Microwave discharge lamps

The related method of atomic resonance fluorescence—the measurement of intensity of fluorescence excited by absorption of resonance radiation—has several advantages over resonance absorption for kinetic studies of reactions of ground state atoms. When the usual strongly self-reversed microwave discharge lamps are used as the sources of resonance radiation, resonance fluorescence is much more sensitive than resonance absorption. The following lower limits of concentration detectable by resonance fluorescence have been found in this laboratory for particular instrumental conditions [Cl] 5 x 10 g atomcm ... 

UV radiation, certainly not sufficient to disrupt the bonds of common organic molecules. We therefore assume that, essentially, photoinitiation is responsible for a chemical change and MW radiation subsequently affects the course of the reaction. The objective of microwave photochemistry is frequently, but not necessarily, connected to the electrodeless discharge lamp (EDL) which generates UV radiation when placed in the MW field. 

A microwave-assisted, high-temperature, and high-pressure UV digestion reactor has been developed by Florian and Knapp [44] for analytical purposes. The apparatus consists of the immersed electrodeless discharge lamp operating as a result of the MW field in the oven cavity (Fig. 14.8). An antenna fixed to the top of EDL enhanced the EDL excitation efficiency. Another interesting MW-UV reactor has... 

Where vapour discharge lamp sources exist (for volatile elements such as Hg, Na, Cd, Ga, In, T1 and Zn) they can be used. Hollow-cathode lamps are insufficiently intense, unless operated in a pulsed mode. Microwave-excited electrodeless discharge lamps are very intense (typically 200-2000 times more intense than hollow-cathode lamps) and have been widely used. They are inexpensive and simple to make and operate. Stability has always been a problem with this type of source, although improvements can be made by operating the lamps in microwave cavities thermostated by warm air currents. A typical electrodeless discharge lamp is shown in Fig. 6.3. 

Currently, there are two types of commercial excimer lamps barrier-discharge- and microwave-driven lamps. 

Kl4n, P., H4jek, M. and Cirkva, V., The electrodeless discharge lamp a prospective tool for photochemistry, Part 3 the microwave photochemistry reactor, /. Photochem. Photobiol, A Chem., 2001, 140, 185. 

Disilene and its isomer silylsilylene were neither available by standard vacuum flash pyrolysis of precursors 59-63, nor by the more elaborate method of pulsed flash pyrolysis of 60-63, a pulsed discharge in mixtures of argon and mono- and disilane74 or by the matrix photolysis of educts 59-66 using various light sources (Hg lamps, excimer laser)69,70,72, the microwave discharge in disilane 66 or the cocondensation of silicon atoms with SiFLt. 

One subcategory of low-pressure lamp that might become more important in the future is the electrodeless discharge lamp, which is energized by an external field. These lamps comprise a quartz tube that has been evacuated, leaving behind a small pressure of argon and mercury or other metal or metal halide. Emission is obtained by placing the lamp in a microwave field, for example. Whilst these lamps are available commercially, they may also be built in-house rather easily [5]. 

For some elements such as arsenic and selenium, which have their main atomic absorption wavelengths lying on the edge of the vacuum UV, the performance of hollow cathode lamps is often poor, the lamps displaying low intensity and poor stability. This, plus the search for more intense sources for AFS (see Chapter 1, section 10), resulted in the development of microwave-powered electrodeless discharge lamps (EDLs) as spectral line sources towards the end of the 1960s.3-5... 

Another approach to the production of UV photons includes the development of electrode-less discharge lamps driven by microwave excitation (e.g. Fassler et al., 2001, Ametepe et al., 1999, He et al., 1998). This type of lamp is shown in Fig. 4-17. In this case, the excitation of mercury vapor within the discharge gap is achieved by coupling in the energy with a water-cooled high-frequency spool. This concept may be a very convenient tool for microwave photochemistry experiments by simultaneous combination of microwave and VUV/UV irradiation of aqueous systems (c.f Klan et al., 2001, 1999). 

Kitamura M, Mitsuka K, Sato H (1994) A Practical High-Power Excimer Lamp Excited by a Microwave Discharge, Appl. Surf. Sci. 79/80 507-513. 

Very recently, a novel microwave-assisted high-temperature UV digestion system for accelerated decomposition of dissolved organic compounds or slurries was developed [95, 96]. The technique is based on a closed, pressurized, microwave decomposition device wherein UV irradiation is generated by immersed electrodeless Cd discharge lamps (228 nm) operated by the microwave field in the oven cavity. The immersion system enables maximum reaction temperatures of up to 250-280 °C, resulting in a tremendous increase in mineralization efficiency. 

Gunning and his coworkers (429a) have in fact succeeded in enriching he reaction product of " Hg preferentially excited with a lamp containing " Hg operated in a microwave discharge. Upon illumination of mixtures if natural mercury and HCI by the ° Hg lamp, the maximum fractional sotopic abundance of HgCl (calomel) obtained is 0.45 with intermittent... 

Electrodeless discharge lamp A source of atomic line spectra that is powered by radio-frequency or microwave radiation. Electrode of the first kind A metallic electrode whose potential is proportional to the logarithm of the concentration (strictly, activity) of a cation (or the ratio of cations) derived from the electrode metal. 

In the hollow cathode lamps of these relatively volatile elements self-absorption at low discharge currents may also be considerable and even self-reversal may take place. This is not the case with electrodeless discharge lamps. They consist of a quartz balloon in which the halogenide of the element is present. The analyte spectra are excited with the aid of a high-frequency (MHz range) or a microwave field (GHz range), supplied e.g. through an external antenna. 

Fig.no. c low discharge lamp with integrated microwave cavity for atomic emission spectrometry. (Reprinted with permission from Ref. [362].)... 

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