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An atom-resolved view of silicon nanoclusters
15 March 1996
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 25 1( 1996) 8- 12
An atom-resolved view of silicon nanoclusters D.W. McComb, B.A. Collings, R.A. Wolkow *, D.J. Moffatt, C.D. MacPherson, D.M. Rayner, P.A. Hackett, J.E. Hulse ’ Steacie Institute for Molecular
Sciences, National Research Council of Canada, IO0 Sussex Drive, Ottawa. Cat&u
KIA OR6
Received 29 November 1995
Abstract Atomically resolved scanning tunneling microscope images of Si clusters deposited onto a Sic11 l)-7 X 7 substrate have been obtained. The clusters display a capping layer of widely spaced surface atoms analogous to the reconstructed surface of a bulk crystal. Site-specific variations in the electronic characteristics of cluster surface atoms are detected. Adsorption studies demonstrate size dependant reactivity of deposited clusters. Clusters are observed to collapse upon annealing to form epitaxial Si islands.
Very small clusters, composed of from several to hundreds of atoms, display fascinating chemical [l] and optical properties [2]. While the characteristics of these nanoclusters bridge the gap between atomic and bulk characteristics, the transition is generally not smooth or predictable. In some instances very different properties are displayed by clusters differing in composition by only a single atom [3]. The goals of the cluster field are structure determination and correlation of structure with observable properties. Soon after the invention of the scanning tunneling microscope @TM) it was recognized that a direct atomic-level view of small supported particles might be attainable. One early study examined silicon clusters that had been deposited onto a gold surface [4]. Protrusions identified as clusters were observed but no atomically resolved features were evident. It was
* Corresponding author. ’Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, Canada KIA 0R6.
unclear whether experimental conditions had to be further optimized, or if innate limitations of the technique were preventing more details from being revealed. More recent studies involving the deposition of metal clusters including Pt, Pd, and Au on carbon substrates have shown profiles that could be interpreted as clusters [5-81, however no atomic features were evident. In this work we report the first atomically resolved images of deposited clusters. In particular, silicon clusters of various sizes were deposited onto a Sic11 11-7 X 7 substrate at room temperature. The STM used in this study is contained in an ultra high vacuum chamber with a base pressure of approximately 5 X lo- ” Torr. Within this environment samples remain free of contamination for many hours. The Si clusters were created in a Smalley type source using a 308 nm XeCl eximer ablation laser [9]. A control experiment was performed in which a clean substrate was exposed to the He carrier gas without the ablation laser being fired. No clusters were observed and there was no significant contami-
ooO9-2614/96/$12.00 0 1996 Elsevier Science B.V. All rights reserved PII SOOO9-2614(96)000681
D. W. McComb et al. /Chemical
nation of the Si surface. The silicon clusters were not mass selected prior to deposition. To achieve a true representation of the surface under study it is essential that a STM tip be formed with a single atom protruding sufficiently far from the apex that tunneling current is exclusively directed to that point. On flat surfaces this is relatively easy to achieve. For structures with sharply sloping features, such as deposited clusters, there is greater opportunity for secondary points on the tip to interact with the surface. This presents a problem because if multiple points on the tip contribute to the tunneling process, the resulting image will include convoluted contributions of tip and surface topography. A measure of the ability of a tip to accurately track sharply sloping features may be obtained by imaging surfaces that contain steps between flat terraces. The substrate used in this study displays well characterized 3.1 A steps [lo]. At a step edge, a sharp tip can accurately resolve lower terrace structure to within = 2 A of the step and can therefore also be expected to accurately image comparably sloped cluster features 2. Characterization of the tip in this way allows us to confidently interpret the top-most region of cluster images as true renditions of the cluster structure. Features toward the extreme edges of clusters are more prone to artifacts and must be interpreted cautiously. Fig. 1 shows an STM image of three deposited silicon clusters 3. The substrate is the extensively studied Si( 111j-7 X 7 surface [ 111. The regular pattern displayed by the substrate atoms serves as a convenient grid, against which cluster structure may be compared. The nearest neighbour spacing of the
* Ideally, tip characterization through step imaging should be done at steps of all angular orientations since a tip will often not be symmetric about its axis. 3 A range of cluster sizes has been deposited and it is not known how many atoms any particular cluster is composed of. Very rough estimates can be made by taking the height as the diameter and by assuming normal bulk density. In this way we find the largest cluster in Fig. 1 contains on the order of 250 atoms. We very roughly estimate that the smaller cluster, toward the left side of the image, is composed of only several atoms, while the cluster on the right contains on the order of 10 atoms. Possible distortions of the clusters and the difficulty in deconvoluting the crystallographic and electronic contributions that make up an STM image further complicate cluster size determinations.
