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The interaction of CO with Al(111) at room temperature
414
Surface Science 126 (1983) 414-421 North-Holland Publishing Company
THE INTERACTION TEMPERATURE * K. KHONDE
OF CO WITH Al(111) AT ROOM
**, J. DARVILLE
and J.M. GILLES
Insiitute for Research in Interface Sciences, LASMOS, Dkpartement de Physique, oersitaires Notre- Dame de la Paix, 61 Rue de Bruxelles, B-5000 Namur, Belgium Received
25 August
1982; accepted
for publication
29 September
FacultPs Uni-
1982
AES, LEED, low resolution ELS and work function measurements were conducted at room temperature on the interaction of CO with AhIll). Three main stages in the adsorption process were observed. Dissociative adsorption led to the incorporation of oxygen and carbon with the formation of amorphous oxidized and carburized aluminium. The initial state of the sample surface strongly determines the adsorption kinetics. In this framework we discuss the effect of structural defects and chemical impurities.
1. Introduction We have previously reported two sets of results on the adsorption of CO on Al( 100) [ 1,2]. Rapid or slow adsorption kinetics were found, depending upon the preparation of the sample surface. However, in both cases the process led to the creation of aluminium carbide and oxide phases. Very recently two papers have appeared that deal with the interaction between CO and monocrystalline aluminium. Flodstrom and Martinsson [3], using AES with a CMA, found a weak molecular adsorption on Al( 111) from room temperature down to 150 K. This adsorbed phase would dissociate and chemisorb upon electron beam irradiation. Bargeron and Nall [4] investigated evaporated films as well as (100) (110) and (111) faces, with CMA-ELS. They essentially reported no adsorption at room temperature for exposures up to 54 kL on the condition that the initial surface be carefully cleaned. They recorded nevertheless some modifications in the ELS spectrum when the electronic beam was turned on during the exposure or if oxygen was initially present on the surface.
* Work performed under the auspices for Science Policy. ** Holder of a grant of the AGCD.
0039-6028/83/0000-0000/$03.00
of the IRIS programme
sponsored
0 1983 North-Holland
by the Belgian Ministry
K. Khonde et al. / Interaction
of CO with AI(II1)
415
In the present work we have extended our previous investigation to the adsorption at room temperature of CO on Al(111). One of our aims was to identify the origin of the discrepancies between the results obtained by the three research groups.
2. Experimental The origin and preparation of the sample were the same as for the (100) crystal previously investigated [2]. The apparatus was equipped with a four-grid LEED-Auger system. Most of the Auger and electron loss spectra were recorded with a grazing incidence electron beam of 1.7 keV energy. The beam current was kept at 2 PA in AES and at 3 PA in ELS. Obviously this was at the expense of the signal-to-noise ratio, which shows up in a noticeable scatter of the experimental points. Work function measurements were realized by means of a Kelvin probe operating in the discontinuous mode in order to avoid shading effects. The adsorption was conducted with the electron beam turned off. The carbon monoxide pressure ranged from 5 X 10e8 up to 10s5 Torr depending upon the required dose, as described in detail in our previous paper [2]. The beam was turned on only after the CO had been evacuated. The base pressure was always under 1 X lo-” Torr. Other details can be found elsewhere [ 1,2].
3. Results 3.1. LEED The normal (111) LEED pattern was recorded during the adsorption sequences without any evidence of superstructure spots but the contrast between the spots and the background continuously decreased and disappeared almost simultaneously with the clean Auger Al signal. 3.2. AES Four different adsorption sequences were conducted. For each of them slow adsorption kinetics were found, leading to a 50% reduction of the clean Al peak at (8 f 4) x lo3 L and a practical disappearance at (6 f 3) x lo4 L. The recorded onset of adsorption varied from the very beginning of the exposure range up to 100 L for carbon and 3 kL for oxygen. The C/O AES signal ratio was systematically higher than or equal to one at the beginning and never smaller than 0.6 at the end of the adsorption sequence. In figs. 1 and 2, two cases are shown where the onset of the adsorption was
K. Khonde et al. / Interaction
EXPOSURE
of CO with A&III)
(L)
Fig. 1. Intensity variation of the aluminium 67 eV (A), carbon 272 eV (M) and oxygen 510 Auger peaks as a function of the logarithm of CO exposure expressed in langmuirs. The show the general behaviour when the initial state of the sample surface leads to a late onset adsorption. E, = 1.7 keV with a beam current of 2 pA in grazing incidence (target area = 4
\ \
\
\n
eV (0) curves of CO mm’).
