Mercury high pressure lamp, emission lines

Figure 2 Emission properties of a) mercury low-pressure lamp (strong 254 line), b) mercury high-pressure lamp and c) RPR-3(XX)A lamp for Rayonet photoreactors (3(X) nm line).

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]. 

High Pressure Lamp.—A lamp filled with mercury or xenon vapor (or a mixture of them) operated at about 10 or more atmospheres pressure. Although the total output is very high, the emission is largely above 3000 A. In mercury or mercury-xenon lamps it consists of broad lines superimposed on a strong continuum. The 2537 A. line is almost entirely lost by self-absorption in the lamp. 

A distinction must be made between continuous sources (hydrogen or deuterium lamps, incandescent tungsten lamps, high pressure xenon lamps) and spectral line sources (mercury lamps), which deliver spectrally purer light in the region of their emission lines. 

Mercaptoethanol reagent 380 Mercury cations 144,311 Mercury lamps 20, 22 ff emission lines 23, 24 -, high pressure 22 ff -, technical data 23 Mercury(I) nitrate reagent 337 Mercury(II) salt reagent 340 Mesaconic acid 61 Mesoporphyrin 101, 102 Metal cations 310—312,398 Metal complexes 248, 398 Methanol, dipole moment 97 Methine dyestuffs 360 4-Methoxyaniline see Anisidine 4-Methoxybenzaldehyde see Anisaldehyde Methoxybenzaldehyde derivatives 72 Methoxycinnamic acid 277... 

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. 

The most intense sources of UV radiation are the high-pressure ( 100 bar) mercury arcs. The spectral lines are broadened due to the high pressure and temperature and they are superimposed on a continuous background of radiation (Figure 3.4). While common mercury xenon [Hg(Xe)] lamps still produce significant mercury emission bands, especially in the UV region, the smoother xenon lamp spectrum finds application in environmental photochemistry experiments because of its resemblance to solar radiation (Figure 1.1). 

Osram lamps for mercury cannot be used in atomic absorption, because they contain vapor at so high a pressure that the emission line is almost completely self-absorbed, and the sensitivity is very small. It is also generally agreed that hollow-cathode lamps for thallium, zinc, and cadmium are superior to the discharge lamps. 

Mercury or xenon arc lamps are used. A schematic of a xenon arc lamp is given in Fig. 5.42. The quartz envelope is filled with xenon gas, and an electrical discharge through the gas causes excitation and emission of light. This lamp emits a continuum from 200 nm into the IR. The emission spectrum of a xenon arc lamp is shown in Fig. 5.43. Mercury lamps under high pressure can be used to provide a continuum, but low-pressure Hg lamps, which emit a line spectrum, are often used with filter fluorometers. The spectrum of a low-pressure Hg lamp is presented in Fig. 5.44. 

In the center spot of a high-current mercury discharge lamp with very high pressure, the number of photons per mode is about 10 at the center frequency of the strongest emission line at A = 253.6 mm. This shows that, even in this very bright light source, the induced emission only plays a minor role. 

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