There are several names given to wavelengths below the Ultra Violet. However, because of the absorption of the air down to few nanometers, and the need to evacuate any part of the environmental experiment, this spectral range is called Vacuum Ultra Violet (VUV).
The VUV regions are surrounded by “above” the ultraviolet (UV) spectral region (from 200 nm) and the long wavelength side of the Hard X-Ray region below (around 1 nm).
Three main regions make up the Vacuum Ultra Violet domain:
These domains have their own physical meanings:
The Far UV ending at around 120 nm, corresponds to the natural barrier of the transmittance of radiation through any window material. The poor reflectivity of optics in soft X-Rays requires the use of large deviation angles up to 170° (extremely low grazing angles).
Fig. 1: Vacuum Ultra-Violet domain
If Vacuum Ultra Violet radiation is blocked by most of gases of the atmosphere, they can propagate partially through transparent gas such as nitrogen, hydrogen or totally through a vacuum. Therefore, VUV analysis requires vacuum technologies of the High Vacuum (~10-6 mbar, 10-4 Pa) level to the Ultra High Vacuum (~10-9 mbar, 10-7Pa).
Energy
The energy E, the frequency n, and the wavelength λ of a
photon are related by:
E = hν = hc/λ
Electron Volt conversion: E (eV) = 1240/ λ (nm)
Vacuum and vacuum units
The quality of the vacuum is related to the amount of particles remaining in a system.
The average distance that gas molecules travel before collisions with each other characterizes the vacuum level. Increasing this distance by pumping the chambers improves the quality of the vacuum.
Pressure
The International System of Units (SI) of the pressure is the Pascal (Pa), equal to a force of 1 N/m2, but the CGS unit called Bar (bar) is largely employed, too. The vacuum pressure can also be measured in torrs (Torr) or in atmospheres (Atm). For conversions, professionals prefer to use units such as millibars (mbar) and hectoPascal (hPa) which are the easiest way to convert those units in torrs with an approximate factor. The below approximations for the unit conversion is commonly accepted.
Vacuum range
Low (rough) vacuum Atmospheric pressure to 1 mbar
Medium vacuum 1 to 10-3 mbar
High vacuum (HV) 10-3 to 10-8 mbar
Ultrahigh vacuum (UHV) 10-8 to 10-12 mbar
Extreme High vacuum (EHV) Less than 10-12 mbar
High vacuum (HV)
A level of the High Vacuum of about 10-5 to 10-6 mbar (10-3 to 10-4 Pa) is sufficient for most of the VUV applications applied for deposition of thin films, semiconductors, most of the characterizations by reflectance or absorbance methods, and spectral emissions … where a none strict vacuum is targeted, leaving free passage of light, particles and electrons.
Ultra high vacuum (UHV)
The Ultra High Vacuum is more dedicated to the research domains, such as fusion studies, electron sciences, Free Electron Lasers or Synchrotrons, where contaminations with molecules of residual gas may be an issue in the experiments are good examples.
