Ultra-high vacuum

Common card · No. 436140

Ultra-high vacuum (often spelled ultrahigh in American English, UHV) is the vacuum regime characterised by pressure lower than about 1×10−9 torrs (1×10−9 mbar; 1×10−7 Pa). UHV conditions are created by pumping the gas out of a UHV chamber, and in some cases by cooling the chamber to cryogenic temperatures. In general, there are two regimes of gas flow, laminar and free molecular, defined by whether the dominant interactions are particle-particle (laminar) or particle-chamber (molecular). At UHV pressures the mean free path of a gas molecule is greater than 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, on average. Almost all molecular interactions therefore take place on various surfaces in the chamber. 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...

Read on Wikipedia