10
i
Fig. 2. Intensity variation of the aluminium 67 eV (A), carbon 272 eV (m) and oxygen 510 eV (0) Auger peaks as a function of the logarithm of CO exposure expressed in langmuirs. We observe in this case an early onset of CO adsorption. EP = 1.7 keV with a beam current of 2 pA in grazing incidence (target area = 4 mm’).
K. Khonde et al. / Interaciion of CO with AI(III)
411
either late or very early. The peak-to-peak intensity of the Al peak at 67 eV, of the carbon peak at 272 eV and of the oxygen peak at 5 10 eV are plotted as a function of the logarithm of the exposure. The first point at the left of the scale relates to the clean surface. Due to a suitable normalization, the Al signals can be directly compared with the C and 0 signals. During the course of adsorption, signals of oxidized and carburized aluminium are developing at 54 and 58 eV, respectively. Although they provide pertinent information, their intensities are difficult to measure at the beginning since they overlap the plasmon loss signals of the clean Al peak at 67 eV. They appear around 2 kL. In spite of the relatively high variability of the results, we can identify three broad stages in the adsorption process. During stage 1 (0 to 6 x lo2 L), the signals vary only slightly. In stage 2 (6 X lo2 to 6 x lo3 L), the signals of C and 0 increase more rapidly and the Al peak decreases faster. In stage 3, (6 X lo3 to lo5 L) the Al peak rapidly decreases and disappears, whereas the 0 and C signals are strongly developing.
3.3. ELS The ELS spectra were recorded in grazing incidence (E, = 1.7 keV) for three adsorption sequences and in normal incidence (E, = 400 eV) for a fourth
Fig. 3. Intensity variation of the aluminium bulk (A) and surface (0) plasmons, normalized to the elastic peak intensity, as a function of the logarithm of CO exposure. E, = 1.7 keV with a beam current of 3 PA in grazing incidence.
418
K. Khonde et al. / Interaction
of CO with A&III)
one. The bulk plasmon loss signal remained essentially constant during stages 1 and 2. It rapidly decreased during stage 3 to 50% at (2.5 f 1) X lo4 L and vanished at about (7 f 3) x IO4 L. The surface plasmon loss peak decreased from the middle of stage 2 to 50% at (5 + 3) x IO3 L and was extinguished at (2 + 1) x lo4 L. Fig. 3 displays results that were spoiled by abnormally high experimental fluctuations but which correspond to the mean behaviour. No interface plasmon signal could be identified in the course of the experiments. 3.4. Work function The variation of the work function was followed for three adsorption sequences and in contrast with AES and ELS techniques, very reproducible results were obtained. The three plots are indistinguishable from that given in fig. 4. The stages introduced for the Auger spectrum can also be identified here. During stage 1, A+ decreased monotonically to - 80 mV. In stage 2 it increased rapidly at first up to 1 kL where it was again equal to zero and then more slowly up to 200 mV. During stage 3 we recorded a fast increase up to 10 kL where AC/I= 600 mV then a slower variation to 1300 mV at 7 X lo4 L where saturation occurred.
-0,20 II o
' n I""' 10
'
' ' ""I' lo2
'
' ' I""' 3
EXPOSURE Fig.4. Work function variation as a function of the
'
' ' ""
I
I
IO'
I‘;
logarithm of CO
exposure.
