Low pressure lamps

In contrast to the low-pressure lamps (1—130 Pa) which primarily emit at the resonance line at A = 254nm, high-pressure lamps (lO —10 Pa) also produce numerous bands in the UV and VIS regions (Fig. 16). Table 3 lists the emission lines and the relative spectral energies of the most important mercury lamps (see also [44]). The addition of cadmium to a mercury vapor lamp increases the numbei of emission lines particularly in the visible region of the spectrum [45] so that it i. also possible to work at A = 326, 468, 480, 509 and 644 nm [46]. 

Fig. 16 Relative intensity distribution of a mercury NK 4/4 low pressure lamp (A) and of a mercury St 41 or St 48 lamp (B).

The dimensions and properties of the lamp envelope are based on the discovery that the volume of Hg is critical for the effective UV operation [43]. Higher Hg pressures result in the need to use higher MW power levels. To focus the MW field efficiently into the EDL a special Cd low-pressure lamp with a metal antenna (a molybdenum foil) was developed by Florian and Knapp [44],... 

The addition of bis(2,6-diisopropylphenyl)silylene, as a reactive divalent species, to CgQ or Cyo yields the [2+1] cycloadduct 351 as the ring-closed 1,2-bridged isomer (Scheme 4.73) [405-407]. The silylene was prepared in situ from the trisilane 350 by photolysis with a low-pressure lamp in toluene solution. 

Low-pressure mercury lamps consist most frequently of a quartz cylinder with electrodes on both ends, although other shapes are also produced. Inside the lamp is a mixture of mercury and argon at a pressure of 10 to 10 Torn The emission from this lamp is 254 nm, and with high-quality quartz, some light with 189 nm wavelength is produced. Low-pressure lamps are of low power, and therefore are not used for the cure of coatings, but are well suitable for applications where slow cure rate is tolerated, such as liquid crystal displays or in resist technology for the production of microchips. 

Resonance Lamp.—Such lamps (sometimes called low pressure lamps) are often used as line sources in photochemical studies. These usually contain a small amount of a metal vapor (e.g., mercury, cadmium, zinc, etc.) and several mm pressure of a rare gas. They operate at relatively low current (ca. 100 ma.) and high voltages (several thousand volts). This is in contrast to a typical medium pressure lamp which may operate off a 110-220 v. power supply delivering ca. 3-5 amp. The most common example in photochemistry is the mercury resonance lamp which has strong emission of the unreversed resonance lines at 2537 A. and 1849 A. (ca. 90% or more of the total) along with other, much weaker lines ( resonance lines are those which appear both in absorption and emission). There is little continuum. Sources of this type are widely used for photosensitized reactions. 

Both lamps generate ozone and oxides of nitrogen in air, hence the inner lamp area should be flushed slowly with nitrogen via G and H as described above. It should be noted however that low-pressure lamps give maximum light output at a wall temperature of 40 °C so excessive cooling in the lamp region is... 

Photodecomposition of the benzenesulfonamide derivative 183 in aqueous solution was investigated by irradiation from a 254 nm low pressure lamp under a helium atmosphere. Homolytic fission of the S-N bond gives arylsulfonyl and arylsulfonylamino radicals. Gyclization from the latter radical via C-N bond formation and hydrogen atom abstraction afforded the /3-sultam 47 in 7% yield. Other photodissociation processes resulted in the formation of compounds 184-186 (Scheme 59) <2002JPH109>. 

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]. 

Benzoyl azide (1 g, 6.8 mmol) and 6-methoxy-5,6-dihydropyran (11.4 g, 100 mmol) dissolved in acetonitrile (100 mL) are placed in 10 quartz test tubes and irradiated using light of wavelength 254 nm (mercury low pressure lamp) for 9h using a merry-go-round photoreactor. After removing the solvent and the olefin excess by vacuum evaporation, the reaction mixture... 

Lamps containing free metallic sodium (e.g. sodium vapour low pressure lamps, see 5.1 in Table 6.23, Section 6.7.4) are not permitted. 

In Table 6.23, a survey of lamps is given for general lighting in industrial plants. Sodium vapour low pressure lamps (point 5.1 in Table 6.23) are not within the scope of the standards for explosion protected luminaires. Incandescent lamps (1.1 in Table 6.23) and blended lamps (2 in Table 6.23) are unimportant for general lighting applications in industry due to their poor luminous efficiency. Therefore, the following luminaire-lamp combinations are the state of the art ... 

Depending on the pressure in the lamp, both line and continuum emission may be observed. Low-pressure lamps operated at low current densities and temperatures produce sharp atomic lines with little or no continuous background. Increasing pressure and temperature cause the lines to broaden and increase the intensity of the background continuum. 

UvlmerTM 530 and the other members of the UvlmerTM series may be cured with benzoin ether type photoinitiators and with diethoxy acetophenone. Table III presents data on the cure speed of UvlmerTM 530 formulated with butyl benzoin ether. These data were obtained in air without the use of Inert blanketing. Films of 3 mil thickness were irradiated on a moving belt using a 5000 watt Addalux uv lamp (Berkey Photo, Inc.) focussed on one linear Inch, This Is a low pressure lamp Incorporated In an experimental unit. Faster cure speeds (up to 200 ft. per mlnute/lamp) were obtained in later studies with a unit employing two medium pressure, 200 watts/llnear inch Hanovia lamps. 

The most common sources of radiation for kinetic photochemical studies are mercury lamps of which there are three types. The low-pressure lamp is used mainly for mercury-sensitised studies. For general photochemical studies the most useful lamp is the medium-pressure lamp having several lines of reasonable... 

Low-pressure lamps. These lamps operate at or close to room temperature. This means that the vapour pressure of Hg is 10 torr. In addition to mercury about 6 torr of inert gas, usually neon, is added. This makes for easy firing but there is some evidence that a higher intensity is produced due to reaction (13). 

The lamps may be operated on AC or DC, but for polymerisation studies and radical lifetime measurements DC must be used. For a low-pressure lamp the intensity falls to nearly zero 120 times per second for a 60 cycles AC source " the high-pressure lamp also tends to be unstable with AC . Circuits (Fig. 35) are shown for medium-pressure and high-pressure lamps operating procedures are given quite clearly in these last two references. The low-pressure lamp reaches stable operating conditions quite quickly, but the other two types require some time to do so (10-15 min). The high-pressure lamp also has a very short lifetime. 

Fig. 34. Effect of current and mean pressure (torr) on output for a low-pressure lamp. From ref. 115.

Likewise, explorative studies are conveniently carried out in very simple apparatus. This may be built with the lamp(s) outside a reaction vessel (made of a material transparent to the desired wavelength, e.g., Pyrex absorbs below ca. 300 nm), including test tubes, and irradiating by means of some external low-pressure lamps (10-20 W, see above). Alternatively, with the lamp inside, a vessel fitted with a refrigerated immersion well (of the correct material) is used in which is inserted a mercury arc (125 or 400 W - the former is sufficient for the great majority of cases). 

In contrast to the low-pressure lamps (1 — 130 Pa) which primarily emit at the resonance line at i = 54nm, high-pressure lamps (10 —10 Pa) also produce... 

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