Electron induced stimulated emission

Electron Microprobe A.na.Iysis, Electron microprobe analysis (ema) is a technique based on x-ray fluorescence from atoms in the near-surface region of a material stimulated by a focused beam of high energy electrons (7—9,30). Essentially, this method is based on electron-induced x-ray emission as opposed to x-ray-induced x-ray emission, which forms the basis of conventional x-ray fluorescence (xrf) spectroscopy (31). The microprobe form of this x-ray fluorescence spectroscopy was first developed by Castaing in 1951 (32), and today is a mature technique. Primary beam electrons with energies of 10—30 keV are used and sample the material to a depth on the order of 1 pm. X-rays from all elements with the exception of H, He, and Li can be detected. 

Figure 10-1. Energy scheme of the relevant electronic levels that are affecting stimulated emission in an isolated dye molecule in solution (a) and in a conjugated polymer (b). In both systems absorption of a photon induces a transition from the singlet ground state Sq to a vibronically excited state within the Si-manifold. After vibronic relaxation, a red-shifted emission can be observed. This transition can occur in a spontaneous as well as in a stimulated manner. A photoinduced absorption may occur as competing processes in both systems (PIA) due to the population of the triplet state T. A pair of oppositely charged carriers (GP) can be generated in the conjugated polymer due to dissociation of the 5i-state. This can lead to additional photoinduced absorption bands that compete with the stimulated emission.

Often the super-high spectral resolution of the induced resonant Raman transitions obtained with single-mode lasers is not necessary if the levels m) are separated by more than one Doppler width. Then pulsed lasers can be used for stimulated emission pumping [600]. Many experiments on high vibrational levels in the electronic ground state of polyatomic molecules have been performed so far by SEP with pulsed lasers. Compilations may be found in [601-604]. 

A very interesting optical cooling technique starts with the selective excitation of a collision pair of cold atoms into a bound level in an upper electronic state (Fig. 9.17). While this excitation occurs at the outer turning point of the upper-state potential, a second laser dumps the excited molecule down into a low vibrational level of the electronic ground state by stimulated emission pumping (photo-induced association). In favorable cases the level u = 0 can be reached. If the colliding atoms... 

Although quite successful as a spectroscopic tool [40-48] (see Chapter 5 by W. Stwalley, P. Gould, and E. Eyler), as a preparatory tool, two-step PA suffers from losses due to spontaneous emission. Even in the presence of the stimulated emission process induced by a dump pulse [39], sponaneous emission populates in an incoherent fashion a large number of vibrational and rotational levels of the ground electronic state (or the low metastable electronic state), resulting in a translationally cold but internally hot ensemble of molecules. 

Besides various detection mechanisms (e.g. stimulated emission or ionization), there exist moreover numerous possible detection schemes. For example, we may either directly detect the emitted polarization (oc PP, so-called homodyne detection), thus measuring the decay of the electronic coherence via the photon-echo effect, or we may employ a heterodyne detection scheme (oc EP ), thus monitoring the time evolution of the electronic populations In the ground and excited electronic states via resonance Raman and stimulated emission processes. Furthermore, one may use polarization-sensitive detection techniques (transient birefringence and dichroism spectroscopy ), employ frequency-integrated (see, e.g. Ref. 53) or dispersed (see, e.g. Ref. 54) detection of the emission, and use laser fields with definite phase relation. On top of that, there are modern coherent multi-pulse techniques, which combine several of the above mentioned options. For example, phase-locked heterodyne-detected four-pulse photon-echo experiments make it possible to monitor all three time evolutions inherent to the third-order polarization, namely, the electronic coherence decay induced by the pump field, the djmamics of the system occurring after the preparation by the pump, and the electronic coherence decay induced by the probe field. For a theoretical survey of the various spectroscopic detection schemes, see Ref. 10. 

Stimulated desorption, whether photon, electron, or ion induced, is an inelastic sputtering process as it is the energy associated with the formation of a core hole that results in the emission of the element in question. Indeed, the formation of F " and Cl ions on electron irradiation of Aluminum and Silicon surfaces is accepted to arise through core hole formation followed by ejection through the Coulom-bic repulsion induced. In the case of ion-irradiated surfaces, it has been suggested that stimulated emission arises from Auger electrons formed in relatively distant neighbors (Williams 1981). 

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