Induced emission process

Fig. 14. Schematic of the Auger electron emission process induced by creation of a K level electron hole.

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. 

Induced, or stimulated, emission. This is a different type of emission process from that of type 2 in that a quantum of radiation of wavenumber v given by Equation (2.2) is required to induce, or stimulate, M to go from n to m. The process is represented by... 

The X-ray emission process followii the excitation is the same in all three cases, as it is also for the electron-induced X-ray emission methods (EDS and EMPA) described in Chapter 3. The electron core hole produced by the excitation is filled by an electron falling from a shallower level, the excess energy produced being released as an emitted X ray with a wavelength characteristic of the atomic energy levels involved. Thus elemental identification is provided and quantification can be obtained from intensities. The practical differences between the techniques come from the consequences of using the different excitation sources. 

Despite such limitations, plasma-deposited a-C(N) H films were found to be used in a number of applications. The stress reduction induced by nitrogen incorporation [12] and consequent adhesion improvement, allowed the development of a-C(N) H antireflective coatings for Ge-based infrared detectors [13]. It was also found that N can electronically dope a-C H films, and can strongly decrease the defect density, which gives prospects on its use as a semiconductor material [14]. Nitrogen incorporation was also found to decrease the threshold electric field in electron-field emission process [15], making possible the use of a-C(N) H films as an overcoat on emission tips in flat-panel display devices [16]. 

It is useful to view optical absorption and emission processes in such a system in terms of transitions between distinct vibrational levels of the ground and excited electronic states of a metal atom-rare gas complex or quasi-molecule. Since the vibrational motions of the complex are coupled with the bulk lattice vibrations, a complicated pattern of closely spaced vibrational levels is involved and this results in the appearance of a smooth, structureless absorption profile (25). Thus the homogeneous width of the absorption band arises from a coupling between the electronic states of the metal atom and the host lattice vibrations, which is induced by the differences between the guest-host... 

Spectroscopic techniques look at the way photons of light are absorbed quantum mechanically. X-ray photons excite inner-shell electrons, ultra-violet and visible-light photons excite outer-shell (valence) electrons. Infrared photons are less energetic, and induce bond vibrations. Microwaves are less energetic still, and induce molecular rotation. Spectroscopic selection rules are analysed from within the context of optical transitions, including charge-transfer interactions The absorbed photon may be subsequently emitted through one of several different pathways, such as fluorescence or phosphorescence. Other photon emission processes, such as incandescence, are also discussed. 

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... 

Using the same method that led to Eq. (5.27), it is easy to establish the rule of multiplication of depolarization factors when several processes inducing successive rotations of the transition moments (each being characterized by cos2 C,) are independent random relative azimuths, the emission anisotropy is the product of the depolarization factors (3 cos2 c, — l)/2 ... 

When a radiation source is placed inside a closed cavity, its radiation energy is distributed among all of the modes following Equations (2.1) and (2.2), once the system has reached equilibrium. As we have seen in Example 2.1, in spite of the large number of modes in such a closed cavity, the mean number of photons per mode corresponding to the optical region is very small. Specifically, it is very small compared to unity. This is the ultimate reason why, in thermal radiation fields, the spontaneous emission per mode by far exceeds the stimulated emission. (Remember that the stimulated emission process requires the presence of photons to induce the transition, opposite to the case of the spontaneous emission process.)... 

Provided that a transition is forbidden by an electric dipole process, it is still possible to observe absorption or emission bands induced by a magnetic dipole transition. In this case, the transition proceeds because of the interaction of the center with the magnetic field of the incident radiation. The interaction Hamiltonian is now written as // = Um B, where is the magnetic dipole moment and B is the magnetic field of the radiation. 

A third source of misassignment has its roots in the existence of nuclear-nuclear cross- relaxation." Again, depending on the mechanism of cross-relaxation and on the polarization of the originally polarized nucleus, this may result in enhanced absorption or emission. This process induces nuclear spin polarization in nuclei without hfc, or alters the nuclear spin polarization of nuclei with weak hfcs. On the other hand, the magnitude of these effects may be quite small and fall below the threshold of chemical significance. 

Capture and emission processes at a deep center are usually studied by experiments that use either electrical bias or absorbed photons to disturb the free-carrier density. The subsequent thermally or optically induced trapping or emission of carriers is detected as a change in the current or capacitance of a given device, and one is able to deduce the trap parameters from a measurement of these changes. 

When one considers the role of the matrix in the particle-induced emission of secondary ions it is no wonder that it is so difficult to unravel all the processes that take place. The matrix is the medium in which the primary excitation occurs. It must also disperse some of that energy to sites at the surface where secondary ion emission occurs. It must provide the species to be desorbed and at the same time mediate the ionization process. In an attempt to understand these complex coupled processes we have tried to simplify the system by first selecting a homogeneous substrate for the energy deposition and then studying the ionization-emission process for species that are present as a submonolayer on the surface (26). 

Figure 2.2 (a) Absorption and emission processes between states m and n. (b) Seeding of a rain cloud with silver iodide (Agl) to induce a shower of rain... 

There are other electron emission processes which lie between such well-defined limiting cases, e.g., resonance affected two-electron emission which lies between direct double photoionization and photon-induced two-step double ionization (photoelectron and Auger electron emission). 

Electron emission can also be induced by the presence of large electric fields. When a potential V is applied between a sharp metal tip and a plate, a large electric field is generated at the tip (i.e. E 10 V cm for a potential of 1000 V apphed to a tip of r = 10 cm). This electron emission process is known as field emission. In the field emission microscope (FEM), emitted electrons are detected at a phosphor plate as a function of potential and are used to measure the work function of different crystallographic planes of the metal tip. 

On semiconductors light emission is induced by injection of electrons into the conduction band and subsequent band-to-band radiative recombination with holes (Fig. 38a). The process is reminiscent of electroluminescence or cathodolumines-cence and works with p-type substrates only (at n-type specimens no hole is available at the surface). Tunnel biases of 1.5-2 V are necessary in the case of GaAs, for instance. Figure 38b is a photon map of a GaAlAs/GaAs multiquantum well obtained by Alvarado et al. [140], The white stripes are regions where photons are emitted and correspond to the GaAs layers. The lateral resolution is about 1 nm and is limited by the diffusion distance of minority carriers. In Sec. 5.1 we have seen an example of the application of this technique in the case of porous silicon layers. 

Stimulated Raman gain Stimulated Raman gain (SRG) and inverse Raman scattering (IRS) are closely related. While one involves stimulated gain at a Stokes-shifted frequency, the other involves stimulated loss at an anti-Stokes-shifted frequency. SRG can be viewed as an induced emission process at the Stokes frequency. Both SRG and IRS are coherent processes. 

When we apply 10 V to a typical 100-nm-thick organic layer, we need to watch for an unexpected high electric field (106 V/cm2), which enables us to induce carrier injection and SCLC. First, we turn our attention to the behavior of current injection from electrodes. We have two possible mechanisms to inject charge carriers Schottky thermal emission and the tunneling injection processes, both of which are based on the theory of inorganic semiconductors. The Schottky emission process is described by41... 

As an illustration of the strong perturbation introduced by an ion in a metal and of the theoretical description of it in nonlinear theory of screening, we show some results obtained for the interaction of Ne ions with metals. Multicharged Ne ions have been widely used in the experimental study of ion neutralization and electron emission processes at metal surfaces [14-16], We plot in Fig. 1 the electronic density induced by a Ne ion in a FEG of... 

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