Metal Oxide Reactions in Complex Environments: High Electric Fields and Pressures above Ultrahigh Vacuum Page: 16
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where T is temperature, and M is the average molecular weight of the gas/surface species.
Thus, most gas/surface experiments involve changes in surface composition as a function
of exposure (= Px time), since this gives the number of single-molecule/surface
interactions. The most common unit of exposure is the Langmuir (L; 1L = 10-6 torr. sec).
Substituting P = 3 x 10-6 torr and using the values of M = 28 g/mol and T = 300 K in Eqn.
(1-7), we can see that F 1015 molecules/cm2/sec.
The residence time (x) for a molecule on a surface is related to the heat of
adsorption (AHads) by [16]
AHl
R=Troexp( " ) (1-8)
where or0 is correlated with surface atom vibrational time. T is the temperature and R is
the gas constant. A typical value for or0 is 10-12 sec [16]. Assuming the heat of adsorption
is equal to the heat of desorption obtained from temperature programmed desorption
(TPD), AHads for H20 on the surface of 5 A thick A1203/NiAl(110) is 43 KJ/mol [89].
The average surface coverage (0) of adsorbed molecules on an initially clean surface is
determined by the product of the incident flux F and the residence time i:
0 = F(1-9)
At 300 K and a partial pressure of water vapor (PH20) of 10-9 Torr, OH2o 10-
Monolayer (ML). At such low coverage, cooperative surface interaction is not possible.
MD calculations [88] indicate that H20 dissociation is stabilized by adjacent H20
molecules, which may provide a substantial probability for H20-H20 cooperative surface16
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Qin, Feili. Metal Oxide Reactions in Complex Environments: High Electric Fields and Pressures above Ultrahigh Vacuum, dissertation, August 2005; Denton, Texas. (https://digital.library.unt.edu/ark:/67531/metadc4843/m1/27/: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; .