Light induced emission

Optical immunosensors are based on the measurement of the absorption or emission of light induced by the immunoreactants [9], They can also be based on evanescent... 

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. 

Gamma counters are a type of ectrttillatlfri counter add measure gamma radiation by detectlngthe emission of light Induced in a scintiBsnt... 

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

Luminescence is a well-established class of analytical spectroscopic techniques where a species emits light after excitation. Emission is an elecnonic nansition from an excited state as opposed to the ground state as is the case in most other spectroscopies. Photoluminescence, or light-induced fluorescence (LIE), is the most common route to induce emission where sufficient incident photons of a particular energy excite the target species via absorption. Although less common, nomadiative excitation can also occur via a chemical reaction termed chemiluminescence. Unless otherwise stated, the terms luminescence and fluorescence within this review infers excitation by light induction. 

Within the context of supramolecular devices, re-emission of the radiation by luminescence is of interest in sensing and signalling applications, while chemical reactions are of interest in applications such as molecular switches and photocatalysis. Strictly speaking absorption and re-emission type processes are termed molecular photophysics while light-induced chemical reactions or chemical processes are termed photochemistry . 

Galperin M, Nitzan A. Current induced light emission and light induced current in molecular tunelling junctions. Phys Rev Lett 2005 95 206802. 

Einstein obtained coefficients for induced absorption B , induced emission Bu i, and spontaneous emission Au, of light by the following thermodynamic arguments, based on Arrhenius 116 law. 

Here we have a combination of emission (a — 1), which will occur even in the dark, and induced emission (a n), which occurs when light emission stimulates even more light emission. Einstein had predicted this in a thermodynamic analysis of blackbody radiation. 

Faulkner and coworkers (27., ), have also studied the interaction of Ru(bpy)32+ with zeolite X. Luminescence lifetime measurements and emission spectra were used to study electron transfer quenching of the electron donors N,N,N, N -tetramethyl-p-phenylenediamine and 10-phenyl-phenothiazine. Lifetime measurements show at least two modes of quenching for the interaction of Ru(bpy)32+ ions with these donors. Products of these electron transfer reactions were isolated and these experiments show that the zeolite can separate the products of light induced electron transfer. 

The optical absorption arising from the defect transitions is weak because of the low defect densities and in a thin film cannot be measured by optical transmission. The techniques of PDS, CPM and photoemission yield, described in Section 3.3, have sufficient sensitivity. Photocapacitance, which measures the light-induced change in the depletion layer capacitance, is similarly sensitive to weak absorption (Johnson and Biegelsen 1985). PDS measures the heat absorbed in the sample and detects all of the possible optical transitions. At room temperature virtually all the recombination is non-radiative and generates heat by phonon emission. CPM detects photocarriers and so is primarily sensitive to the optical transitions which excite electrons to... 

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