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.
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.
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