VACUUM ULTRAVIOLET SOURCES
RESONANCE LINE SOURCES
OPTHOS resonance line sources are electrodeless lamps filled with H, Kr, or Xe gas at pressures of about 1 Torr. They provide intense sources of radiation at the wavelengths of the resonance lines of the filling gas. Lamps made with mercury or iodine as the filling material are also available.
Lamps are excited by a microwave generator such as the OPTHOS GMS200 2450 MHz generators. The microwave energy is coupled to the discharge by means of an antenna or tuned cavity.
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Fig. 1 shows a rare gas VUV lamp. The lamp is made to fit a standard 1 1/8" quick-disconnect, shown in the figure, thus permitting rapid mounting and demounting of the lamp from spectrographs or other chambers. When the lamp is mounted in this way a stop should be used inside the disconnect to prevent the lamp from being pulled in by the vacuum.
The lamp housing consists of a quartz body to which a suitable window is sealed. Crystal windows such MgF2 or LiF are sealed with an epoxy having extremely low vapor pressure. Quartz windows (Suprasil) are fused directly to the body. Rare gas lamps have a side arm containing an evaporated getter material that maintains high gas purity in the lamp over a long duration. Because gettering action is stimulated by running the discharge, the spectral purity of these lamps actually improves with use. By the same token, storage of lamps for long periods without use is not recommended. Hydrogen lamps are made with the same body, but have no getter.
Most of our rare gas lamps use MgF2 windows. MgF2 has excellent transmission down to 1100 A and is not degraded by irradiation from high energy photons, as is LiF. MgF2 has the added advantage of being harder and less hydroscopic than LiF.
In common usage lamps are run with an Evenson cavity placed between the side arm and the enlarged portion near the front of the lamp. The discharge takes place in the central tube; diameter 12 mm, length 150mm.
About 20 watts of microwave power usually suffice to operate resonance line lamps. Reports in the literature [H. Okabe, J. Opt. Soc. Am. 54, 478 (1964)] indicate that intensities of up to 10 15 photons/sec can be obtained with lamps of this type. The VUV spectrum of a krypton resonance line lamp is reproduced in Fig. 2. Details of resonance line lamps are given in Table 1.
CONTINUUM SOURCES
OPTHOS continuum sources are electrodeless lamps filled with rare gases at pressures of about 200 Torr. At this pressure the discharge appears as one or more filaments running along the length of the lamp. The lamp housing is the same as is used for resonance line lamps. An Evenson cavity with 20-50 watts of microwave power is usually used for excitation.
Continuum lamps are made with filling gases of Ar, Kr or Xe. The properties of the continua of these gases are shown in Fig. 3. The curves have been normalized to show the shapes and general locations of the continua. Further details are given in Table 2.
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