Physics Letters 251 (I 996) 8-12
Fig. 1. STM constant current image of deposited Si clusters on the Si( 1 1 1j-7 X 7 surface. The sample bias was + 1.O V, the tunneling current was 0.04 nA.
substrate surface atoms is 7.7 A. The clusters in Fig. 1 have widely spaced surface atoms (6 to 8 A>, not unlike those on the substrate. Si(l1 l)-7 X 7 exhibits a surface reconstruction. That is, surface atoms adopt a configuration that serves to ‘cap’ more closely spaced internal atoms, thereby minimizing the number of dangling bonds. The widely spaced cluster features strongly suggest that a capping layer also forms on deposited silicon clusters. Theoretical models predict the existance of capping atoms on silicon clusters [12]. It is important to note that, unlike three-coordinate capping atoms which have dangling bond electronic character, four-coordinate (and higher) atoms in the surface vicinity are not expected to be visible in STM images, accounting for the large spaces between observed surface features. The effect of cluster-surface interaction is a major concern in studying deposited clusters. We cannot state with certainty if, or to what extent, the clusters imaged here have been modified upon deposition, however, it does seem that all of the clusters are flattened somewhat. For example, the large cluster in Fig. 1 is 21 f 0.1 A high and 40 * 5 8, wide. The large uncertainty in width derives from the problem of knowing where true cluster structure ends and tip-cluster convoluted structure begins. In any case, all the clusters appear to be approximately twice as wide as high. It is conceivable that the clusters are imbedded into the surface but the lack of
10
D.W. McComb et al./ Chemical Physics Letters 251 (1996) 8-12
Fig. 2. Two STM images of a Si cluster obtained with a tunneling current of 0.04 nA and a bias voltage of (a) + 1 V and (b) - I V. A gradient view is shown to enhance the visibility of local features.
structural changes. This is not to say that substrate temperature is not an important parameter. At sufficiently low temperatures thermodynamically unstable structures can be frozen in place. Indeed the present study provides an example of such a system. Fig. 2 shows silicon clusters that have collapsed to form epitaxial islands upon annealing to approximately 700 K. Evidence is also found (not shown here) of atom migration to substrate step edges. This result also verifies that the deposited clusters are composed of silicon and not an impurity. It will be necessary to perform mass selected cluster depositions to better understand the structure of deposited clusters. A composite three-dimensional view of clusters may emerge from images of many identical clusters, in various orientations. The range of electronic energy levels that contribute to an STM image is determined by the bias voltage between the tip and sample. By acquiring images at both polarities, details of the occupied and unoccupied electronic states may be examined. The + 1V STM image in Fig. 3a provides a view of the unoccupied states of a cluster while the - 1 V image in Fig. 3b shows the occupied states of the same cluster. The grid has been overlaid to aid in examining identical points in the two images. The arrow
a crater or displaced atoms surrounding the clusters tends to discount this view. Further understanding of the observed structures will only come from future experiments varying; (1) the impact energy, (2) the substrate, and, (3) the substrate temperature. In the present configuration the clusters have a velocity of approximately 1700 m/s 4. In the future, various soft landing schemes may be explored including deposition onto a mattress of condensed xenon [ 13,141, and electrostatically controlled landing. By changing the substrate it will be possible to examine the effect of the substrate-cluster strength of interaction. In the present case the clusters likely form several Si-Si bonds with a bond energy of = 2 eV/bond. It is worth noting that independent of impact energy and substrate temperature, this ‘heat of adsorption’ will be present, and it might lead to
4 This corresponds to 0.29 eV/Si would have a 2.9 eV kinetic energy.
atom.
Si,,
for example,
Fig. 3. A Si cluster which has been annealed to 700 K for 2 min. The image was obtained with a tunneling current of 0.04 nA and a bias voltage of + 1.O V.