I I111111 ld
_
K. Khonde et al. / Interaction of CO with Al(lI1)
3.5. Checking
419
experiments
A series of short adsorption sequences were performed in order to evaluate the effect on the adsorption kinetics of the impinging electron beam or of the impurities or structural defects left initially at the surface. After annealing the clean sample at 150°C during 40 min, the LEED pattern was practically identical to that obtained after 3 h at 350°C. However, the work function was still 27 mV higher than for the normal annealing. CO was then admitted. At 400 L the clean Al peak was reduced by 50%. At 2600 L it was decreased by 90% and the LEED spots had disappeared. The work function was then 832 mV with respect to the reference electrode. The oxygen signal appeared at about 200 L. In a second series of experiments, the sample preparation was normal, corresponding to a relative work function of 94 mV. Then the sample was submitted to (a) about 1 L oxygen exposure, (b) the same oxygen exposure and 300 PA min of electron beam irradiation. In a third series the sample was left with some contamination carbon before annealing and (c) used as such or (d) subjected to 300 PA min electron beam irradiation. Treatment (a) led to a measurable decrease of the work function and to AES oxygen signal intensities of about 3 X lop3 of that of clean Al. Electron beam irradiation in treatment (b) induced a detectable decrease of the Auger 0 signal and a slight increase of the work function towards the clean Al value. After treatments (c) and (d), the carbon signal was also 3 x low3 of that of clean Al. The work function was practically that of the clean surface. Electron beam irradiation had no significant influence on the signals. Exposure to 2500 L of CO at 10e6 Torr after treatments (a), (b), (c) and (d) led to the disappearance of the LEED spots and to a Acp of 1065 + 15 mV. The clean Al Auger peak was drastically reduced and the C and 0 Auger signals rose to values of 30 to 40 in the scale of figs. 1 and 2.
4. Discussion It is obvious from the work function measurements that we observe a significant adsorption in the absence of electron beam effects. Furthermore, in spite of fluctuations in the Auger measurements, a good correlation exists between both types of results. Hence, although some influence on the adsorption process of the electron beam irradiation during the AES recordings cannot be excluded, it may safely be concluded that in our case the main phenomenon is not due to electron beam stimulation as suggested by Flodstrom and Martinsson [3]. The next question to be answered is then to reconcile our observations with those of Bargeron and Nall [4]. These authors claim that CO does not adsorb
420
K. Khonde et al. / Interaction of CO with AI(III)
on a clean and perfectly reconstructed surface and they suggest implicitly that any adsorption should be due to the presence of impurities or defects. Our checking experiments showed indeed that lattice disorder left by low temperature annealing decreased the efficient exposures by an order of magnitude. The same reduction was observed when small levels of impurities were left at the surface. We thus agree that a small impurity or defect concentration at the surface has a drastic influence on the adsorption kinetics. It is thus tempting to explain the adsorption on our “clean” samples as being triggered by a small defect concentration which would not give rise to detectable changes in the AES or CPD results. A permanent dislocation background could act in this way and lead to reproducible work function measurements upon CO adsorption. Starting from this hypothesis one can also explain the various stages in the adsorption process. During stage 1, adsorption would take place only at the defects where dissociation would occur. This can be deduced from the fact that the work function decreases, which indicates that oxygen is lying below the surface. This is also suggested by the fact that the carbon signal is initially always higher than that of oxygen. Often carbon is detected whereas no oxygen signal is visible (fig. 1). If adsorption were molecular, both signals should always appear together and the oxygen signal should be higher than that of carbon. One cannot exclude, however, that both mechanisms be present at the same time where both signals are comparable (fig. 1). During stage 2, small clusters of aluminium oxide and/or carbide appear as indicated by the AES results. They would form around the initial adsorption spots. This agrees also with the decrease of the surface plasmon signal and with the stabilization of the work function. As proposed earlier for the oxidation of magnesium [5], the boundary of a well developed oxide island can act as an efficient dissociation site. This new mechanism could be responsible for the sudden increase in adsorption rate in the third stage. The whole surface is covered by the islands when the surface plasmon disappears (2 x lo4 L) and the Al Auger signal is reduced to a few percent of its original amplitude. The strong increase in work function after the completion of the first layer may be due either to the increase in thickness of the aluminium oxide islands or to on-top adsorption of oxygen. This saturates, however, when the work function reaches its plateau. The origin of the fluctuations in the AES and ELS spectra is not yet well understood. If the initial state is really reproducible, as suggested by the work function results, these fluctuations could be due to variations in electron beam irradiation in the course of the experiment. We are investigating this point.
