Electromagnet fluorescence

A number of surface-sensitive spectroscopies rely only in part on photons. On the one hand, there are teclmiques where the sample is excited by electromagnetic radiation but where other particles ejected from the sample are used for the characterization of the surface (photons in electrons, ions or neutral atoms or moieties out). These include photoelectron spectroscopies (both x-ray- and UV-based) [89, 9Q and 91], photon stimulated desorption [92], and others. At the other end, a number of methods are based on a particles-in/photons-out set-up. These include inverse photoemission and ion- and electron-stimulated fluorescence [93, M]- All tirese teclmiques are discussed elsewhere in tliis encyclopaedia. 

A dye molecule has one or more absorption bands in the visible region of the electromagnetic spectrum (approximately 350-700 nm). After absorbing photons, the electronically excited molecules transfer to a more stable (triplet) state, which eventually emits photons (fluoresces) at a longer wavelength (composing three-level system.) The delay allows an inverted population to build up. Sometimes there are more than three levels. For example, the europium complex (Figure 18.15) has a four-level system. 

A modem use of uranine is in the manufacture of fluorescent laminates, eg, sheets, glass, and plastic films, that are transparent to electromagnetic waves and visible light rays (45). Such material might be used in windows, viewing partitions, and optical lenses. 

The scintillators are a special type of fluorescence indicators they are employed for the fluorimetric detection of radioactively labelled substances. They are stimulated by ) -radiation to the emission of electromagnetic radiation and will be discussed in Volume 2. 

Electromagnetic radiation energy can be used to stimulate substances to fluorescence after separation by thin-layer chromatography. Its action makes it possible to convert some nonfluorescent substances into fluorescent derivatives. The active sorbents often act as catalysts in such processes (cf. Chapter 1.1). 

Nishitani et al. observed the tip-enhanced fluorescence of 8 nm thick meso-tetralds (3,5-di-tertiarybutyl-phenyl)porphyrin (H2TBPP) films on ITO with an Ag tip [38]. They reported that the fluorescence was enhanced by the locally confined electromagnetic field in the vicinity of the tip. The enhancement factor is evaluated to be larger than 2000. 

The physical basis of spectroscopy is the interaction of light with matter. The main types of interaction of electromagnetic radiation with matter are absorption, reflection, excitation-emission (fluorescence, phosphorescence, luminescence), scattering, diffraction, and photochemical reaction (absorbance and bond breaking). Radiation damage may occur. Traditionally, spectroscopy is the measurement of light intensity... 

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. 

Direct Photolysis. Direct photochemical reactions are due to absorption of electromagnetic energy by a pollutant. In this "primary" photochemical process, absorption of a photon promotes a molecule from its ground state to an electronically excited state. The excited molecule then either reacts to yield a photoproduct or decays (via fluorescence, phosphorescence, etc.) to its ground state. The efficiency of each of these energy conversion processes is called its "quantum yield" the law of conservation of energy requires that the primary quantum efficiencies sum to 1.0. Photochemical reactivity is thus composed of two factors the absorption spectrum, and the quantum efficiency for photochemical transformations. 

Little attention has been devoted to the effects of time-dependent magnetic fields (created by electromagnetic waves) in the absence of a strong magnetic field. Hore and his coworkers [123-125] recently described this effect as the oscillating magnetic field effect (OMFE) on the fluorescence of an exciplex formed in the photochemical reaction of anthracene with 1,3-dicyanobenzene over the frequency range 1-80 MHz. 

There are two principle ways for optical detection of protein concentrations either the macromolecule or its label emits energy (after excitation by light) -then a fluorescence signal can be measured or it absorbs energy from electromagnetic waves passing the sample - then the optical absorption of the sample can be measured by UV/Vis spectroscopy and concentrations can be calculated according to Lambert-Beers Law. 

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