Nature of Light Emission

When the temperature of atoms or molecules is raised, they are promoted to excited states.0-3] Radiation energy is then emitted when they fall back to their normal 

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When it comes to the study of more concrete phenomena like the strength of various materials, for instance steel and concrete, we find laws at a still lower level. It is not possible to deduce the strength properties of conaete within a weU-defined hypothetico-deductive system. We are, in a way, in a similar situation as Bohr was in when he studied the nature of light emission. The laws which control say the... 

Nuclear magnetic resonance spectroscopy is a technique that, based on the magnetic properties of nuclei, reveals information on the position of specific atoms within molecules. Other spectroscopic methods are based on the detection of fluorescence and phosphorescence (forms of light emission due to the selective excitation of atoms by previously absorbed electromagnetic radiation, rather than to the temperature of the emitter) to unveil information about the nature and the relative amount specific atoms in matter. 

Hilgenfeldt S, Grossmann S, Lohse D (1999) A simple explanation of light emission in sonoluminescence. Nature (London) 398 402-405... 

Fundamentally, the properties of laser light are concomitants of its coherence, which is in turn a consequence of the nature of stimulated emission. Most of these properties, especially brightness, monochromaticity, directionality, polarization, and coherence itself, are useful (for many applications, indis-pensible) in a spectroscopic light source. The spectroscopic potential of lasers was recognized even before they were invented. Actual applications remained very specialized until tunable lasers were devised. 

Some of the major questions that semiconductor characterization techniques aim to address are the concentration and mobility of carriers and their level of compensation, the chemical nature and local structure of electrically-active dopants and their energy separations from the VB or CB, the existence of polytypes, the overall crystalline quality or perfection, the existence of stacking faults or dislocations, and the effects of annealing upon activation of electrically-active dopants. For semiconductor alloys, that are extensively used to tailor optoelectronic properties such as the wavelength of light emission, the question of whether the solid-solutions are ideal or exhibit preferential clustering of component atoms is important. The next... 

The lamp above is more properly called a low-pressure sodium lamp. Such lamps are ideal for street and road illumination, but the monochromatic nature of the emission makes seeing in colour impossible. An adaptation which emits a range of colours is the high-pressure sodium-vapour lamp, which is similar to that described above but contains a mixture of mercury and sodium. Such lamps emit a whiter light and are useful for extra-bright lighting in places such as road intersections, car parks and sports stadia. 

J.S. Kim, P.K.H. Ho, C.E. Murphy, N. Baynes, and R.H. Friend, Nature of non-emissive black spots in polymer light-emitting diodes by in-situ micro-Raman spectroscopy, Adv. Mater., 14 206-209, 2001. 

The nature of the emission by these three lanthanide ions is phosphorescence, since the emission of light is accompanied by a change in spin multiplicity. For example, the emission by the Eu3+ cation involves a change in the spin multiplicity from 5 to 7 on going from the excited state to the ground state (5Eu —> 7Eu). 

Of the many types of bioluminescence in nature, that of the firefly represents the most thoroughly studied and best understood biological luminescent process. The molecular mechanism of light emission by the firefly was elucidated in the 1960s in which a dioxetanone (a-peroxy lactone) was proposed as an intermediate, formed by the luciferase-catalyzed enzymatic oxidation of the firefly luciferin with molecular oxygen (Scheme 15). This biological reaction constitutes one of the most efficient luminescent processes known to date . Hence, it is not surprising that the luciferin-luciferase system finds wide use... 

In 1917 Einstein [18] wrote a paper on the dualistic nature of light in which he discusses emission without excitation from external causes, in other words stimulated emission and also spontaneous absorption and emission. He derives Planck s formula but also discusses the recoil of molecules when they emit photons. It is the latter discussion that Einstein regarded as the most significant aspect of the paper If a radiation bundle has the effect that a molecule struck by it absorbs or emits a quantity of energy hv in the form of radiation (ingoing radiation), then a momentum hvlc is always transferred to the molecule. For an absorption of energy, this takes place in the direction of propagation of the radiation bundle for an emission, in the opposite direction. ... 

A bacterial assay, the Photobacterium test, based on the inhibition of light emission of a bioluminescent bacterium Vibrio fisheri (ISO, 1998), originally developed to test the toxicity of industrial effluents, gives an indication of the effects of the test agent on the oxidative metabolism of the cell. As a bacterial assay its advantages are rapidity and relatively low costs, but naturally the differences between bacterial and mammalian systems make the interpretation of the results even more difficult than with other short-term tests. 

By 1930, these paradoxes had been resolved by quantum mechanics, which superseded Newtonian mechanics. The classical wave description of light is adequate to explain phenomena such as interference and diffraction, but the emission of light from matter and the absorption of light by matter are described by the particlelike photon picture. A hallmark of quantum, as opposed to classical, thinking is not to ask What is light but instead How does light behave under particular experimental conditions Thus, wave-particle duality is not a contradiction, but rather part of the fundamental nature of light and also of matter. 

Part V covers spectroscopic methods of analysis. Basic material on the nature of light and its interaction with matter is presented in Chapter 24. Spectroscopic instruments and their components are described in Chapter 25. The various applications of molecular absorption spectrometric methods are covered in some detail in Chapter 26, while Chapter 27 is concerned with molecular fluorescence spectroscopy. Chapter 28 discusses various atomic spectrometric methods, including atomic mass spectrometry, plasma emission spectrometry, and atomic absorption spectroscopy. 

G. Vazquez and K. Weninger, Is there a simple theory of sonoluminescence Nature (London) 409,782-3 (2001) B. D. Storey and A. J. Szeri, Argon rectification and the cause of light emission in single-bubble sonoluminescence, Phys. Rev. Lett. 88, 074301/1-3 (2002). 

In conventional LEDs, the spectral characteristics of the devices reflect the thermal distribution of electrons and holes in the conduction and valence band. The spectral characteristics of light emission from microcavities are as intriguing as they are complex. However, restricting our considerations to the optical axis of the cavity simplifies the cavity physics considerably. If we assume that the cavity resonance is much narrower than the natural emission spectrum of the semiconductor, then the on-resonance luminescence is enhanced whereas the off-resonance luminescence is suppressed. The on-axis emission spectrum should therefore reflect the enhancement, that is, the resonance spectrum of the cavity. The experimental results shown in Fig. 1.9 confirm this conjecture. 

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