Chemistry Reference
In-Depth Information
yielding a reaction probability of 0.05 for H abstraction by atomic hydrogen from
hydrocarbon film surfaces.
Since a reaction of Eley-Rideal type proceeds directly, the products are not in
thermal equilibrium and, hence, the desorbed particles are hyperthermal. The cross
sections of such reactions are rather small, because the interaction time of the direct
reactions is quite small. The reaction rate scales proportionally with the coverage
until a complete surface coverage is reached.
An incoming species
A
can also adsorb at the surface and thermalize prior to
reacting with another adsorbed species
B
to form the molecule
AB
, which desorbs.
This is called a
Langmuir-Hinshelwood-type
reaction. The occurrence of such a
reaction is indicated by several criteria:
1. The reaction obeys second-order kinetics and is proportional to the coverage
of species
A
and
B
.
2. The incoming species
A
thermalize at the surface, which leads to a kinetic
energy of the desorbing molecules
AB
corresponding to the substrate
temperature.
3. The cross section for this reaction is large, since both species
A
and
B
react
in their adsorbed states, leading to a much longer interaction time compared
to an Eley-Rideal-type reaction.
4. The reaction rate is insensitive on the used isotopes for reactants
A
and
B
,
since the species react in thermal equilibrium with the surface and any initial
momentum of reactant
A
is lost during its thermalization with the surface.
The rate of a Langmuir-Hinshelwood reaction with cross section σ
reaction
of impinging
species
A
leading to a coverage
A
with adsorbed species
B
with a coverage
B
at a
surface with a total number of adsorption sites
n
0
can be written as
Reaction rate
∼
A
B
n
0
σ
reaction
.
(5.11)
In summary, the reaction rate for the adsorption of growth precursors on surfaces can
be estimated on the basis of the following:
1. Neutral, stable precursors have to overcome in many cases an activation
barrier to transfer from the physisorbed into the chemisorbed state. Often,
this activation barrier cannot be overcome by thermal activation at room
temperature. However, since the potential well of the physisorbed state is
shallow compared to the thermal energy at typical substrate temperatures
(
room temperature), the adsorbed precursor desorbs thermally activated.
As a result, stable neutral precursors are mainly reflected and their sticking
coefficient is usually small.
2. Radicalic growth precursors have high potential energy at large distances
above the surface, corresponding to the half of the dissociation energy,
necessary for the formation of the radical. Upon approaching the surface,
the radical converts its potential energy into kinetic energy, which has to be
released via energy transfer to the atoms of the solid upon impact, in order
∼
