Emission of light

Such a compound, though it absorbs light, appears to human eyes as colorless, as the absorption would not disturb the visible range. 

We can use the phenomenon of interaction between light and a compound to explore the details of its energy states and strncture. Scientists have devised means to mea-snre the wavelength and the intensity at which light is absorbed by a compound. The device is known as spectrophotometer, and the technique is spectroscopy. Absorption of light in the visible and ultraviolet region has been shown to be caused by the movement of electron between different energy states. 

We do see water an aggregate of tiny water molecules. We are able to see the molecules, all right, but not an individual molecule. It is too minute to see with our naked eyes. So, are the atoms and the molecules for real Chemists assume and also believe that they are. What do atoms or molecules look like Chemists picture them based on the atoms and their combination, as we did in the last two chapters and do throughout this book. Can we see them Seeing is believing, isn t it But the atoms and molecules are too small to be seen with our naked eyes. So what should we do Magnify them  

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A1.6.1.3 ABSORPTION, STIMULATED EMISSION AND SPONTANEOUS EMISSION OF LIGHT... 

Classic examples are the spontaneous emission of light or spontaneous radioactive decay. In chemistry, an important class of monomolecular reactions is the predissociation of metastable (excited) species. An example is the fonnation of oxygen atoms in the upper atmosphere by predissociation of electronically excited O2 molecules [12, 13 and 14] ... 

Finally, one may want to control the emission of light from molecules. 

In a cascade process, one incident electron (e ) collides with a neutral atom ((S)) to produce a second electron and an ion ( ). Now there are two electrons and one ion. These two electrons collide with another neutral atom to produce four electrons and three ions. This process continues rapidly and — after about 20 successive sets of collisions — there are millions of electrons and ions. (The mean free path between collisions is very small at atmospheric pressures.) A typical atmospheric-pressure plasma will contain 10 each of electrons and ions per milliliter. Some ions and electrons are lost by recombination to reform neutral atoms, with emission of light. 

Luminance is the luminous intensity divided by the area of emission of light (Iumens/steradian/m2). This is the power density emitted per unit area. 

These collisions can be sufficiently energetic such that the gas molecules become electronically excited, and, as the excited atoms return to their ground state, they emit light. Thus, passage of electrons (an electric current) through a gas under the right conditions leads to the emission of light from the gas. 

Analysis. Lithium can be detected by the strong orange-red emission of light in a flame. Emission spectroscopy allows very accurate determination of lithium and is the most commonly used analytical procedure. The red emission line at 670.8 nm is usually used for analytical determinations although the orange emission line at 610.3 nm is also strong. Numerous other methods for lithium determinations have been reviewed (49,50). 

Chemiluminescence. Chemiluminescence (262—265) is the emission of light duting an exothermic chemical reaction, generaUy as fluorescence. It often occurs ia oxidation processes, and enzyme-mediated bioluminescence has important analytical appHcations (241,262). Chemiluminescence analysis is highly specific and can reach ppb detection limits with relatively simple iastmmentation. Nitric oxide has been so analyzed from reaction with ozone (266—268), and ozone can be detected by the emission at 585 nm from reaction with ethylene. 

Johansson, C.H., Emission of Light at Detonation of High Explosives. Translated from Explosivstoffe 11, 251 (1963). 

Cathodoluminescence (CL), i.e., the emission of light as the result of electron-beam bombardment, was first reported in the middle of the nineteenth century in experiments in evacuated glass tubes. The tubes were found to emit light when an electron beam (cathode ray) struck the glass, and subsequendy this phenomenon led to the discovery of the electron. Currendy, cathodoluminescence is widely used in cathode-ray tube-based (CRT) instruments (e.g., oscilloscopes, television and computer terminals) and in electron microscope fluorescent screens. With the developments of electron microscopy techniques (see the articles on SEM, STEM and TEM) in the last several decades, CL microscopy and spectroscopy have emerged as powerfirl tools for the microcharacterization of the electronic propenies of luminescent materials, attaining spatial resolutions on the order of 1 pm and less. Major applications of CL analysis techniques include ... 

NO2 refers to the excited nitrogen oxide molecule. These molecules can decay by emission of light of wavelengths longer than 600 nm.- ... 

The first known laser was made by Theodore Maiman at Hughes Research Laboratories in Malibu, California, in 1960, but the seeds of this breakthrough were planted years before. In 1917 Albert Einstein, through his work on the quantum theory of light, theorized that stimulated emission of light radiation could occur. The idea was forgotten, though, until the middle of the century. 

In order to summarize this section it must be noted that the possibility to evaporate conjugated oligomers in such a way that their chain axes point predominantly in one direction allows us to determine the anisotropic electronic properties of conjugated molecules. From these studies it becomes evident that in order to obtain polarized absorption or emission of light as required in polarization Filters or LCD-backlights, the application of conjugated molecules is one of the most attractive areas of application for these materials. 

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