Thermally stimulated electron emission

PTTL phototransferred (faermoluminescence TSEE thermal stimulated electron emission... 

The Sm " " ion (4/ ) demonstrates 5d-4f broad emission together with intra-conflgurational 4/ line emission (Fig. 4.18). It is worth noting that, despite their different origins, the broad band and narrow Hnes have a similar decay time. The possible reason is a thermally stimulated electron exchange between the lower 4/ and higher 5d excited states. It ceases at 77 K and the 5d-4f broad emission is absent. Using different excitations several types of Sm are detected in anhydrite. 

In this discussion, electrochemical reactions with semiconductors are referred to as thermal. The reason is that semiconductors are particularly sensitive to incident light, which stimulates electron emission and causes photocurrents to flow. They are... 

This method is based on the emission of light by atoms returning from an electronically excited to the ground state. As in atomic absorption spectrometry, the technique involves introduction of the sample into a hot flame, where at least part of the molecules or atoms are thermally stimulated. The radiation emitted when the excited species returns to the ground state is passed through a monochromator. The emission lines characteristic of the element to be determined can be isolated and their intensities quantitatively correlated with the concentration of the solution. 

It should be noted that in the above examples significant energy migration can occur between thermal stimulation and recombination since the electron is free to diffuse throughout the sample. Other electron-hole recombination processes occur, however, in which the excited states of the charge carrier are not to be found in the delocalized bands. An example of this, recently noted in the literature, concerns the TL emission from oligoclase feldspar in which the thermal stimulation involves the transition of the electron to a localized, intermediate state - see Figure 8... 

The nature of the participating defects in EE from LiF has been studied by thermally stimulated luminescence (TSL) ( ). TSL peaks from LiF samples pulverized in a mortar and pestle are quite similar to the peaks from LiF irradiated with UV light, suggesting that the same defects participate in both emissions. TSL studies of MgO powders suggest that above room temperature the rate controlling process involves electron traps. phE and EE decay from MgO follow very similar kinetics, as seen in Figure 1, suggesting that phE and EE are rate limited by the concentration of the same defect. 

In response to criticism from members of the electronics industry that thermal analysis methods are often incompatible with the rapid pace of corporate research development, new sections have been introduced on thermally stimulated current spectroscopy, thermal conductivity, optothermal transient emission radiometry and micro-thermal analysis (/ TA). 

Atomic emission spectroscopy Spectroscopy in which thermal exdtatirm is used to stimulate electronic transitions and photon emission. 

TSL is observable in most dielectrics in polymers the sample is commonly irradiated at liquid nitrogen temperature and heated to room temperature at a rate of approximately 3" C/min. TSL emission in many commercial polymers is negligible above room temperature and the information, which can be extracted from a single TSL measurement on the molecular environment of the trapped electrons, is not as precise as from ESR. The TSL spectrum of a polymer may contain both fluorescent and phosphorescent components. TSL provides information about ageing processes and can be used as a method for early recognition of damage in polymers. Fleming [488] has reviewed thermally stimulated luminescence (TSL) for the analysis of polymers. 

A description of the emission and capture processes at a trap will be useful before discussing the various experimental methods. Figure 1 depicts the capture and emission processes that can occur at a center with electron energy ET. The subscripts n and p denote electron and hole transitions, and the superscripts t and differentiate between thermally and optically stimulated processes. It is assumed here that only thermal capture processes are occurring. 

Nonlaser light sources emit radiation in all directions as a result of the spontaneous emission of photons by thermally excited solids (filament lamps) or electronically excited atoms, ions, or molecules (fluorescent lamps, etc.). The emission accompanies the spontaneous return of the excited species to the ground state and occurs randomly, Le. the radiation is not coherent, in a laser, the atoms, ions, or molecules are first pumped to an excited state and then stimulated to emit photons by collision of a photon of the same energy. This is called stimulated emission. In order to use it, it is first necessary to create a condition in the amplifying medium, called popidatlon inversion, in which the majority of the relevant entitles are excited. Random emission firom one entity can then trigger coherent emission firom the others that it passes. In this way amplification is achieved. 

Polyfluorenes are an important class of LEP with high thermal, photo and environmental stability and efficient bright blue emission. This stimulated a number of researchers to develop fluorene-thiophene copolymers for light-emitting applications. In addition to an expected increase in PL quantum etSciency, such a combination of electron-rich thiophene units with relatively electron-deficient fluorene units should modify the bandgap of the material (and thus tune the emission) and improve the charge injection/transport balance, compared with fluorene homopolymers. 

To obtain stimulated emission between two energy levels, a population inversion is necessary. This is usually achieved by excitation into a third level (or levels) which rapidly and efficiently transfers its energy to a metastable upper laser level. A generalized energy level scheme for laser action is shown in fig. 35.3. If the terminal laser level is the ground state, then more than one-half of the ions must be excited to obtain an inverted population. If, instead, the terminal level 2 is above the ground state, then only an excited-state population sufficient to overcome the Boltzmann thermal equilibrium population in the terminal level is needed. This reduces the pumping requirements. In phonon-terminated lasers, level 2 is a vibrational-electronic state. The four-level laser scheme depicted in fig. 35.3 is representative of that employed for most rare earth lasers. 

We assume the optimistic case that the vibrational and rotational manifolds of the upper electronic state can be collisionally relaxed on a time scale short with respect to the upper electronic state lifetime. In that case the v J manifold is thermalized. In this limit, typical values are Fy -- 0.8, Fj 0.02, Qy y -- 0.2, and Sj/2J+1 0.5 this implies a dilution factor of nearly three orders of magnitude. In other words, for comparable radiative lifetimes and emission wavelengths, the single line stimulated emission cross section for a diatomic molecule will be -1000 times smaller than for an atomic emitter. 

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