D.W. McComb et al./ Chemicul Physics Letters 251 (1996) 8-12
Fig. 4. Silicon clusters (a) the day of preparation, and (b) 7 days later, changes have occurred as a result of adsorption of background gases. Imaging conditions were 0.04 nA and + 1 V.
marks a feature in the occupied state image that is not evident in the unoccupied state image. This observation demonstrates that STM may be employed to detect the electronic inequivalence of cluster surface atoms. STM studies of bulk surfaces have revealed similar site-specific electronic characteristics and these differences have been correlated with spatial variations in reactivity [ 151. Through analogy with the bulk surface results we expect that deposited silicon clusters will also have site-specific reactive character. STM has been successfully used to monitor reactions at individual surface sites and it is of interest to see if a similar view may be gained of cluster reactivity, particularly so since in cluster-beam flow reactor studies, sharp variations in reactivity as a function of number of constituent atoms have been demonstrated [16]. Our first attempt to observe size dependent reactivity involved observing a group of clusters over many hours to see if reaction would occur with background gases in the vacuum chamber (consisting primarily of H,, H,O, and CO). Fig. 4
Ii
shows three clusters; (a) on the day of preparation and, (b) 7 days later. The largest cluster, cluster 1, becomes smaller in appearance after adsorption has occurred, its height changing from 23.8 f 0.1 to 20.7 + 0.1 A. Cluster 2, shows a small change in height, measuring 19.5 + 0.1 A initially and finally 19.3 k 0.1 A. Cluster 3 becomes larger, changing from 18.4 &-0.1 A to 21.2 IL-0.1 A. This striking range of behavior provides a clear demonstration of size-dependent reactivity of deposited clusters. Normally on silicon, chemically bound adsorbates lead to elimination of protrusions associated with surface atom dangling bonds. This occurs as a dangling bond state is replaced by a chemical bond with a binding energy outside the energy window probed by STM. A reacted cluster is therefore expected to appear smaller. Based on this simple scenario we might conclude that cluster 1 reacted most readily (ie. more molecules adsorbed) with background gases and cluster 2 reacted to a lesser degree. The enlarged appearance of cluster 3 in Fig. 4 is however not understood at this point. We have described only gross structural changes related to adsorption in this work. Studies probing adsorption at individual sites are in progress.
References
[ll
M.F. Jarrold, Science 252 (1991) 1085, and references therein. Dl T.T. Rantala, M.I. Stockman, D.A. Jelski and T.F. George, J. Chem. Phys. 93 (1990) 7427. [31 S.A. Mitchell, L. Lian, D.M. Rayner and P.A. Hackett, J. Chem. Phys.. in press. [41 Y. Kuk, M.F. Jarrold, P.J. Silverman, J.E. Bower, W.L. Brown, Phys. Rev. B 39 (1989) I I 168. G.N. Subbanna and C.N.R. [51 H.N. Aiyer, V. Vijayakrishnan, Rao, Surface Sci. 313 (1994) 392. [61F.J. Aires, P. Sautet, G. Fuchs, J.-L. Rousset and P. Mt?linon, Microsc. Microanal. Microstnrct. 4 (I 993) 44 I. [7] A. Humbert, R. Pierrisnard, S. Sangay, C. Cbapon, C.R. Henry and C. Claeys, Europhys. Letters 10 (1989) 533. [8] Z. Ma, C. Zbu, J. Shen, S. Pang and Z. Xue, Vacuum 43 (1992) 111.5. [9] T.G. Diets, M.A. Duncan, D.E. Powers and R.E. Smalley, J. Chem. Phys. 74 (1981) 6511. [IO] R.S. Becker, J.A. Golovchenko, E.G. McRae,and B.S. Swartzentruber, Phys. Rev. Letters 55 (1985) 2032. 1111 K. Takayanagi, Y. Tanishiro, M. Takahashi and S. Takahashi, J. Vacuum Sci. Technol. A 3 (1985) 1502.
12
D. W. McComb et al. / Chemical Physics Letters 251 (1996) 8-12
[12] K. Raghavachari, J. Chem. Phys. 84 (1986) 5672;
K. Raghavachari and C.M. Rohlfmg, Chem. Phys. Letters 143 (1988) 428; J. Chem. Phys. 89 (1988) 2219. [ 131 H.-P. Cheng and U. Landman, Science 260 (1993) 1304. [ 141 CL. Cleveland and U. Landman, Science 257 (1992) 356.
[15] R.A. Wolkow and Ph. Avouris, Phys. Rev. Letters 60 (1988) 1049. [16] M.F. Jarrold, in: Advances in gas-phase photochemistry and kinetics; bimolecular collisions, eds. M.N.R. Ashford and J.E. Baggott (Royal Society of Chemistry, 1989).