K. Khonde et al. / Interaction of CO with AI(lII)
421
5. Conclusions Within our experimental conditions of sample surface preparation, CO adsorbs at room temperature on Al( 111) in a dissociative and amorphous way, leading to the formation of oxidized and carburized aluminium. A similar behaviour has been previously observed on the Al(100) face [2]. It has also been shown that initial structural and chemical surface defects lead to much faster adsorption kinetics. It is thus possible that our results be due to the presence of undetected defects after the preparation of our sample.
Acknowledgements The authors are grateful to Dr. J.M. Welter (KFA-Jtilich) for the gift of the aluminium single crystal and to Mr. M. Renier for his skilful technical assistance. K. Khonde thanks the Administration Gtnerale pour la Cooperation et le Dtveloppement for a grant. J.M. Gilles thanks the Fonds National pour la Recherche Scientifique for an equipment grant. This research was supported by the Belgian Ministry for Science Policy (IRIS programme).
References [l] [2] [3] [4] [5]
K. Khonde, J. K. Khonde, J. S.A. Flodstrijm C.B. Bargeron H. Namba, J.
Darville and J.M. Gilles, Vacuum 31 (1981) 499. Darville and J.M. Gilles, J. Vacuum Sci. Technol. 20 (1982) 834. and C.W.B. Martinsson, Appl. Surface Sci. 10 (1982) 115. and B.H. Nall, Surface Sci. 119 (1982) L319. Darville and J.M. Gilles, Surface Sci. 108 (1981) 446.
Surface Science 126 (1983) 414-421 North-Holland Publishing Company
THE INTERACTION TEMPERATURE * K. KHONDE
OF CO WITH Al(111) AT ROOM
**, J. DARVILLE
and J.M. GILLES
Insiitute for Research in Interface Sciences, LASMOS, Dkpartement de Physique, oersitaires Notre- Dame de la Paix, 61 Rue de Bruxelles, B-5000 Namur, Belgium Received
25 August
1982; accepted
for publication
29 September
FacultPs Uni-
1982
AES, LEED, low resolution ELS and work function measurements were conducted at room temperature on the interaction of CO with AhIll). Three main stages in the adsorption process were observed. Dissociative adsorption led to the incorporation of oxygen and carbon with the formation of amorphous oxidized and carburized aluminium. The initial state of the sample surface strongly determines the adsorption kinetics. In this framework we discuss the effect of structural defects and chemical impurities.
1. Introduction We have previously reported two sets of results on the adsorption of CO on Al( 100) [ 1,2]. Rapid or slow adsorption kinetics were found, depending upon the preparation of the sample surface. However, in both cases the process led to the creation of aluminium carbide and oxide phases. Very recently two papers have appeared that deal with the interaction between CO and monocrystalline aluminium. Flodstrom and Martinsson [3], using AES with a CMA, found a weak molecular adsorption on Al( 111) from room temperature down to 150 K. This adsorbed phase would dissociate and chemisorb upon electron beam irradiation. Bargeron and Nall [4] investigated evaporated films as well as (100) (110) and (111) faces, with CMA-ELS. They essentially reported no adsorption at room temperature for exposures up to 54 kL on the condition that the initial surface be carefully cleaned. They recorded nevertheless some modifications in the ELS spectrum when the electronic beam was turned on during the exposure or if oxygen was initially present on the surface.
* Work performed under the auspices for Science Policy. ** Holder of a grant of the AGCD.
