Emission induced

In die presence of an electromagnetic field of energy of about our systems can undergo absorjDtive transitions from to E2, extracting a photon from die electric field. In addition, as described by Einstein, die field can induce emission of photons from 2 lo E (given E2 is occupied). Let die energy density of die external field be E(v) dren. 

This equation illustrates the important point that spontaneous emission increases rapidly relative to induced emission as v increases. Since lasers (Section 9.1) operate entirely by induced emission the equation is particularly relevant to laser design. 

The kinetic energy of charge earners in a solid increases with increasing temperature and therefore the probability that a charge carrier passes a given potential barrier also increases. The thermally induced current flow of the charge earners from a metal contact into a polymer film can be derived from the Richardson equation, which describes the temperature-induced emission of hot charge carriers from a metal surface... 

R. E. Honig and J. R. Woolston. Laser-Induced Emission of Electrons, Ions, and Neutral Atoms from Solid Surfaces. Appl. Phys. Lett., 2(1963) 138-139. 

The kets Rik> and vRjn> are two eigenstates, one associated to the Hamiltonians Hc(i) and the other related with Hc(j)- They describe different regimes of electro-nuclear fluctuations. The labels i andj are there to indicate that by spontaneous or/and induced emission processes, there is a subset of corresponding excited states that would relax... 

Ray D, Iyer ESS, Sadhu KK et al (2009) Ag(I) induced emission with azines having donor-acceptor-donor chromophore. Dalton Trans 5683-5687... 

Hong Y, Jacky WYL, Tang BZ (2009) Aggregation-induced emission phenomenon, mechanism and applications. Chem Commun 4332-4353... 

Qian Y, Li S, Zhang G, Wang Q, Wang S, Xu H, Li C, Li Y, Yang G (2007) Aggregation-induced emission enhancement of 2-(2 -hydroxyphenyl)benzothiazole-based excited-state intramolecular proton-transfer compounds. J Phys Chem B 111 5861-5868... 

Let us consider a molecule and two of its energy levels E) and f 2- The Einstein coefficients are defined as follows (Scheme B2.2) Bn is the induced absorption coefficient, B2i is the induced emission coefficient and A21 is the spontaneous emission coefficient. 

The coefficients for spontaneous and induced emissions will be discussed in Chapter 3 (see Box 3.2). 

The condition for observing induced emission is that the population of the first singlet state Si is larger than that of So, which is far from the case at room temperature because of the Boltzmann distribution (see above). An inversion of population (i.e. NSi > Nso) is thus required. For a four-level system inversion can be achieved using optical pumping by an intense light source (flash lamps or lasers) dye lasers work in this way. Alternatively, electrical discharge in a gas (gas lasers, copper vapor lasers) can be used. 

In contrast to spontaneous emission, induced emission (also called stimulated emission) is coherent, i.e. all emitted photons have the same physical characteristics - they have the same direction, the same phase and the same polarization. These properties are characteristic of laser emission (L.A.S.E.R. = Light Amplification by Stimulated Emission of Radiation). The term induced emission comes from the fact that de-excitation is triggered by the interaction of an incident photon with an excited atom or molecule, which induces emission of photons having the same characteristics as those of the incident photon. 

Figure 15 Laser-induced emission spectra of polymer 3 before (solid line) and after (dashed line) a one second exposure to DNT vapor.

The spatial coherence of the induced emission which renders it possible to focus the laser output into a nearly parallel light beam. 

Its temporal coherence, causing spectral linewidths of the induced emission to be smaller by several orders of magnitude than those of fluorescence lines emitted by spectral lamps. 

In order to realize the extremely small linewidths, attainable because of the temporal coherence of induced emission, care must be taken to ensure that the threshold condition for induced oscillation is fulfilled for only one mode. The transverse modes can be eliminated by an appropriate choice of the cavity dimensions, introducing... 

The single-mode laser naturally gives less output power than a multimode laser with the same active volume since its induced emission is concentrated into a smaller frequency range. This loss in intensity, however, is much less than one would expect from the ratio of linewidths or from the reduction in oscillating mode number 3i. 32,41) jbis is due to the fact, that not only atoms with the exact transition frequency can contribute to the induced emission, but also those inside the homogeneous linewidth which is determined by collision processes in the case of gas lasers or by crystal line broadening in solid lasers... 

Another technique for measuring the lifetime of excited activator atoms in solid-state lasers has been published by Gilrs h If the pulsed laser is operated close above threshold, only a single spike (i.e. a short pulse of induced emission) appears, whereas many spikes are emitted when the laser ist running well above threshold. This... 

The observation of the induced emission, its time behavior and threshold conditions allow to study with new techniques details of chemical reactions which lead to specific states of the molecular or atomic reaction product A quantitative study of such laser systems will also yield information about collisional deactivation rates of the excited states (see also Section 111.4). 

Several other chemical lasers with similar excitation schemes soon followed vibrationally excited CO was formed during flash photolysis of a CSj-Oj mixture at 1 torr with 150 torr He buffer gas Investigations of the line spectrum (270 new laser lines have been found 00) gain and power output of induced emission under various helium pressures and with the addition of CO, established that CO was being selectively excited by different mechanisms into different vibrational levels. The experimental results enabled a few reaction schemes to be selected out of several other possibilities, which could be excluded. 

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