Ultraviolet, Visible and Near Infrared

Analytical Instrumentation A Guide to Laboratory, Portable and Miniaturized Instruments G. McMahon 

Absorption of NIR radiation corresponds to certain vibrations of the molecule and is due to overtones and combinations of parent absorption bands in the mid-infrared (IR) region. Generally, these absorptions are weaker than the parent absorptions in the IR but the decrease in intensities is not the same for all molecnles. It can be seen as a complementary techniqne to conventional IR, exploiting a different region of the electromagnetic spectrum. For example, water has less absorption in the NIR compared with mid IR, so NIR spectra of aqneons samples are often sharper. A NIR absorption spectrum is usually a plot of absorbance versns wavelength and has more fine structure than a UV or UV-Vis spectrum. NIR absorbance also follows the Beer-Lambert Law, so can be used as a quantitative techniqne. 

Deuterium lamps are commonly used as a UV radiation source in the range 200-400 nm and tungsten incandescent lamps as sonrces for the visible and NIR regions covering the range 400-2500 nm. For the NIR work, the sonrce is operated at 2500-3000 K, which results in more intense radiation. 

A monochromator is usually used as a wavelength selector. Monochromators are composed of a dispersing medium to separate the wavelengths of the polychromatic radiation from the source, slits to select the narrow band of wavelengths of interest and lenses or mirrors to focus the chosen radiation. 

The greatest receut improvemeut in spectrophotometers has been in the detector. PMTs are monochannel detectors and are still very popular. They cousist of a photosensitive surface and a series of electrodes (dynodes), each at an increased potential compared to the one before. When a photon strikes the photosensitive surface, a primary electron is emitted and accelerates towards the first dynode. This electron impacts the dynode and causes the release of a number of secondary electrons, which hit the next electrode and so on, until the signal is amplified many times over. Even extremely small signals can 

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Laser Photochemistry. Photochemical appHcations of lasers generally employ tunable lasers which can be tuned to a specific absorption resonance of an atom or molecule (see Photochemical technology). Examples include the tunable dye laser in the ultraviolet, visible, and near-infrared portions of the spectmm the titanium-doped sapphire, Tfsapphire, laser in the visible and near infrared optical parametric oscillators in the visible and infrared and Line-tunable carbon dioxide lasers, which can be tuned with a wavelength-selective element to any of a large number of closely spaced lines in the infrared near 10 ]lni. 

Recently was estimated an expected impact on the global chemistry of the atmosphere of the indirect heterogeneous photocatalytic reactions under the much more abundant near ultraviolet, visible and near infrared solar light [2]. As photocatalysts may serve atmospheric aerosols, i.e. ultrasmall solid particles that sometimes are embedded into liquid droplets. Aerosols are known to contain Ti02, Fc203, ZnO and other natural oxides, as well as metal sulfides of volcanic or antropogenic origin, that may serve as semiconductor photocatalysts (see Fig.5). Aerosols are known to be concentrated mainly in the air layers near the surface of the Earth, i.e. in the troposphere, rather than stratosphere. 

Standard Practice for Describing and Measuring Performance of Ultraviolet, Visible, and Near-Infrared Spectrophotometers, ASTM E 257-93, 1998. 

COLOR CENTERS. Certain crystals, such as the alkali halides, can be colored by the introduction of excess alkali metal into the lattice, or by irradiation with x-rays, energetic electrons, etc. Thus sodium chloride acquires a yellow color and potassium chloride a blue-violet color. The absorption spectra of such crystals have definite absorption bands throughout the ultraviolet, visible and near-infrared regions. The term color center is applied to special electronic configurations in the solid. The simplest and best understood of these color centers is the F center. Color centers are basically lattice defects that absorb light. 

The ideal high-throughput analytical technique would be efficient in terms of required resources and would be scalable to accommodate an arbitrarily large number of samples. In addition, this scalability would be such that the dependence of the cost of the equipment to perform the experiments would scale in a less than linear manner as a function of the number of samples that could be studied. The only way to accomplish this is to have one or more aspects of the experimental setup utilize an array-based approach. Array detectors are massively multiplexed versions of single-element detectors composed of a rectangular grid of small detectors. The most commonly encountered examples are CCD cameras, which are used to acquire ultraviolet, visible and near-infrared (IR) photons in a parallel manner. Other examples include IR focal plane arrays (FPAs) for the collection of IR photons and channel electron multipliers for the collection of electrons. 

Mercury-xenon lamp (Arc) An intense source of ultraviolet, visible, and near infrared radiation produced by an electrical discharge in a mixture of mercury vapor and xenon under high pressure. 

Apparatus. Ultraviolet, visible and near infrared spectra were recorded with a Cary 17 spectrophotometer, y spectroscopy was carried out with a Ge-Li detector and a Zoomax (Sein) multichannel analyzer. pH measurements were taken with an Aries 10000 (Tacussel) potentiometer, a spectroscopy was carried with a solid state a detector and a (Intertechnique) multichannel analyzer. 

Figure 3.110 Ultraviolet, visible, and near-infrared light transmittance spectra of PVDF film (96.5 pm thickness). ...

In this study, an advanced textured PS/Si stmcture for the light detection in ultraviolet, visible and near infrared ranges is discussed. 

This is the so-called principal series of lines. If the lithium atoms are excited in an arc or flame, the upper states become populated and light is emitted as the atoms undergo transitions to the lower states. Four series appear in the ultraviolet, visible, and near infrared ... 

Instruments for measuring the absorption of ultraviolet. visible, and near-infrared radiation are intide up ol one or more (I) sources, (2) wavelength selectors, (3) sample containers. (4) radiation transducers, and (.5) signal processors and readout devices. The design and peifoi mance of components (2). f4). and (5) were described in considerable tletail in Oiapier 7 and thus arc not discussed further here. We will, however, consider briefly the characteristics of sources and sample containers for the region of l9l) to nm. 

Matched Absorption Cells. For routine investigations in the ultraviolet and visible regions, absorption cells of silica or glass are commercially available. Commercial products offered are (1) quartz cells usable in the ultraviolet, visible, and near-infrared range between 220 and 2400 nm and (2) fused-silica cells with extremely high... 

Energy provided by ultraviolet, visible and near infrared radiation. 

Photomultiplier Tubes (Photomultipliers, PMT) Photomultiplier tubes are extremely sensitive detectors of light in the ultraviolet, visible and near infrared (Fig. 1). These detectors multiply the signal produced by incident light by as much... 

Ultraviolet, visible, and near-infrared radiation from lamps and lasers in the laboratory can produce a number of hazards. Medium-pressure Hanovia 450 Hg lamps are commonly used for ultraviolet irradiation in photochemical experiments. Powerful arc lamps can cause eye damage and blindness within seconds. Some compounds, for example, chlorine dioxide, are explosively photosensitive. 

Slavin W (1963) Stray Light in Ultraviolet, Visible, and Near-infrared Spectrophotometry. Analytical Chemistry 35(4) 561-566. 

Brown, C. Ultraviolet, visible and near-infrared spectrophotometers. a Analytical Instrumentation Flandbook, 2nd edn. Ewing, G.W. (ed.) Marcel Dekker, Inc. New York, 1997. 

A. Fix, T. Schroder, R. Wallenstein New sources of powerful tunable laser radiation in the ultraviolet, visible and near infrared. Laser und Optoelektronik 23, 106 (1991)... 

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