Source ultraviolet continuum

Promotion of an electron in Hc2 from the (7 15 to a bonding orbital produces some bound states of the molecule of which several have been characterized in emission spectroscopy. For example, the configuration ((J l5 ) ((7 l5 ) ((7 25 ) gives rise to the 2i and bound states. Figure 7.24(a) shows the form of the potential curve for the state. The A-X transition is allowed and gives rise to an intense continuum in emission between 60 nm and 100 nm. This is used as a far-ultraviolet continuum source (see Section 3.4.5) as are the corresponding continua from other noble gas diatomic molecules. 

The Continuum-Source Correction Method Figure 9-14 illustrates a second method for background corrections that is widely used. In this technique, a deuterium lamp provides a source of continuum radiation throughout the ultraviolet region. The configuration of the chopper is such that radiation from the continuum source and the hollow-cathode lamp arc passed alternately through the electrothermal atomizer. The absorbance of the deuterium radiation is then subtracted from that of the analvte beam. 

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). 

Deuterium lamp A source that provides a spectral continuum in the ultraviolet region of the spectrum radiation results from application of about 40 V to a pair of electrodes housed in a deuterium atmosphere. 

In fluorescence spectrometry, the intensity of fluorescence is proportional to the intensity of the radiation source (see Section 16.15). Various continuum UV sources are used to excite fluorescence (see below). But the use of lasers has gained in importance because these monochromatic radiation sources can have high relative intensities. Table 16.5 lists the wavelength and power characteristics of some common laser sources. Only those that lase in the ultraviolet region are generally useful for exciting fluorescence. The nitrogen laser (337.1 nm), which can only be operated in a pulsed mode (rather than continuous wave, or CW, mode), is useful... 

X-ray sources, like ultraviolet and visible emitters, often produce both continuum and fine spectra both types are of importance in analysis. Continuum radiation is also called whhe radiation or hremsstrahfung. Brenisstrahlung means radiation fhat arises from retardation of particles such radiation is generally a spectral continuum. 

Atomic fluorescence flame spectrometry is receiving increased attention as a potential tool for the trace analysis of inorganic ions. Studies to date have indicated that limits of detection comparable or superior to those currently obtainable with atomic absorption or flame emission methods are frequently possible for elements whose emission lines are in the ultraviolet. The use of a continuum source, such as the high-pressure xenon arc, has been successful, although the limits of detection obtainable are not usually as low as those obtained with intense line sources. However, the xenon source can be used for the analysis of several elements either individually or by scanning a portion of the spectruin. Only chemical interferences are of concern they appear to be qualitatively similar for both atomic absorption and atomic fluorescence. With the current development of better sources and investigations into devices other than flames for sample introduction, further improvements in atomic fluorescence spectroscopy are to be expected. 

Another fruit of modern technology is the advent of synchrotron radiation from electron storage rings This produces a pulsed, polarized tunable continuum source extending into the X-ray region, having a typical pulse duration 0.4 ns and a repetit ion (1—10) MHz, and is an ideal source for time-resolved spectroscopy in the vacuum ultraviolet. The expected intensity of the system at Daresbury, for example,... 

Continuous spectral emission sources also are useful in atomic fluorescence if they have sufficient intensity. The most commonly used continuous source has been the xenon arc lamp, the 450-W xenon lamp being especially useful. These lamps emit a continuum in the visible and near ultraviolet. Their intensities, however, decrease rapidly below about 2500 A,... 

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