Emission radiative

We assume that at the grain boundaries aggregates, that are responsible of the radiative emission, are randomly oriented and therefore giving rise to isotropic emission. 

At present there is no small-scale test for predicting whether or how fast a fire will spread on a wall made of flammable or semiflammable (fire-retardant) material. The principal elements of the problem include pyrolysis of solids char-layer buildup buoyant, convective, tmbulent-boundary-layer heat transfer soot formation in the flame radiative emission from the sooty flame and the transient natme of the process (char buildup, fuel burnout, preheating of areas not yet ignited). Efforts are needed to develop computer models for these effects and to develop appropriate small-scale tests. 

The light-emission characteristics of a white-light-emitting EL device with a doubly doped ZnS Pr,Ce,F phosphor layer have been described. It was observed that optimization of the co-doping of Ce enhances the emission characteristics compared to an EL device with a singly doped ZnS Pr,F layer.22 An electrical characterization of Ce-doped ZnS TbOF EL thin films has been reported Ce doping was seen to improve the radiative emission efficiency leading to improved performance of Ce co-doped film.23... 

Let N(j,Ni,N2, and Nj, be the equilibrium population densities of the states 0, 1,2, and 3, respectively (reached under continuous wave excitation intensity Iq), and let N = NQ + Ni+N2 + N3he the total density of optical absorbing centers. The up-converted luminescence intensity ho (corresponding to the transition 2 0) depends on both N2 and on the radiative emission probability of level 2, A2. This magnitude, which is dehned below, is proportional to the cross section a20 (called the emission cross section and equal to the absorption cross section ao2, as shown in Chapter 5). Thus we can write... 

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. ... 

One of the major applicahons of pyrolants is to produce smoke clouds or smoke curtains by chemical reactions.I -s] Smoke is defined as condensed particles that can remain in the atmosphere for at least several seconds. The radiative emission from smoke itself is small because of its low-temperature nature. Thus, no visible emission is seen in the dark by the human eye. The applications of smoke are ... 

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 instrumentation described in the foregoing sections is all based On mass analysis of the charged or neutral products of ion-neutral interactions. A different type of apparatus, which has been extensively utilized to characterize internally excited products from ion-neutral processes, is that in which an optical detector is employed to observe radiative emissions... 

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...

When A and B are identical atomic species (e.g., H + -H259 or He+-He260), radiative emissions may result from neutral target excitation as well as from electron capture by the ionic projectile into the same excited state. Both processes yield Lyman a radiation in the case of H+-H collisions,259... 

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. 

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