0039-6028/83/0000-0000/$03.00
of the IRIS programme
sponsored
0 1983 North-Holland
by the Belgian Ministry
K. Khonde et al. / Interaction
of CO with AI(II1)
415
In the present work we have extended our previous investigation to the adsorption at room temperature of CO on Al(111). One of our aims was to identify the origin of the discrepancies between the results obtained by the three research groups.
2. Experimental The origin and preparation of the sample were the same as for the (100) crystal previously investigated [2]. The apparatus was equipped with a four-grid LEED-Auger system. Most of the Auger and electron loss spectra were recorded with a grazing incidence electron beam of 1.7 keV energy. The beam current was kept at 2 PA in AES and at 3 PA in ELS. Obviously this was at the expense of the signal-to-noise ratio, which shows up in a noticeable scatter of the experimental points. Work function measurements were realized by means of a Kelvin probe operating in the discontinuous mode in order to avoid shading effects. The adsorption was conducted with the electron beam turned off. The carbon monoxide pressure ranged from 5 X 10e8 up to 10s5 Torr depending upon the required dose, as described in detail in our previous paper [2]. The beam was turned on only after the CO had been evacuated. The base pressure was always under 1 X lo-” Torr. Other details can be found elsewhere [ 1,2].
3. Results 3.1. LEED The normal (111) LEED pattern was recorded during the adsorption sequences without any evidence of superstructure spots but the contrast between the spots and the background continuously decreased and disappeared almost simultaneously with the clean Auger Al signal. 3.2. AES Four different adsorption sequences were conducted. For each of them slow adsorption kinetics were found, leading to a 50% reduction of the clean Al peak at (8 f 4) x lo3 L and a practical disappearance at (6 f 3) x lo4 L. The recorded onset of adsorption varied from the very beginning of the exposure range up to 100 L for carbon and 3 kL for oxygen. The C/O AES signal ratio was systematically higher than or equal to one at the beginning and never smaller than 0.6 at the end of the adsorption sequence. In figs. 1 and 2, two cases are shown where the onset of the adsorption was
K. Khonde et al. / Interaction
EXPOSURE
of CO with A&III)
(L)
Fig. 1. Intensity variation of the aluminium 67 eV (A), carbon 272 eV (M) and oxygen 510 Auger peaks as a function of the logarithm of CO exposure expressed in langmuirs. The show the general behaviour when the initial state of the sample surface leads to a late onset adsorption. E, = 1.7 keV with a beam current of 2 pA in grazing incidence (target area = 4
\ \
\
\n
eV (0) curves of CO mm’).
10
i
Fig. 2. Intensity variation of the aluminium 67 eV (A), carbon 272 eV (m) and oxygen 510 eV (0) Auger peaks as a function of the logarithm of CO exposure expressed in langmuirs. We observe in this case an early onset of CO adsorption. EP = 1.7 keV with a beam current of 2 pA in grazing incidence (target area = 4 mm’).
K. Khonde et al. / Interaciion of CO with AI(III)
411
either late or very early. The peak-to-peak intensity of the Al peak at 67 eV, of the carbon peak at 272 eV and of the oxygen peak at 5 10 eV are plotted as a function of the logarithm of the exposure. The first point at the left of the scale relates to the clean surface. Due to a suitable normalization, the Al signals can be directly compared with the C and 0 signals. During the course of adsorption, signals of oxidized and carburized aluminium are developing at 54 and 58 eV, respectively. Although they provide pertinent information, their intensities are difficult to measure at the beginning since they overlap the plasmon loss signals of the clean Al peak at 67 eV. They appear around 2 kL. In spite of the relatively high variability of the results, we can identify three broad stages in the adsorption process. During stage 1 (0 to 6 x lo2 L), the signals vary only slightly. In stage 2 (6 X lo2 to 6 x lo3 L), the signals of C and 0 increase more rapidly and the Al peak decreases faster. In stage 3, (6 X lo3 to lo5 L) the Al peak rapidly decreases and disappears, whereas the 0 and C signals are strongly developing.
