# Why You Need a High Vacuum for a VUV System

Wavelengths below the Ultra Violet range are given several different names. However, due to the absorption of the air down to very few nanometers and the need to evacuate any part of the environmental experiment, this particular spectral range is known as Vacuum Ultra Violet (VUV).

VUV regions are encompassed on either side by the ultraviolet (UV) spectral region (from 200 nm) or the longer wavelength side of the hard X-ray region (around 1 nm). There are three key regions that make up the specific Vacuum Ultra Violet domain. These are:-

• The Far Ultra Violet (FUV): 200-120 nm
• The Extreme Ultra Violet (EUV): 120-10 nm
• The Soft X-Rays (Soft X): 15-0.2 nm

Each of these domains possesses its own physical meaning:-

The Far UV range that ends around 120 nm refers to the natural barrier stopping the transmission of radiation through any window material. Optics in X-rays have poor reflectivity, meaning that they need to use large deviation angles of up to 170° and therefore very low grazing angles.

Figure 1. Vacuum Ultra-Violet domain.

Where Vacuum Ultra Violet radiation is blocked by many of the gases present in the atmosphere, these are able to propagate at least partially via transparent gases such as hydrogen or nitrogen or, most ideally, totally through a vacuum.

To achieve the best results, VUV analysis needs to use vacuum technologies that are of the High Vacuum (~10-6 mbar, 10-4 Pa) level, through to the Ultra High Vacuum (~10-9 mbar, 10-7 Pa).

## What Units are Used in VUV?

### 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

A vacuum’s quality is directly linked to the number of particles remaining within the system. Other factors include the average distance that gas molecules must travel before colliding with each other. This particular factor is the key determinant in the vacuum level itself. Increasing the distance between molecules by pumping the chambers therefore improves the vacuum’s quality.

### Pressure

Pascal (Pa) is the International System of Units (SI) for pressure. This is equal to a force of 1 N/m2. The CGS unit known as the Bar (bar) is frequently used and it is also possible to measure vacuum pressure in torrs (Torr) or atmospheres (Atm).

Professionals tend to prefer units such as hectoPascal (hPa) and millibars (mbar) for conversions, as these are simpler to convert to torrs with an approximate factor. The approximations for the unit conversion highlighted below are the ones that are most 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

Most VUV applications can be conducted at the High Vacuum level (10-5 to 10-6 mbar or 10-3 to 10-4 Pa) where a none strict vacuum is targeted, therefore leaving free passage of electronics, light and particles. This is suitable for the deposition of thin films, spectral emissions, semiconductors and most of the characterizations obtained by absorbance or reflectance methods.

Figure 2. High vacuum (HV).

More specialist research domains such as Free Electron Lasers or Synchrotrons, electron sciences or fusion studies are likely to utilize the Ultra High Vacuum. This is especially the case where there may be an issue with residual gas or molecules causing contamination.

Figure 3. Ultra high vacuum (UHV).

This information has been sourced, reviewed and adapted from materials provided by HORIBA Scientific.

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