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.
The langmuir is defined by multiplying the pressure of the gas by the time of exposure.
[1][2] 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. Since both different pressures and exposure times can give the same langmuir (see Definition) it can be difficult to convert Langmuir (L) to exposure pressure × time (Torr·s) and vice versa.
is an integer allowing different magnitudes of pressure or exposure time to be used in conversion.
Using the prior example, for a dose of 100 L a pressure of 2.5 × 10−6 Torr can be applied for 40 seconds, thus,
Exposure of a surface in surface physics is a type of fluence, that is the integral of number flux (JN) 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:[3] where C is the number density of the gas, and
Hence The proportionality between number flux and pressure is only strictly valid for a given temperature and a given molecular mass of adsorbing gas.
However, the dependence is only on the square roots of m and T. Gas adsorption experiments typically operate around ambient temperature with light gases, and so the langmuir remains useful as a practical unit.
Assuming that every gas molecule hitting the surface sticks to it (that is, the sticking coefficient is 1), one langmuir (1 L) leads to a coverage of about one monolayer of the adsorbed gas molecules on the surface[citation needed].
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.
This also illustrates why ultra-high vacuum (UHV) 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.