3.3. ELS The ELS spectra were recorded in grazing incidence (E, = 1.7 keV) for three adsorption sequences and in normal incidence (E, = 400 eV) for a fourth
Fig. 3. Intensity variation of the aluminium bulk (A) and surface (0) plasmons, normalized to the elastic peak intensity, as a function of the logarithm of CO exposure. E, = 1.7 keV with a beam current of 3 PA in grazing incidence.
418
K. Khonde et al. / Interaction
of CO with A&III)
one. The bulk plasmon loss signal remained essentially constant during stages 1 and 2. It rapidly decreased during stage 3 to 50% at (2.5 f 1) X lo4 L and vanished at about (7 f 3) x IO4 L. The surface plasmon loss peak decreased from the middle of stage 2 to 50% at (5 + 3) x IO3 L and was extinguished at (2 + 1) x lo4 L. Fig. 3 displays results that were spoiled by abnormally high experimental fluctuations but which correspond to the mean behaviour. No interface plasmon signal could be identified in the course of the experiments. 3.4. Work function The variation of the work function was followed for three adsorption sequences and in contrast with AES and ELS techniques, very reproducible results were obtained. The three plots are indistinguishable from that given in fig. 4. The stages introduced for the Auger spectrum can also be identified here. During stage 1, A+ decreased monotonically to - 80 mV. In stage 2 it increased rapidly at first up to 1 kL where it was again equal to zero and then more slowly up to 200 mV. During stage 3 we recorded a fast increase up to 10 kL where AC/I= 600 mV then a slower variation to 1300 mV at 7 X lo4 L where saturation occurred.
-0,20 II o
' n I""' 10
'
' ' ""I' lo2
'
' ' I""' 3
EXPOSURE Fig.4. Work function variation as a function of the
'
' ' ""
I
I
IO'
I‘;
logarithm of CO
exposure.
I I111111 ld
_
K. Khonde et al. / Interaction of CO with Al(lI1)
3.5. Checking
419
experiments
A series of short adsorption sequences were performed in order to evaluate the effect on the adsorption kinetics of the impinging electron beam or of the impurities or structural defects left initially at the surface. After annealing the clean sample at 150°C during 40 min, the LEED pattern was practically identical to that obtained after 3 h at 350°C. However, the work function was still 27 mV higher than for the normal annealing. CO was then admitted. At 400 L the clean Al peak was reduced by 50%. At 2600 L it was decreased by 90% and the LEED spots had disappeared. The work function was then 832 mV with respect to the reference electrode. The oxygen signal appeared at about 200 L. In a second series of experiments, the sample preparation was normal, corresponding to a relative work function of 94 mV. Then the sample was submitted to (a) about 1 L oxygen exposure, (b) the same oxygen exposure and 300 PA min of electron beam irradiation. In a third series the sample was left with some contamination carbon before annealing and (c) used as such or (d) subjected to 300 PA min electron beam irradiation. Treatment (a) led to a measurable decrease of the work function and to AES oxygen signal intensities of about 3 X lop3 of that of clean Al. Electron beam irradiation in treatment (b) induced a detectable decrease of the Auger 0 signal and a slight increase of the work function towards the clean Al value. After treatments (c) and (d), the carbon signal was also 3 x low3 of that of clean Al. The work function was practically that of the clean surface. Electron beam irradiation had no significant influence on the signals. Exposure to 2500 L of CO at 10e6 Torr after treatments (a), (b), (c) and (d) led to the disappearance of the LEED spots and to a Acp of 1065 + 15 mV. The clean Al Auger peak was drastically reduced and the C and 0 Auger signals rose to values of 30 to 40 in the scale of figs. 1 and 2.
