Source ultraviolet line

Figure 10-3 shows the basic features of a hollow cathode lamp source. Here A is the anode (the plus electrode) and C is the cathode, terminated in the lamp as a hollow cup. The anode can be a wire, such as tungsten, and the cathode cup may be constructed from the element whose spectrum is desired or it may be an inert material into which the desired element or a salt of the desired element is placed. The lamp envelope is made of glass and IT is a window of suitable properties. If an ultraviolet line spectrum is desired, the window may be quartz or a high silica glass. The hollow cathode has an inert gas present, usually neon or argon, at low pressure. 

In the ultraviolet and visible region of the spectrum, however, most laboratory sources emit lines whose shape and width are determined mainly by the Doppler effect due to the random thermal motion of the emitting atoms. 

Until the advent of lasers the most intense monochromatic sources available were atomic emission sources from which an intense, discrete line in the visible or near-ultraviolet region was isolated by optical filtering if necessary. The most often used source of this kind was the mercury discharge lamp operating at the vapour pressure of mercury. Three of the most intense lines are at 253.7 nm (near-ultraviolet), 404.7 nm and 435.7 nm (both in the visible region). Although the line width is typically small the narrowest has a width of about 0.2 cm, which places a limit on the resolution which can be achieved. 

A monochromator is useful not only for removing unwanted lines from the X-ray source but also for narrowing the otherwise broad lines. For example, each of the MgXa and AlXa doublets is unresolved and about 1 cY wide at half-intensity. A monochromator can reduce this to about 0.2 cY This reduction of the line width is very important because in an XPS specttum, unlike an ultraviolet photoelectron specttum, the resolution is limited by the line width of the ionizing radiation. Unfortunately, even after line narrowing to 0.2 cY... 

Dye lasers, frequency doubled if necessary, provide ideal sources for such experiments. The radiation is very intense, the line width is small ( 1 cm ) and the wavenumber may be tuned to match any absorption band in the visible or near-ultraviolet region. 

Ultraviolet light sources are based on the mercury vapor arc. The mercury is enclosed ia a quart2 tube and a potential is appHed to electrodes at either end of the tube. The electrodes can be of iron, tungsten, or other metals and the pressure ia a mercury vapor lamp may range from less than 0.1 to >1 MPa (<1 to >10 atm). As the mercury pressure and lamp operating temperatures are iacreased, the radiation becomes more iatense and the width of the emission lines iacreases (17). 

Because of the potential hazard of release of unignited hydrocarbons at ground level, a flame scanner with alarm in the control house is included for each pilot. The flame scanner must be located so that interference of ultra violet rays from the main flame or other sources do not cause false readings. Ultraviolet detectors should be mounted such that they are looking straight down through the pilots toward the ground. The installation should also provide strainers in each gas or oil line to pilots. 

The ultraviolet (UV) absorption HPLC detector is basically a UV spectrophotometer that measures a flowing solution rather than a static solution. It has a light source, a wavelength selector, and a phototube like an ordinary spectrophotometer. The cuvette is a flow cell, through which the column effluent flows. As the mobile phase elutes, the chromatogram traces a line at zero absorbance, but when a mixture... 

Perhaps all the elements present in the periodic table might be excited to yield respective emission spectra by employing a huge energetic source. However, it has a serious drawback because most of the spectral lines invariably fall within the vacuum-ultraviolet region thereby rendering their critical studies rather difficult. Hence, the emission spectroscopy is exclusively limited to metals and metalloids. The non-metals, for instance Phosphorus, Sulphur, Carbon etc. are not limited to these studies. 

Makovsky et al. (70) and Low (77, 72) succeeded in exciting optical-fluorescence spectra of rare earths and transition metals by means of X rays. All the crystals studied showed a wealth of fluorescent lines. In general, they observed emissions from 200 A in the ultraviolet to 20,000 A in the infrared. Many fluorescences were observed that could not be excited optically. By pulsing the X-ray source for 1 /xsec, they were able to measure decays from levels with the longer lifetimes. They point out that the X-ray beam could be pulsed for times on the order of 10 9 sec, thus allowing much shorter decay times to be measured. It is also possible to modulate the X-rays... 

Figure 6.46 Representation of two mechanisms of repair of DNA damage such as ultraviolet light-induced dimerization. The upper line represents cut and patch repair, the lower sister-strand exchange. Thick lines represent newly synthesized DNA. Source From Ref. 12.

Before the laser, the light sources used in infrared, visible, and ultraviolet spectroscopy were heated solids or gas discharge tubes. These sources are based on spontaneous emission. Such light is emitted in random directions with random phases. Even lines of gas discharge tubes lack true monochromaticity due to pressure-, Doppler-, and naturalbroadening. Thus light from these traditional sources does not possess the above-mentioned four qualities of laser light, which is based on stimulated emission. 

Medium Pressure Lamp.—A lamp filled with mercury vapor and operated at a pressure of about 1 atmosphere. The total intensity in the near ultraviolet and visible is lower than that of high pressure lamps, but photochemically useful light at wavelengths less than 3000 A. is produced. The ultraviolet spectrum consists of reasonably narrow lines with only a weak continuum. Hence, in conjunction with a filter or monochromator it is a good source for monochromatic radiation including reversed 2537-A. radiation (see definition below). 

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