2.1  Vacuum requirements

We can quickly estimate the vacuum requirements for surface science. Let’s imagine a surface in the vacuum. The number of gas molecules impinging on the surface is

where m is the molecular mass in kg and M is the molecular mass in units of the atomic mass constant. The usual units for the pressure in vacuum technology are torr or mbar (1 torr = 1.3332 mbar = 133.32 Pascal). For a pressure of 10⁻⁶mbar and a temperature of 300K we find

As an order of magnitude value a surface has 10¹⁵ atoms per square centimetre. This means that if every rest-gas molecule at the above conditions sticks to the surface the latter will only stay clean for a second or so. If we are not willing to tolerate more than, say, a percent of contaminating rest-gas molecules on the surface then the pressure has to be in the UHV region.

It is also interesting to calculate the mean free path of the molecules at a given pressure, i.e. the mean distance before hitting another molecule. It is

where ξ is the molecular diameter. For typical UHV pressures the mean free path of the molecules is many meters. This means that it is much more likely that the molecule hits the walls of the vacuum vessel than another molecule. We come back to this later.