Temperature induced emission

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... 

In a more general application, thermoluminescence is used to study mechanisms of defect annealing in crystals. Electron holes and traps, crystal defects, and color-centers are generated in crystals by isotope or X-ray irradiation at low temperatures. Thermoluminescent emission during the warmup can be interpreted in terms of the microenvironments around the various radiation induced defects and the dynamics of the annealing process (117-118). ... 

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. 

Z. Y. Zhang, K. T. V. Grattan, and A. W. Palmer, Thermal characteristics of alexandrite fluorescence decay at high temperatures, induced by a visible laser diode emission, J. Appl. Phys. 73(1), 3493-3498 (1993). 

At low pressure, the only interactions of the ion with its surroundings are through the exchange of photons with the surrounding walls. This is described by the three processes of absorption, induced emission, and spontaneous emission (whose rates are related by the Einstein coefficient equations). In the circumstances of interest here, the radiation illuminating the ions is the blackbody spectrum at the temperature of the surrounding walls, whose intensity and spectral distribution are given by the Planck blackbody formula. At ordinary temperatures, this is almost entirely infrared radiation, and near room temperature the most intense radiation is near 1000 cm". ... 

Dipole moments that absorb will also emit radiation. Especially at elevated temperatures collision-induced emission (CIE) spectra may be expected and have in fact been found in the laboratory using shock tubes. Stellar atmospheres are also known to emit radiation by interatomic (e.g., H-He) and intermolecular (H2-H2, etc.) interactions. 

Fig. 3.49. Collision-induced emission in the fundamental band of hydrogen after [116]. 20% hydrogen in argon at the temperature T = 2844 K.

Laser techniques can be used to produce extreme temperatures for short durations, possibly as high as several thousand degrees centigrade. Honig and Woolston described a combined laser mass spectrographic technique to study laser-induced emission from solids (3). 

Figure 7.9 The solid-state linear emission (a) and multiphoton absorption induced emission/SHG (b) spectra of 4 at room temperature.

If the effective "temperature" T of the radiation field is so low that hv 1 kBT, then induced emission is less important than spontaneous emission if, instead, hv 1 kBT, then induced emission dominates over spontaneous... 

In most experiments designed to measure the intensity of spectral absorption, the measurement gives the net absorption due to the effects of absorption from the lower energy level m to the upper energy level n> less induced emission from n to m. Since the populations depend on the temperature, so will the measured net absorption. This comment applies to all the quantities defined in the table to measure absorption intensity, although for transitions where hc0v kTthe temperature dependence... 

Mercuiy in the Arctic cycles with the seasons between the atmosphere and snow on the ground. In the spring, as the sun reappears after the winter darkness, mercury levels in the troposphere decline for about 3 months. At the same time, the level of mercury in the snow increases 100-fold, both as methylmercury and as inorganic compounds of mercury. Later in the year, as the snow melts, the levels in the snow drop and mercury reappears in the troposphere. The elemental mercury in the atmosphere is converted to particulates or reactive species, parallelling a decrease in atmospheric ozone, and is then deposited in the snow. Later in the summer, the mercury levels in the atmosphere increase, probably due to temperature- or sunlight-induced emission of volatile mercury species from the surface. 

For the energy of excitation discussed above (3.98 x 10" J) and a temperature of 20°C (293 K), the ratio of excited states to ground states is n(A )/n(A ) = 10". In other words, there is only one excited-state atom or molecule for every 5 x 10 molecules in the ground state Thus, the chances of observing a natural or stimulated emission are vanishingly small because there are almost no atoms or molecules in the excited state. For every photon entering such an assembly of molecules, there are billions of chances that it will be absorbed and only one that it will induce emission. Any photons so emitted would meet only ground-state molecules and not another excited-state atom or molecule required to start a cascade. Thus, an incipient cascade would stop before it could develop. 

Fig. 9 (A) shows for reference purposes again the absorption spectrum of Rb. sphaeroides with the regions of the various absorption bands appropriately identified as their origins in the different pigment molecules. Fig. 9 (B) shows the absorbance changes measured at room temperature as a function oftime at 920, 785 and 545 nm. A linear time scale was used for delay times (to) up to 1 ps, and a logarithmic one for delay times greater than 1 ps. The profile of date points collected at 920 nm shows an instantaneous decrease due to the depopulation of the ground state of the primary donor P plus a contribution from the induced emission of the excited states of the primary donor, P. The decay of P occurs with a...

Besides the dust clouds, changes of contents of carbon dioxide and certain other components in the air can also affect the mean temperature of the atmosphere close to the earth s surface. The assumed increase of the surface temperature induced by increasing concentration of certain trace components of the atmosphere is shown in Table 5.6. The problem of effects of increasing CO2 emissions is discussed in more detail in Section 5.2.4.3, where possible effects of CF2CI2 and CFCI3 on the thickness of the ozone layer in the stratosphere are also considered. 

Picosecond fluorescence studies were applied by Winnik and co-workers [72] for studies of temperature-induced phase transition of pyrene-labelled hydroxypropylcellulose (HPC-Py) in water. Temperature dependence of the fluorescence emission ratio of excimer to monomer emission (Ie/Im) showed a significant increase of excimer emission in a temperature range 283-313 K, then a decrease to a constant value at 319 K. Two excimer bands were observed when time-resolved spectroscopy was used i) a broad, structureless band with a maximum at 420 nm and a corresponding lifetime of 250 ps and ii) the well-known band of pyrene excimer, with a maximum at 470 nm and a lifetime of 68 ns. In the initial time region, 0-150 ps, monomer emission was observed, with a simulation by a superposition of three components (377, 398 and 421 nm). They observed only one excimer emission above the LCST and that was with a maximum at 470 nm. They concluded that the LCST implies a complete disruption of the ordered microstructures, which were created in cold water. 

Finally, it is interesting to compare how the distributions of EET rates develop when the temperature is changed. As seen in Fig. 22, at low temperature the distribution of EET rates is significantly broader and shifted to smaller values compared to room temperature. The narrowing of the spectra at low temperature induces strong modifications of emission and absorption spectral shapes and results in many different spectral configurations where the spectral overlap can be more or less favored. On average, the EET slows down by approximately a factor of three when the temperature is lowered to 1.4 K. 

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