surface
science / reaction
dynamics
/ laser photochemistry /
laser-surface
interactions / nanotechnology
/ physical
chemistry / chemical
physics
Dynamics of Adsorption and
Desorption
I have long studied the simplest of surface chemical reactions, the
adsorption and thermal desorption of small molecules from surfaces, for
example, nitric
oxide on platinum, hydrogen
on silicon, oxygen
on palladium, and methane
on platinum. Also I have recently reviewed stimulated
desorption of hydrogen from silicon. A superb review of the
dynamics of hydrogen adsorption onto and desorption from silicon has
recently been published by Dürr
and Höfer.
These studies attempt to under how chemical bonds are made and broken
in the most elemental of surface reactions: that of a molecule
approaching a surface and sticking to it or of a molecule detaching
from a surface. We also investigate the dynamics of how electronic
excitations induced by electron and photon irradiation affect surface
chemical reactions.
Adsorption is either activated or nonactivated. In nonactivated
adsorption, low energy molecules stick better than high energy
molecules. For activated adsorption, a molecule must have a minimum
amount of energy before it is able to stick. But it is details of the
potential energy surface (PES) that determine what types of energy
(kinetic, vibrational, rotational and electronic) are most effective at
enhancing activation adsorption or reducing nonactivated adsorption.
How the molecule moves over this potential energy surface is what we
meant by dynamics. What is the path taken by the molecule and how do
the initial conditions of the molecule and the surface affect the
sticking probability of the molecule? This is the type of fundamental
question asked.
Desorption occurs over the same PES as adsorption with the motions, in
a sense, reversed. Thus, adsorption and desorption are related by
microscopic reversibility so long as the two process are carried out at
the same conditions. In particular, it is important that the coverage
of surface bound species (adsorbates) is the same when comparing
adsorption and desorption using microscopic reversibility. For hydrogen
on silicon this is especially important.
Hydrogen adsorption leads to changes in the silicon surface. The
surface structure relaxes and both its geometry and vibrational
properties change in response to adsorption. This effect is generally
greater on the surfaces of covalent solids such as semiconductors.
While such effects may also be important on metal surfaces, the most
studied case of hydrogen on copper does not exhibit a similarly strong
dependence on coverage. For hydrogen on silicon, the changes introduced
are so strong that there appears to be one mechanism that describes
dissociative adsorption of H2 into two H atoms on the
surface at low coverage (with little adsorbed H on the surface, denoted
H2* in the figure below or intradimer desorption) as compared to high
coverage (when there is nearly one adsorbed H atom on every surface Si
atom, denoted H4 in the figure below or interdimer desorption). The
adsorption and desorption of hydrogen is of great interest from a
fundamental viewpoint since it is (supposedly) the simplest reaction of
a molecule with the most important semiconductor. However, it is also
of practical interest because the adsorption and desorption of hydrogen
play an important role in the chemical vapor deposition (CVD) of
silicon from silanes (such as SiH4 and Si2H6),
which is important in the integrated circuit industry.
Methane (CH4) is a
molecule that is very important in
a range of catalytic chemistry. Methane is the primary component of natural
gas and steam reforming of methane over a nickel catalyst is the primary industrial
source of hydrogen. We examined the dynamics of methane and ethane adsorption
on
metal surfaces using effusive molecular beam techniques. Effusive beams
from a Knudsen source have a distribution of rotational, vibrational
and translational energy that is described by an equilibrium thermal
distribution (a Maxwell-Boltzmann distribution). Methane activated
adsorption is quite interesting because it has a very low sticking
coefficient and exhibits vibrational mode-specific chemistry. In other
words, excitation of the molecule is required to get it to stick and
the amount of sticking coefficient enhancement depends on the
vibrational mode that is excited. Not all modes are equally active at
enhancing sticking. While this is very interesting from a fundamental
scientific standpoint, it is still unclear whether the mode specificity
is relevant for the kinetics of the industrial process.
For further information on related topics,
try these sites:
Surface
Dynamic Group Essen
SFB:
Energiedissipation an Oberflächen
Dynamics of
Gas-Surface Interactions
Labs
working in Surface
Science, Nanotechnology and Catalysis
