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J. Chem. Phys., Vol. 109, No. 1, 1 July 1998
S. Wehner and J. Kuppers
clusion on the operating mechanism is, however, not pos-
sible from that phenomenological feature. Neither HD not H2
are expected as products from reactions between D and ad-
instead of H directed at CH3I adlayers, showed no gaseous
reaction products, which is expected at an 85 K substrate
temperature.
Restricting at first to the CH3I/Pt(111) and
CH3I/H/Pt(111) systems, the measured reaction kinetics data
will be analyzed within the following framework. An incom-
ing D atom is assumed to stick with probability ps if its
impact on the Pt surface occurs at an empty site. Alterna-
*
sorbed CH3I if D –CH3I encounters do not cause the disso-
ciation of CH3I. This is confirmed in Fig. 2 ͑dotted lines͒.
It is important to realize that in an experimental setup for
the measurement of product rates which does not allow us to
monitor the initial reaction region or does not provide a strict
separation between the sample and the atom source prior to
the reaction start, the outcome of a reaction kinetics measure-
ment would confirm the ER reaction scheme, as only kinetic
data from the late reaction period would be available.
The essential input for rationalizing the measured kinet-
ics in the early reaction period is given by the competition
between the sticking and reaction of hot atoms. In the CH3I
multilayer regime the hot-atom picture does not apply since a
multilayer does not provide a deep adsorption well for in-
coming D atoms which supplies potential energy to the
atom.2 However, D atoms are small particles and can migrate
through a not very dense layer of stacked molecules. The rate
of CH3D formation should become proportional to the con-
centration of CH3I molecules available for a reaction in the
target, and, accordingly, the rate step in the multilayer re-
gime should be proportional to the CH3I coverage. This was
observed experimentally. Necessarily, the reaction kinetics
assumes ER phenomenology, i.e., the rates strictly follow an
exponential decay law. The experimentally determined reac-
tion cross section is smaller than for the hot-atom case in the
monolayer regime, ϭ0.4 Å2, as compared to ϭ0.9 Å2.
The small reaction cross section implies that a considerable
fraction of the incoming D atoms do not react.
*
tively, it is transformed into the hot-atom D state. At sites
occupied with H or D or methyl iodide, it either becomes a
*
*
*
hot D atom or generates a hot H or D atom with specific
*
*
probabilities. Hot D ͑H ͒ atoms travel across the surface,
and in collisive encounters with adsorbed CH3I they abstract
methyl to form CH3D (CH4) with a probability pr . At the
given temperature of 85 K, methane products desorb imme-
diately and I remains on the surface. Upon encounter with an
empty surface site, hot atoms stick with a probability phs . In
the further discussion it is assumed that the probabilities ps
and phs are bigger than pr .
The repulsive interaction between methyl iodide mol-
ecules adsorbed on Pt͑111͒ causes the monolayer coverage
to be only about 0.2. This leaves many empty sites for D
*
adsorption and D sticking, if D atoms impinge at a sub-
monolayer or monolayer CH3I covered surface. Accordingly,
*
there is a considerable chance for hot D atoms to stick at
these sites while moving on the surface. The assumption
phsϾpr makes sticking of hot atoms a very efficient step
competitive to reactive events. As long as a surface exhibits
empty sites, this competing step will lower the rate of meth-
ane formation significantly below that value which would be
achieved without hot-atom sticking.
The reaction yield in the limit of infinite thickness of the
adsorbed methyl iodide layers, available through the time-
integrated CH3D signal and the accumulated D flux, was not
measured. However, even with the top rate curve in Fig. 1
the height of the initial rate jump indicates that only every
sixth incoming D atom reacted. The reflection of atoms
might be a non-negligible process.
A comparison of the rates measured at adsorbed CH3I
and coadsorbed H and CH3I, full and dotted lines in Fig. 2, is
illustrative concerning the action of coadsorbed H on the
kinetics of CH3D formation. On Pt͑111͒ surfaces, covered
with H at saturation, sticking of hot atoms is suppressed to a
great extent. Sticking is not completely excluded because a
H saturation coverage of only about 0.7 is achieved through
dosing of molecular H2 to Pt͑111͒ surfaces.16 Therefore, not
all sites at the Pt͑111͒ surface are blocked by adsorbed H.
However, the 17 amu full line rate curve in Fig. 2 illustrates
that the reduction of the number of sites available for hot-
atom sticking has a significant effect on the CH3D rate in the
early reaction period. The rate jump becomes bigger and the
atom fluence necessary to achieve the rate maximum is re-
duced. Both phenomena are expected within the present
framework.
The action of this scenario is seen in the kinetics mea-
sured in the monolayer CH3I/Pt(111) regime, as depicted in
Figs. 1 and 2 ͑dotted lines͒. The rate jumps are small because
the probability pr is small and there are many empty sites
available on the surface. The rates stay small as long as the
surface is not fully covered, which corresponds to the early
reaction period. Only after the surface is completely covered
with D and CH3I through adsorption of D from the gas phase
*
or through sticking of D species, the methane rate starts to
grow substantially. This situation is met after a fluence of ca.
1.5 Ml D. In this early reaction period, the rate of methane
formation strongly contradicts the operation of an ER
mechanism, since the methane rate increases although the
CH3I concentration on the surface decreases. Beyond the re-
action rate maximum, in the late reaction period, the total
coverage, given by the fractional coverages of CH3I and D,
has assumed saturation. Stationary conditions with respect to
the total coverage are maintained under a continuous D flux.
Accordingly, no or almost no empty sites are present on the
surface in the late reaction period, and hot-atom sticking is
*
suppressed. Each hot D atom has to react eventually with an
adsorbed methyl iodide toward CH3D or an adsorbed D to-
ward D2. Since the amount of CH3I on the surface is limited,
blocking of competitive hot-atom sticking in the late reaction
period causes the CH3D rates to decrease exponentially. In
this late reaction regime, the phenomenology of hot-atom
processes necessarily lead to an ER-like exponential depen-
dence between rate and time ͑atom fluence͒. A con-
The presence of the coadsorbed H adlayer has additional
consequences which are easy to understand within the cur-
*
rent scenario. Incoming D atoms produce hot H atoms
which can react with adsorbed H to form H2. These mol-
ecules are monitored in the 2 amu signal in Fig. 2. Further-
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