Chemistry Reference
In-Depth Information
(b)
(a)
A
1/2
E
dissociation
E
act
A
2
Distance
E
physisorption
E
chemisorption
FIGURE 5.1
1D potential energy diagram showing a possible transition from molecular
physisorption to dissociative chemisorption. Potential curves are presented for the molecule
(a) and the atom (b).
A
and
A
2
at large distances from the surface corresponds to one half of the dissociation
energy
E
dissociation
of molecule
A
2
. The adsorption of the molecule
A
2
can be described
as follows:
A
2
approaches the surface at the vacuum level. Upon collision with the
surface atom, it transfers kinetic energy to the solid and reduces thereby its potential
energy becoming trapped into the physisorbed state with binding energy
E
physisorption
.
If the crossover of the potential curve for
A
(b) with the potential curve for
A
2
(a) is
below the vacuum level, the molecule
A
2
can directly dissociate and arrive into the
chemisorbed state with binding energy
E
chemisorption
as two atoms
A
. This mechanism
is called dissociative chemisorption. If the crossover is above the vacuum level, as
illustrated in Figure 5.1, the molecule
A
2
can only chemisorb if it overcomes an
activation barrier
E
act
. This may occur at higher substrate temperature via thermal
activation or if the kinetic energy of the impinging molecule
A
2
is sufficiently high.
Finally, the desorption of a particle from adsorption potential can be regarded
as an excited or activated process again, which is described by an Arrhenius-like
rate. In many cases, the thermal energy at room temperature is sufficient for the
desorption of a physisorbed molecule back into the gas phase via thermal activation.
As a consequence, the sticking coefficient for chemisorption of molecules at room
temperature is often very small.
5.2 STICKING COEFFICIENTS AND SURFACE
LOSS PROBABILITIES
Adsorption processes can be quantified using macroscopic parameters such as stick-
ing coefficients and surface loss probabilities. The sticking coefficient γ describes
the probability of an incoming particle to be trapped in a potential above the surface
[2], for example, the rates of desorption and adsorption just differ by the sticking
coefficient.
The sticking coefficient depends on various quantities of the gas/plasma-surface
system, for example, the sticking probability decreases as the surface coverage