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 25 1( 1996) 8- 12
An atom-resolved view of silicon nanoclusters D.W. McComb, B.A. Collings, R.A. Wolkow *, D.J. Moffatt, C.D. MacPherson, D.M. Rayner, P.A. Hackett, J.E. Hulse ’ Steacie Institute for Molecular
Sciences, National Research Council of Canada, IO0 Sussex Drive, Ottawa. Cat&u
KIA OR6
Received 29 November 1995
Abstract Atomically resolved scanning tunneling microscope images of Si clusters deposited onto a Sic11 l)-7 X 7 substrate have been obtained. The clusters display a capping layer of widely spaced surface atoms analogous to the reconstructed surface of a bulk crystal. Site-specific variations in the electronic characteristics of cluster surface atoms are detected. Adsorption studies demonstrate size dependant reactivity of deposited clusters. Clusters are observed to collapse upon annealing to form epitaxial Si islands.
Very small clusters, composed of from several to hundreds of atoms, display fascinating chemical [l] and optical properties [2]. While the characteristics of these nanoclusters bridge the gap between atomic and bulk characteristics, the transition is generally not smooth or predictable. In some instances very different properties are displayed by clusters differing in composition by only a single atom [3]. The goals of the cluster field are structure determination and correlation of structure with observable properties. Soon after the invention of the scanning tunneling microscope @TM) it was recognized that a direct atomic-level view of small supported particles might be attainable. One early study examined silicon clusters that had been deposited onto a gold surface [4]. Protrusions identified as clusters were observed but no atomically resolved features were evident. It was
* Corresponding author. ’Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, Canada KIA 0R6.
unclear whether experimental conditions had to be further optimized, or if innate limitations of the technique were preventing more details from being revealed. More recent studies involving the deposition of metal clusters including Pt, Pd, and Au on carbon substrates have shown profiles that could be interpreted as clusters [5-81, however no atomic features were evident. In this work we report the first atomically resolved images of deposited clusters. In particular, silicon clusters of various sizes were deposited onto a Sic11 11-7 X 7 substrate at room temperature. The STM used in this study is contained in an ultra high vacuum chamber with a base pressure of approximately 5 X lo- ” Torr. Within this environment samples remain free of contamination for many hours. The Si clusters were created in a Smalley type source using a 308 nm XeCl eximer ablation laser [9]. A control experiment was performed in which a clean substrate was exposed to the He carrier gas without the ablation laser being fired. No clusters were observed and there was no significant contami-
ooO9-2614/96/$12.00 0 1996 Elsevier Science B.V. All rights reserved PII SOOO9-2614(96)000681
D. W. McComb et al. /Chemical
nation of the Si surface. The silicon clusters were not mass selected prior to deposition. To achieve a true representation of the surface under study it is essential that a STM tip be formed with a single atom protruding sufficiently far from the apex that tunneling current is exclusively directed to that point. On flat surfaces this is relatively easy to achieve. For structures with sharply sloping features, such as deposited clusters, there is greater opportunity for secondary points on the tip to interact with the surface. This presents a problem because if multiple points on the tip contribute to the tunneling process, the resulting image will include convoluted contributions of tip and surface topography. A measure of the ability of a tip to accurately track sharply sloping features may be obtained by imaging surfaces that contain steps between flat terraces. The substrate used in this study displays well characterized 3.1 A steps [lo]. At a step edge, a sharp tip can accurately resolve lower terrace structure to within = 2 A of the step and can therefore also be expected to accurately image comparably sloped cluster features 2. Characterization of the tip in this way allows us to confidently interpret the top-most region of cluster images as true renditions of the cluster structure. Features toward the extreme edges of clusters are more prone to artifacts and must be interpreted cautiously. Fig. 1 shows an STM image of three deposited silicon clusters 3. The substrate is the extensively studied Si( 111j-7 X 7 surface [ 111. The regular pattern displayed by the substrate atoms serves as a convenient grid, against which cluster structure may be compared. The nearest neighbour spacing of the
* Ideally, tip characterization through step imaging should be done at steps of all angular orientations since a tip will often not be symmetric about its axis. 3 A range of cluster sizes has been deposited and it is not known how many atoms any particular cluster is composed of. Very rough estimates can be made by taking the height as the diameter and by assuming normal bulk density. In this way we find the largest cluster in Fig. 1 contains on the order of 250 atoms. We very roughly estimate that the smaller cluster, toward the left side of the image, is composed of only several atoms, while the cluster on the right contains on the order of 10 atoms. Possible distortions of the clusters and the difficulty in deconvoluting the crystallographic and electronic contributions that make up an STM image further complicate cluster size determinations.
