Energy radiative emission

By Equation (5.17), we see that the probability of spontaneous emission is proportional to I/LXIso we can use the same selection rules as previously established for absorption and stimulated emission to predict the allowance of spontaneous emission. Moreover, it can be noted that A is proportional to ci>l and so, for a small energy separation between the two levels of our system, the radiative emission rate A should also be small. In this case, noimadiative processes, described by A r (see Equation (1.17)), can be dominant so that no emitted light is observed. 

There is an alternative mechanism, radiative emission, that can lead to stabilized addition products at low pressures (see Chapters 2 and 3). The excited intermediate can radiate an IR photon to lose sufficient excess energy (though not necessarily all the excess energy) to stabilize itself with respect to dissociation. This process typically has a rate of5-1000 s , so normally is not competitive with dissociation, but there are cases known where certain structural features enhance radiative emission and it can become an important competitor to other chemistry. For methoxide addition to acrylonitrile, nevertheless, it is imlikely to be an operating mechanism. ... 

Not every mineral shows liuninescence. The reason is that the radiative emission process has a competitor, namely, the nonradiative return to the ground state. In that process the energy of the excited state is used to excite the vibrations of the host lattice, i.e. to heat the host lattice. 

The electronically excited mercury atom generated by the recombination of the mercury cation with an electron loses its energy radiatively. The above are only a few of fhe processes fhaf fake place in the lamp, but the combined effect is the emission of light in the UV and visible regions and the generation of heat The heat vaporizes some of fhe mercury mefal. The mercury cations are conducting and the current passing across the electrodes rises until a steady state is reached. 

Figure 26. Apparent cross section for collisional dissociation reaction, N2+(N2 N2,N)N+, as function of energy of electrons producing Nj" (solid curve and data points). Laboratory kinetic energy of primary ions was 10 eV. Cross section for radiative emissions from long-lived, excited states formed in electron impact on N- is also indicated (dashed line).36a...

The two processes illustrated here, direct excitation and charge transfer, are distinguishable because the radiative emission from the translationally thermal electronically excited target atom is not shifted in wavelength, whereas that produced by charge transfer exhibits a Doppler shift because of the high kinetic energy of the projectile. 

Reported data for atomic charge-transfer reactions that produce luminescence are summarized in Table IV.A. Many of the initial studies were concerned with reactions of He+ with rare-gas atoms. These were also the first processes yielding luminescence to be studied over a wide translational-energy range. Very large cross sections for radiative emissions were observed from reactions such as... 

A fluorescence spectrum is characteristic of a given compound. It is observed as a result of radiative emission of the energy absorbed by the molecule. The observed spectrum does not depend on the wavelength of the exciting light, except that the spectrum will be more intense if irradiation occurs at the absorption maximum. The spectral intensity is called the quantum efficiency and is usually abbreviated as . The quantum yield or quantum efficiency, d>, which is solvent dependent, is the ratio Approximate values of quantum efficiencies are as follows naphthalene, 0.1 anthracene, 0.3 indole, 0.5 and fluorescein, 0.9. 

Dexter mechanism Energy transfer by radiative emission... 

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