Langmuir (unit)

Langmuir is a measure of gas exposure (or dosage) to a surface ("e.g." of a crystal) and is used in ultra-high vacuum (UHV) surface physics. Assuming that every gas molecule which hits the surface sticks to it ("i.e.," the sticking coefficient is 1.00), one Langmuir (1 L) leads to a coverage of about one monolayer of the adsorbed gas molecules on the surface. The exact definition is:

: 1 L = 10⁻⁶ Torr s

For example, exposing a surface to a gas pressure of 10⁻⁸ Torr for 100 seconds corresponds to 1 L.Similarly, keeping the pressure of oxygen gas at 2.5 10⁻⁶ mbar for 53 seconds will give a dose of 100L.

In general, the sticking coefficient varies depending on the reactivity of the surface and the molecules, so that the Langmuir gives a lower limit of the time it needs to completely cover a surface.

The Langmuir definition is a very intuitive way of explaining why ultra-high vacuum must be used to study solid-state surfaces, nanostructures or even single molecules. The typical time to perform physical experiments on sample surfaces is in the range of one to several hours. In order to keep the surface free of contaminations, the pressure of the residual gas in a UHV chamber should be below 10⁻¹⁰ Torr.

References

H. Lueth, "Surfaces and Interfaces of Solid Materials" (3^rd edition, Springer Berlin Heidelberg New York, 1997).