Spontaneous emission lifetime

Mc/s and the spontaneous emission lifetime is 10 sec. Obviously this lifetime is too long and the transitions will be saturated exceedingly easily. In other words, the populations of the two levels become essentially equal and no net transition can be observed. Fortunately there are a number of nonradiative relaxation mechanisms open to the upper spin level including interactions with other electrons, with nuclei having nuclear magnetic moments, and with the lattice. The latter process is often known as spin-lattice relaxation. The term "lattice" generally refers to the degrees of freedom of the system other than those directly related with spin. Spin... 

Forbidden pure rotational transitions of H3, following the selection rules Ak = +3, occur in the wide region from millimetre wave to mid-infrared.These transitions are caused by centrifugal distortions of the symmetric structure. No laboratory observation of them has been reported so far. These transitions are much weaker than the usual dipole-allowed rotational transitions in polar molecules, and their spontaneous emission rates range from ca. 10" s" to ca. 10" s". Nevertheless, such weak transitions may be observable in low-density regions just like the Hj quadrupole transitions. Also, the spontaneous emission lifetimes are short compared with the collisional time in low-density areas, making the forbidden rotational transitions important processes for cooling the rotational temperature of Hj. ... 

Suzuki M., Yokoyama H., Brorson S. D., Ippen E. P. Observation of spontaneous emission lifetime change of dye-containing Langmuir-Blodgett films in optical microcavities Appl. Phys. Lett. 58, 998 (1991). 

Intraband relaxation in most systems occurs on timescales that are short relative to the spontaneous emission lifetimes of the intraband transition, and this has hindered potential applications of intraband transitions for infrared detection and emission. Quantum dots with large number of surface traps have however been shown to rapidly extract electrons from the P levels of the nanocrystal. Mid infrared photons incident on such a material induce a PL quench at room temperatures, allowing for the visualization of mid-infrared light. 

First, the change in spontaneous emission lifetime with refractive index of the surrounding medium has been modeled by several theories as well as being investigated by experimental work, which ideally requires the measurement of emission lifetimes over a wide range of refractive index without other changes in... 

Vibrationally hot molecules, on the other hand, have been created using laser ablation and molecular beam loading and have been observed in our group. We were able to translationally cool CaH(v = 1) to <500 mK and saw no evidence of vibrational quenching, giving a limit of acaH,u=i < 10 cm on the helium induced vibrational quenching cross-section [4]. We also magnetically trapped NH (v = 1) and found that the lifetime of the trapped vibrationally excited molecules was limited by spontaneous emission, not collisional thermalization. The fact that the vibrational temperature remained extremely hot allowed us to perform a precise measurement of the NH (X E , w = 1 0) spontaneous emission lifetime. We measured a limit... 

R. M. Amos, and W. L. Barnes, Modification of spontaneous emission lifetimes in the presence of... 

Note that spontaneous emission is not coupled to the optical field and therefore does not depend on the photon density in the system having the same energy. The above rate equation simply indicates that the photon population in level 2 would decrease with a rate of J 2i = l ii + B2iP(v)]N2 by which we can argue that the spontaneous emission lifetime is r p = (A2i). Typically, this lifetime is on the order of 10 s. Similarly, the rate equation for the stimulated emission that involves transitions from level 2 to level 1 can be written as... 

The line width A/ is just the range of radiation frequencies that have a high probability of interacting with the molecule. The time At can be vmderstood as the spontaneous emission lifetime of the transition discussed in Section 3.5. The spontaneous decay has an exponential dependence, and therefore the line shape due to natural line broadening is Lorentzian. The natural line width. 

The interpretation of emission spectra is somewhat different but similar to that of absorption spectra. The intensity observed m a typical emission spectrum is a complicated fiinction of the excitation conditions which detennine the number of excited states produced, quenching processes which compete with emission, and the efficiency of the detection system. The quantities of theoretical interest which replace the integrated intensity of absorption spectroscopy are the rate constant for spontaneous emission and the related excited-state lifetime. 

Apart from the natural lifetime due to spontaneous emission, both uni- and bimolecular processes can contribute to the observed value of T. One important contribution comes from coiiisionai broadening, which can be distmguished by its pressure dependence (or dependence upon concentration [M] of tlie collision partner) ... 

Here, t is the time taken for to fall to 1 /e of its initial value (where e is the base of natural logarithms) and is referred to as the lifetime of state n. If spontaneous emission is the only process by which M decays, comparison with Equation (2.9) shows that... 

For a typical sodium atom, the initial velocity in the atomic beam is about 1000 m s1 and the velocity change per photon absorbed is 3 crn-s. This means that the sodium atom must absorb and spontaneously emit over 3 x 104 photons to be stopped. It can be shown that the maximum rate of velocity change for an atom of mass m with a photon of frequency u is equal to hu/lmcr where h and c are Planck s constant and the speed of light, and r is the lifetime for spontaneous emission from the excited state. For sodium, this corresponds to a deceleration of about 106 m s"2. This should be sufficient to stop the motion of 1000 m-s 1 sodium atoms in a time of approximately 1 ms over a distance of 0.5 m, a condition that can be realized in the laboratory. 

It is interesting to note that for a resonant transition (i.e. coinciding absorption and emission frequencies), the reciprocal of the radiative lifetime is equal to the Einstein coefficient Ai for spontaneous emission (see Box 3.2). 

Films of this material can be optically pumped to induce amplified spontaneous emission at 535 nm, as shown in Figure 14. The lasing threshold (Eth) h the pump energy at which amplified spontaneous emission is observable, and depends strongly, among other factors, upon the lifetime of the polymer excited state. A longer excited state lifetime allows more emissive excitons to build up in... 

Whenever the absorbing species undergoes one or more processes that depletes its numbers, we say that it has a finite lifetime. For example, a species that undergoes unimolecular dissociation has a finite lifetime, as does an excited state of a molecule that decays by spontaneous emission of a photon. Any process that depletes the absorbing species contributes another source of time dependence for the dipole time correlation functions C(t) discussed above. This time dependence is usually modeled by appending, in a multiplicative manner, a factor exp(-ltl/x). This, in turn modifies the line shape function I(co) in a manner much like that discussed when treating the rotational diffusion case ... 

B. Spontaneous Emission and the Mean Radiative Lifetimes of the Halogen Atoms in the np5 2Pyi States... 

From the data of Hoogschagen and Gorter (104), the oscillator strength of the 5D4-+7F6 transition was obtained. By means of the Ladenburg formula, the spontaneous coefficient A46 was calculated. Using the relative-emission intensities, the rest of the A4J spontaneous-emission coefficients could be calculated. From these and a measured lifetime of 5.5 x 10 4 sec at 15°C, he calculated a quantum efficiency of 0.8 per cent. Kondrat eva concluded that the probability of radiationless transition for the trivalent terbium ion in aqueous solution is approximately two orders of magnitude greater than for the radiation transition. 

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