Physics Letters 251 (I 996) 8-12
Fig. 1. STM constant current image of deposited Si clusters on the Si( 1 1 1j-7 X 7 surface. The sample bias was + 1.O V, the tunneling current was 0.04 nA.
substrate surface atoms is 7.7 A. The clusters in Fig. 1 have widely spaced surface atoms (6 to 8 A>, not unlike those on the substrate. Si(l1 l)-7 X 7 exhibits a surface reconstruction. That is, surface atoms adopt a configuration that serves to ‘cap’ more closely spaced internal atoms, thereby minimizing the number of dangling bonds. The widely spaced cluster features strongly suggest that a capping layer also forms on deposited silicon clusters. Theoretical models predict the existance of capping atoms on silicon clusters [12]. It is important to note that, unlike three-coordinate capping atoms which have dangling bond electronic character, four-coordinate (and higher) atoms in the surface vicinity are not expected to be visible in STM images, accounting for the large spaces between observed surface features. The effect of cluster-surface interaction is a major concern in studying deposited clusters. We cannot state with certainty if, or to what extent, the clusters imaged here have been modified upon deposition, however, it does seem that all of the clusters are flattened somewhat. For example, the large cluster in Fig. 1 is 21 f 0.1 A high and 40 * 5 8, wide. The large uncertainty in width derives from the problem of knowing where true cluster structure ends and tip-cluster convoluted structure begins. In any case, all the clusters appear to be approximately twice as wide as high. It is conceivable that the clusters are imbedded into the surface but the lack of
10
D.W. McComb et al./ Chemical Physics Letters 251 (1996) 8-12
Fig. 2. Two STM images of a Si cluster obtained with a tunneling current of 0.04 nA and a bias voltage of (a) + 1 V and (b) - I V. A gradient view is shown to enhance the visibility of local features.
structural changes. This is not to say that substrate temperature is not an important parameter. At sufficiently low temperatures thermodynamically unstable structures can be frozen in place. Indeed the present study provides an example of such a system. Fig. 2 shows silicon clusters that have collapsed to form epitaxial islands upon annealing to approximately 700 K. Evidence is also found (not shown here) of atom migration to substrate step edges. This result also verifies that the deposited clusters are composed of silicon and not an impurity. It will be necessary to perform mass selected cluster depositions to better understand the structure of deposited clusters. A composite three-dimensional view of clusters may emerge from images of many identical clusters, in various orientations. The range of electronic energy levels that contribute to an STM image is determined by the bias voltage between the tip and sample. By acquiring images at both polarities, details of the occupied and unoccupied electronic states may be examined. The + 1V STM image in Fig. 3a provides a view of the unoccupied states of a cluster while the - 1 V image in Fig. 3b shows the occupied states of the same cluster. The grid has been overlaid to aid in examining identical points in the two images. The arrow
a crater or displaced atoms surrounding the clusters tends to discount this view. Further understanding of the observed structures will only come from future experiments varying; (1) the impact energy, (2) the substrate, and, (3) the substrate temperature. In the present configuration the clusters have a velocity of approximately 1700 m/s 4. In the future, various soft landing schemes may be explored including deposition onto a mattress of condensed xenon [ 13,141, and electrostatically controlled landing. By changing the substrate it will be possible to examine the effect of the substrate-cluster strength of interaction. In the present case the clusters likely form several Si-Si bonds with a bond energy of = 2 eV/bond. It is worth noting that independent of impact energy and substrate temperature, this ‘heat of adsorption’ will be present, and it might lead to
4 This corresponds to 0.29 eV/Si would have a 2.9 eV kinetic energy.
atom.
Si,,
for example,
Fig. 3. A Si cluster which has been annealed to 700 K for 2 min. The image was obtained with a tunneling current of 0.04 nA and a bias voltage of + 1.O V.
