaneous charge shift to A2, pyromellitimide (PI), is thermodynamically uphill and does not occur. The mechanism for switching makes use of the large absorption cross-section of the NI- anion radical at 480 nm, (e = 28,300). A second laser pulse at 480 nm can selectively excite this chromophore and provide the necessary energy to move the electron from NI- to PI. These systems do not rely on electrochemical oxidation-reduction reactions at an electrode. Thus, switching occurs on a subpicosecond time scale. 

Initially the focus will be upon producing cold antihydrogen atoms. The rate for spontaneous radiative recombination of antiprotons and positrons is rather low because the emission of photons is a slow process on the time scale of collisions. Laser-stimulated recombination can increase the antihydrogen formation rate by orders of magnitude [14]. Other avenues towards antihydrogen production at low energies are pulsed-field recombination [15] or collisions of antiprotons with positronium [16]. 

Since an electron that has been promoted to the conduction band will have a greater energy than those left in the valence band, there is a possibility for the electron to lose this excess energy. The spontaneous return of electrons in the conduction band to the valance band is known as recombination, and is usually accompanied by light emission and heat (Figure 4.4). This phenomenon happens all the time for excited-state molecules. For instance, consider what happens when one supplies sodium atoms with sufficient energy to promote an electron from the 3s energy level... 

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