Exposure, Langmuir unit

Spectra similar to the top trace in Fig. 2.37, showing the characteristic distortion of the conduction band feature, were obtained after exposure of the iron foil to 2 L (L is the Langmuir unit, 1 L = 10" Torr sec) of oxygen at 300 K. The shoulder around 4 eV is indicative of the presence of ferrous ions. These are not normally formed in the initial phase of oxidation of elemental iron. The catalyst surface is... 

All the work we describe in this chapter was carried out in UHV on the rutile Ti02(l 1 0)1 x 1 surface. Exposures to vapors and gases are given in Langmuirs (L) where 1 L = 1.333 x 10 s mbar s. Coverages of defects or molecules adsorbed at the surface will be given in monolayers (ML), where 1 ML corresponds to the density of primitive surface unit cells. 

Gas exposures are reported in units of Langmuir (1 Langmuir = 1 L = 1x10 torr-sec), uncorrected for ion gauge sensitivity and doser enhancement. Coverages, 0, are reported relative to the unreconstructed Au(lll) surface atom density (0 = 1 corresponds to 1.39x10 5 atoms/cm ). 

The flux of molecules to a surface, JN, determines either the rate at which it would be covered if each molecule stuck, or the rate of effusion through a hole in the surface. In studying the exposure, JNdt, of a surface to a gas, surface scientists find it useful to use the product of pressure and time as a measure of the exposure since this product is proportional to the number flux, JN, times the time JNt = (1/4)Cut = (u/4kT)pt, where C is the number density of molecules, u their average speed, k the Boltzmann constant and T the thermodynamic temperature. The unit langmuir (symbol L) corresponds to the exposure of a surface to a gas at 10" 6 torr for 1 second. 

One Langmuir has been suggested (157) as a descriptive unit of gas exposure. IL = 10" Torr-sec commonly corresponds to incidence of about one monolayer of gas molecules. Many authors have therefore adopted this convenient unit of exposure. 

The first term defines the simple linear partitioning process while the second term is a Langmuir expression defining distribution into the irreversible compartment as a function of compartment capacity, the fraction of this compartment ifaf) filled at the time of exposure and the distribution factor Aioc(irr), which appears to be relatively constant. The capacity of the irreversible compartment, expressed per unit of SOC, shows an inverse relation to Aiow. however, the utility of this relation will depend on further evaluation of how widely it can be applied. 

The determining parameter in these curves is the oxygen dose, defined as the exposure time multiplied by the partial pressure. Its unit is the Langmuir L. One Langmuir represents one second of exposure to a partial pressure of 133 Pa (1 torr) 1... 

The unit of exposure is the langmuir. A pressure of 1 X 10 torr applied for 1 second gives a surface an exposure to molecules that is defined as 1 langmuir (1 L, not to be confused with 1 liter) ... 

For scientists trying to study clean surfaces, this is a useful unit for communicating exposure pressure and time, both of which relate to how long a surface can really be considered clean. Roughly speaking, a monolayer of adsorbed atoms or molecules forms when a surface experiences an exposure of 1 langmuir. 

A vacuum is defined by the pressure, or lack of, within the region of interest. The SI unit for pressure is pascal (Pa) and that most commonly used in vacuum science and technology is torr. The unit mbar is also heavily used. Conversion factors between these units and other units of pressure are listed in Table 4.1. Another term of interest is the Langmuir (L). This is a unit of exposure, i.e. 1L equates to 10 Torr for 1 second. Note the pressure-time integral required to form a monolayer is 1 L assuming a surface with unit sticking coefficient. 

---