4. Discussion It is obvious from the work function measurements that we observe a significant adsorption in the absence of electron beam effects. Furthermore, in spite of fluctuations in the Auger measurements, a good correlation exists between both types of results. Hence, although some influence on the adsorption process of the electron beam irradiation during the AES recordings cannot be excluded, it may safely be concluded that in our case the main phenomenon is not due to electron beam stimulation as suggested by Flodstrom and Martinsson [3]. The next question to be answered is then to reconcile our observations with those of Bargeron and Nall [4]. These authors claim that CO does not adsorb
420
K. Khonde et al. / Interaction of CO with AI(III)
on a clean and perfectly reconstructed surface and they suggest implicitly that any adsorption should be due to the presence of impurities or defects. Our checking experiments showed indeed that lattice disorder left by low temperature annealing decreased the efficient exposures by an order of magnitude. The same reduction was observed when small levels of impurities were left at the surface. We thus agree that a small impurity or defect concentration at the surface has a drastic influence on the adsorption kinetics. It is thus tempting to explain the adsorption on our “clean” samples as being triggered by a small defect concentration which would not give rise to detectable changes in the AES or CPD results. A permanent dislocation background could act in this way and lead to reproducible work function measurements upon CO adsorption. Starting from this hypothesis one can also explain the various stages in the adsorption process. During stage 1, adsorption would take place only at the defects where dissociation would occur. This can be deduced from the fact that the work function decreases, which indicates that oxygen is lying below the surface. This is also suggested by the fact that the carbon signal is initially always higher than that of oxygen. Often carbon is detected whereas no oxygen signal is visible (fig. 1). If adsorption were molecular, both signals should always appear together and the oxygen signal should be higher than that of carbon. One cannot exclude, however, that both mechanisms be present at the same time where both signals are comparable (fig. 1). During stage 2, small clusters of aluminium oxide and/or carbide appear as indicated by the AES results. They would form around the initial adsorption spots. This agrees also with the decrease of the surface plasmon signal and with the stabilization of the work function. As proposed earlier for the oxidation of magnesium [5], the boundary of a well developed oxide island can act as an efficient dissociation site. This new mechanism could be responsible for the sudden increase in adsorption rate in the third stage. The whole surface is covered by the islands when the surface plasmon disappears (2 x lo4 L) and the Al Auger signal is reduced to a few percent of its original amplitude. The strong increase in work function after the completion of the first layer may be due either to the increase in thickness of the aluminium oxide islands or to on-top adsorption of oxygen. This saturates, however, when the work function reaches its plateau. The origin of the fluctuations in the AES and ELS spectra is not yet well understood. If the initial state is really reproducible, as suggested by the work function results, these fluctuations could be due to variations in electron beam irradiation in the course of the experiment. We are investigating this point.
K. Khonde et al. / Interaction of CO with AI(lII)
421
5. Conclusions Within our experimental conditions of sample surface preparation, CO adsorbs at room temperature on Al( 111) in a dissociative and amorphous way, leading to the formation of oxidized and carburized aluminium. A similar behaviour has been previously observed on the Al(100) face [2]. It has also been shown that initial structural and chemical surface defects lead to much faster adsorption kinetics. It is thus possible that our results be due to the presence of undetected defects after the preparation of our sample.
Acknowledgements The authors are grateful to Dr. J.M. Welter (KFA-Jtilich) for the gift of the aluminium single crystal and to Mr. M. Renier for his skilful technical assistance. K. Khonde thanks the Administration Gtnerale pour la Cooperation et le Dtveloppement for a grant. J.M. Gilles thanks the Fonds National pour la Recherche Scientifique for an equipment grant. This research was supported by the Belgian Ministry for Science Policy (IRIS programme).
References [l] [2] [3] [4] [5]
K. Khonde, J. K. Khonde, J. S.A. Flodstrijm C.B. Bargeron H. Namba, J.
Darville and J.M. Gilles, Vacuum 31 (1981) 499. Darville and J.M. Gilles, J. Vacuum Sci. Technol. 20 (1982) 834. and C.W.B. Martinsson, Appl. Surface Sci. 10 (1982) 115. and B.H. Nall, Surface Sci. 119 (1982) L319. Darville and J.M. Gilles, Surface Sci. 108 (1981) 446.