D.W. McComb et al./ Chemicul Physics Letters 251 (1996) 8-12
Fig. 4. Silicon clusters (a) the day of preparation, and (b) 7 days later, changes have occurred as a result of adsorption of background gases. Imaging conditions were 0.04 nA and + 1 V.
marks a feature in the occupied state image that is not evident in the unoccupied state image. This observation demonstrates that STM may be employed to detect the electronic inequivalence of cluster surface atoms. STM studies of bulk surfaces have revealed similar site-specific electronic characteristics and these differences have been correlated with spatial variations in reactivity [ 151. Through analogy with the bulk surface results we expect that deposited silicon clusters will also have site-specific reactive character. STM has been successfully used to monitor reactions at individual surface sites and it is of interest to see if a similar view may be gained of cluster reactivity, particularly so since in cluster-beam flow reactor studies, sharp variations in reactivity as a function of number of constituent atoms have been demonstrated [16]. Our first attempt to observe size dependent reactivity involved observing a group of clusters over many hours to see if reaction would occur with background gases in the vacuum chamber (consisting primarily of H,, H,O, and CO). Fig. 4
Ii
shows three clusters; (a) on the day of preparation and, (b) 7 days later. The largest cluster, cluster 1, becomes smaller in appearance after adsorption has occurred, its height changing from 23.8 f 0.1 to 20.7 + 0.1 A. Cluster 2, shows a small change in height, measuring 19.5 + 0.1 A initially and finally 19.3 k 0.1 A. Cluster 3 becomes larger, changing from 18.4 &-0.1 A to 21.2 IL-0.1 A. This striking range of behavior provides a clear demonstration of size-dependent reactivity of deposited clusters. Normally on silicon, chemically bound adsorbates lead to elimination of protrusions associated with surface atom dangling bonds. This occurs as a dangling bond state is replaced by a chemical bond with a binding energy outside the energy window probed by STM. A reacted cluster is therefore expected to appear smaller. Based on this simple scenario we might conclude that cluster 1 reacted most readily (ie. more molecules adsorbed) with background gases and cluster 2 reacted to a lesser degree. The enlarged appearance of cluster 3 in Fig. 4 is however not understood at this point. We have described only gross structural changes related to adsorption in this work. Studies probing adsorption at individual sites are in progress.
References
[ll
M.F. Jarrold, Science 252 (1991) 1085, and references therein. Dl T.T. Rantala, M.I. Stockman, D.A. Jelski and T.F. George, J. Chem. Phys. 93 (1990) 7427. [31 S.A. Mitchell, L. Lian, D.M. Rayner and P.A. Hackett, J. Chem. Phys.. in press. [41 Y. Kuk, M.F. Jarrold, P.J. Silverman, J.E. Bower, W.L. Brown, Phys. Rev. B 39 (1989) I I 168. G.N. Subbanna and C.N.R. [51 H.N. Aiyer, V. Vijayakrishnan, Rao, Surface Sci. 313 (1994) 392. [61F.J. Aires, P. Sautet, G. Fuchs, J.-L. Rousset and P. Mt?linon, Microsc. Microanal. Microstnrct. 4 (I 993) 44 I. [7] A. Humbert, R. Pierrisnard, S. Sangay, C. Cbapon, C.R. Henry and C. Claeys, Europhys. Letters 10 (1989) 533. [8] Z. Ma, C. Zbu, J. Shen, S. Pang and Z. Xue, Vacuum 43 (1992) 111.5. [9] T.G. Diets, M.A. Duncan, D.E. Powers and R.E. Smalley, J. Chem. Phys. 74 (1981) 6511. [IO] R.S. Becker, J.A. Golovchenko, E.G. McRae,and B.S. Swartzentruber, Phys. Rev. Letters 55 (1985) 2032. 1111 K. Takayanagi, Y. Tanishiro, M. Takahashi and S. Takahashi, J. Vacuum Sci. Technol. A 3 (1985) 1502.
12
D. W. McComb et al. / Chemical Physics Letters 251 (1996) 8-12
[12] K. Raghavachari, J. Chem. Phys. 84 (1986) 5672;
K. Raghavachari and C.M. Rohlfmg, Chem. Phys. Letters 143 (1988) 428; J. Chem. Phys. 89 (1988) 2219. [ 131 H.-P. Cheng and U. Landman, Science 260 (1993) 1304. [ 141 CL. Cleveland and U. Landman, Science 257 (1992) 356.
[15] R.A. Wolkow and Ph. Avouris, Phys. Rev. Letters 60 (1988) 1049. [16] M.F. Jarrold, in: Advances in gas-phase photochemistry and kinetics; bimolecular collisions, eds. M.N.R. Ashford and J.E. Baggott (Royal Society of Chemistry, 1989).