The surface of a solid which is exposed to atmosphere is significantly more complex than the ideal, clean reconstructed or relaxed surfaces. On exposure to the atmosphere, atoms and molecules will adsorb on the surface generally destroying any well-ordered surface structure.

The presence of the contaminant layer prohibits any controlled, systematic investigation of surface properties. Hence, we must work at very low pressures (i.e. ultra high vacuum (UHV)) to maintain clean surfaces for any length of time.

And this is only the first reason.

Ultra high vacuum is required for most surface science experiments for two principal reasons :

1. To enable atomically clean surfaces to be prepared for study, and such surfaces to be maintained in a contamination-free state for the duration of the experiment.
2. To permit the use of low energy electron and ion-based experimental techniques without undue interference from gas phase scattering.

How long will it take for a clean surface to become covered with a complete monolayer of adsorbate ?

This is dependent upon the flux of gas phase molecules incident upon the surface, the actual coverage corresponding to the monolayer and the coverage-dependent sticking probability ... however , it is possible to get a minimum estimate of the time required by assuming a unit sticking probability (i.e. S = 1) and noting that monolayer coverages are generally of the order of 1015 per cm2 or 1019 per m2 . Then

Time / ML ~ ( 1019 / F ) [ s ]

Then at 10^-10 Torr time per 1 monolayer is 10⁴ seconds, at 10^{-6 Torr} 1 second, and at atmospheric conditions it is 10^-9 of second.

Generally, following requirements exist for :

Collision Free Conditions => P < 10-4 Torr
Maintenance of a Clean Surface => P < 10-9 Torr

For surface spectroscopy, the mean free path of probe and detected particles (ions, atoms, electrons) in the vacuum environment must be significantly greater than the dimensions of the apparatus in order that these particles may travel to the surface and from the surface to detector without undergoing any interaction with residual gas phase molecules. This requires pressures better than 10-4 Torr. There are, however, some techniques, such as IR spectroscopy, which are "photon-in/photon-out" techniques and do not suffer from this requirement.
(On a practical level, it is also the case that the lifetime of channeltron and multiplier detectors used to detect charged particles is substantially reduced by operation at pressures above 10-6 Torr).

In order to begin experiments with a reproducibly clean surface, and to ensure that significant contamination by background gases does not occur during an experiment, the background pressure must be such that the time required for contaminant buildup is substantially greater than that required to conduct the experiment i.e. of the order of hours. The implication with regard to the required pressure depends upon the nature of the surface, but for the more reactive surfaces this necessitates the use of UHV (i.e. < 1 x 10-9 Torr).

For specimen preparation and growth techniques (e.g. Molecular Beam Epitaxy (MBE)) UHV conditions required to ensure that a very low level of contaminants is present, since it involves the layer-by-layer growth of atomically precise material layers.