<P> Ultra-high vacuum (UHV) is the vacuum regime characterised by pressures lower than about 10 pascal or 100 nanopascals (10 m bar, ~ 10 torr). UHV conditions are created by pumping the gas out of a UHV chamber . At these low pressures the mean free path of a gas molecule is approximately 40 km, so the gas is in free molecular flow, and gas molecules will collide with the chamber walls many times before colliding with each other . Almost all molecular interactions therefore take place on various surfaces in the chamber . </P> <P> UHV conditions are integral to scientific research . Surface science experiments often require a chemically clean sample surface with the absence of any unwanted adsorbates . Surface analysis tools such as X-ray photoelectron spectroscopy and low energy ion scattering require UHV conditions for the transmission of electron or ion beams . For the same reason, beam pipes in particle accelerators such as the Large Hadron Collider are kept at UHV . </P> <Ul> <Li> Sorption of gases </Li> <Li> Kinetic theory of gases </Li> <Li> Gas transport and pumping </Li> <Li> Vacuum pumps and systems </Li> <Li> Vapour pressure </Li> </Ul>

Why mass spectrometric analysis is performed under ultra high vacuum
find me the text answering this question