4.1 What is Ultra High Vacuum (UHV) ?
Vacuum technology has advanced considerably over the last 25 years and very low pressures are now routinely obtainable.
Firstly, we need to remind ourselves of the units of pressure :
Whilst the mbar is often used as a unit of pressure for describing the level of vacuum, the most commonly employed unit is still the Torr. (The SI unit, the Pa , is almost never used ! )
Classification of the degree of vacuum is hardly an exact science - it very much depends upon who you are talking to - but as a rough guideline :
Rough (low) vacuum :
1 - 10-3 Torr
Medium vacuum :
10-3 - 10-5 Torr
High vacuum (HV) :
10-6 - 10-8 Torr
Ultrahigh vacuum (UHV) :
< 10-9 Torr
Virtually all surface studies are carried out under UHV conditions - the question is why ? This is the question that we will address in Section 4.2 .