Molecular Beam Induced Dissociative Chemisorption at Surfaces
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MOLECULAR BEAM INDUCED DISSOCIATIVE CHEMISORPTION AT SURFACES
ROBERT J. MADIX, Departments of Chemical Engineering and Chemistry, Stanford University, Stanford, CA 94305 ABSTRACT The application of supersonic nozzle beams to the study of the dynamics
of gas-surface scattering is
Examples are chosen to
discussed.
illustrate the use of such beam sources in
synthesizing surface compounds
otherwise unaccessible under normal ambient conditions in
ultrahigh
vacuum and the role of trapping and direct collisional activation in adsorption on surfaces. MOLECULAR BEAM SOURCES Molecular beams are particularly useful for the control of dynamical reactions at surfaces.
parameters that may effect the rates of chemical
There are two basic types of sources for the production of beams of neutral particles,
effusive and supersonic (nozzle) beams.
source consists of a hole in dimension of the hole is vapor in
the chamber.
cell occurs,
smaller than the mean free path of the gas or
Under such conditions free molecular flow from the
and the effusing molecules directly sample the equilibrium
distribution of energies and species in is
An effusive
small chamber such that the characteristic
the source chamber.
Thus,
there
a Boltzmann-like distribution of velocities and a Boltzmann
distribution over the rotational and vibrational states in Collimation of this effusive flow produces a beam. the other hand, are formed by the hydrodynamic, expansion of a gas from a reservoir, energy in
the beam.
Supersonic beams,
resulting in
conversion of internal
the source region to directed flow along the beam axis.
result of this expansion, cooling of the gas in
there is
the flow,
velocity distribution,
is
on
nearly isentropic
As a
both translational and rotational
and a beam is
rotationally cold,
with a high velocity along the beam axis. 5% of the average are easily obtainable.
produced which has a narrow and which is
forward peaked
Velocity spreads of less than The near monoenergeticity and
the higher energies achievable with these beams (1] makes them useful for inducing reactive gas-surface collisions that otherwise might be difficult under normal,
low pressure ambient conditions.
In addition,
a
heavy gas can be introduced into a light carrier gas to generate beams of
Mat. Res. Soc. Symp. Proc. Vol. 201. %!1991 Materials Research Society
heavy molecules of considerable translational energy [2]. Essentially, the heavy molecule isdrug along by the lighter species, achieving the same terminal velocity as the expanding light gas. This seeding technique can easily produce kinetic energies that are an appreciable fraction of intramolecular bond strengths for molecules with molecular weights as low as butane, for example [3). A typical molecular beam apparatus used for the study of reactive gas-surface collisions incorporates a beam source, including several stages of differential pumping, coupled to an ultrahigh vacuum system which contains the necessary spectroscopic tools to determine surface composition
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