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Extreme UV

The portion of the electromagnetic spectrum that can be directly viewed by the human eye is rather small. The colour corresponding to the shortest wavelength we can observe is violet. The part of the electromagnetic energy with a little higher photon energy is called the Ultra Violet region. This light is still transmitted by the air, and we have to protect our eyes and skin from exposure to this light usig sun glasses or aother absorbing materials. "Ordinary glass quickly adsorbs this light. Optical systems designed for the UV musttherefore be designed using quartz-glas which is not only transparent in the visible but also in the UV reagion. At even shorter wavelengths quartz glass must also be avoided, because it absorbs light with a wavelength shorter then 200 nm. In this spectral region (200nm-100nm) only few materials with interest for optics design are available. Magnesioum di flouride and Lithium flouride are two examples.

Light absorption in air

Air does not transmit wavelengths below about 190 nm, because oxygen and water vapour are highly absorbent at these short wavelengths. Some of the strongest emission lines for some very important elements are in this region, notably H I 121.5 nm, O I 130.2 nm, Cl I 133.5 nm & 134.7 nm, N I 149.2 nm, C I 156.1 nm & 165.7 nm, P I 177.4 nm & 178.2 nm and S I 180.7 nm, where I means the neutral atom.

Oxygen Absorption

To use these lines we need to eliminate oxygen and water vapour from both the spectrometer and the optical path to the emission source. This can be achieved either by evacuating the optical system or by purging the system with a gas not containing oxygen or water vapour, the obvious choice is nitrogen, and today about 50% of all new spectrometers working below 200 nm are evacuated and about 50% nitrogen-purged.

N2 Generator

Nitrogen-purging eliminates contamination from vacuum pumps and simplifies alignment of the spectrometer above 200 nm.

People have wondered how it is possible to detect nitrogen signals when there is nitrogen purge-gas in the spectrometer. The answer is that the nitrogen signals comes from nitrogen atoms in the source while the purge gas in the spectrometer is molecular N2 and the energy levels in atomic nitrogen and molecular nitrogen are sufficiently different to prevent nitrogen molecules absorbing emission lines from nitrogen atoms.

First published on the web: 15 December 1999.

Author: Richard Payling