Langmuir (unit)

The langmuir (symbol: L) is a unit of exposure (or dosage) to a surface (e.g. of a crystal) and is used in ultra-high vacuum (UHV) surface physics to study the adsorption of gases. It is a practical unit, and is not dimensionally homogeneous, and so is only used in this one field. It is named after American physicist Irving Langmuir.

The langmuir is defined by multiplying the pressure of the gas by the time of exposure. One langmuir corresponds to an exposure of 10⁻⁶ Torr during one second. 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 Torr for 40 seconds will give a dose of 100 L.

Exposure of a surface in surface physics is a type of fluence, that is the integral of number flux (J_N) with respect to exposed time (t) to give a number of particles per unit area (Φ):

The number flux for an ideal gas, that is the number of gas molecules passing through (in a single direction) a surface of unit area in unit time, can be derived from kinetic theory:

where C is the number density of the gas, and ${\displaystyle {\bar {u}}}$ is the mean speed of the molecules (not the root-mean-square speed, although the two are related). The number density of an ideal gas depends the thermodynamic temperature (T) and the pressure (p):

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