摘要:

Ion sputtering as a probe of liquid surfaces M. A. Gleeson,a T. T. Nuver,b J. Lourenço,c A. M. C. Moutinho,c J. Losa and A. W. Kleyn†a a FOM-Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands b Debye Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands c GIDS-Departamento de Física de Ciéncas e Technologia, Universidade Nova de Lisboa, 2825 Monte de Caparica, Portugal Received 4th October 1999, Accepted 29th November 1999, Published 6th December 1999 We have measured the energy and angular distribution of the ions emitted from a perfluoropolyether liquid polymer surface due to an incident 320 eV Ar+ beam. The anion distributions are comprised almost exclusively of F–. Substantial amounts of highenergy F– recoils are produced, which can be attributed to multiple-collision processes.The production of these high-energy recoils indicates a very open surface structure, which allows hard (small impact parameter) collisions between Ar+ and individual functional groups of the polymer. Scattered Ar+ is only observed for small total scattering angles. room temperature is ~10–15 mbar. Fig. 1 shows a structural representation of a fragment of this molecule containing 7 of the polymer sub-units. SIMS measurements of the low energy (<20 eV) ions emitted from the polymer surface under Ar+ and e– irradiation showed production of mainly F– and fluorocarbon cations.18 Only a small fraction of the scattered ions contained oxygen, suggesting that the PFPE molecule orientates itself such that the oxygen atoms are concealed beneath the outermost layer of the surface.This may be achieved by orienting the -CF3 side-chain groups to form the surface layer. Such an arrangement is consistent with the results of previous studies on this surface. In this report we present measurements of the angular and energy distribution of high energy (>20 eV) ions produced by an incident 320 eV Ar+ beam. The production of substantial quantities of high energy F– recoils is consistent with a surface of open structure. Introduction Scattering of atomic and molecular ions is a common means of obtaining information about the structural and electronic properties of surfaces. Techniques such as secondary ion mass spectrometry (SIMS) and low energy ion scattering (LEIS) are routinely used to determine surface composition.Measuring the angular distribution of scattered particles can yield structural information, while the charge fraction leaving a surface can be used to study charge exchange processes. Apart from a fundamental interest in enhancing the understanding of ion–surface interactions, such studies have relevance to technological processes, particularly plasma processing. Most ion–surface interaction studies have concentrated on single-crystal metal surfaces.1–3 These surfaces have several practical advantages from an experimental point of view, including having a well-ordered periodic structure, being relatively easy to prepare and reproduce, and not suffering from charging problems.However, in recent years an increasing number of studies have been performed on non-metallic surfaces. These have included an increased interest in liquid surfaces, both from a theoretical and an experimental point of view.4–6 Part of this interest has been stimulated by the application of surface-induced dissociation (SID) in mass spectrometry.7 SID requires an inert, reproducible surface preferably with a high ion survival probability. Surfaces that have been considered for SID applications have included selfassembled monolayers8,9 and perfluoropolyether (PFPE) liquids.10,11 The interaction of ions with PFPE surfaces is also of interest in the field of reactive ion etching of Si, since fluorocarbon ion etching leads to the formation of a fluorocarbon polymer layer on the surface.PFPE surfaces have been studied by a variety of techniques, including hyper-thermal atom beam scattering,12 reactive ion scattering13 and angle-resolved XPS.14 These studies indicate that the surface is primarily composed of -CF3 and -CF2 functional groups. Studies on the scattering of CFx+ (x = 0–3) from a PFPE are consistent with scattering by a CF3-based group on the surface.15–17 The PFPE used is sold under the trade name Krytox 16256. Its molecular formula is: F[CF(CF3)CF2O]63(av)CF2CF3. Its average molecular weight is of the order of 11000 u and its vapor pressure at PhysChemComm, 1999, 15 Fig. 1 Structural model of a fragment of the PFPE molecule containing 7 polymer repeat units.Click here to access a 3D model. Experimental A schematic of the experimental set-up is shown in Fig. 2. It consists of three sections: a preparation chamber where samples can be characterized and stored, a low-energy ion beam source, and a scattering chamber in which the measurements are performed. The ions are produced in a low energy-spread plasma source19 and accelerated to 400 eV for transport along the beamline. A Colutron Wien filter (M/DM = 400)20 is used to select the ion of appropriate mass. A 10° bending section ensures the removal of light and neutral particles. The final step in the beam transport is deceleration to the desired kinetic energy and focusing on the sample.The beam energy can be varied between 50 and 400 eV. The final beam-spot at the sample surface was ~2 mm in diameter. The drain current on the sample was typically ~8 nA for the measurements shown in this paper.Fig. 2 Schematic of the experimental set-up. The PFPE liquid was supported on a stainless steel disc (diameter 10 mm, thickness 2 mm) which was mechanically polished to a roughness <0.05 mm. A smooth thin PFPE film was obtained by allowing the liquid to sheet across the support. This could be done under UHV conditions by changing the azimuthal orientation of the disc. The film thickness was of the order of 10 mm. The sample could be transferred by means of a linear translator to a two-axis goniometer in the centre of the scattering chamber.The ions are detected using a 90° cylindrical electrostatic energy analyzer with an energy resolution of DE/E » 0.08. The measurements shown in this report are the raw distributions uncorrected for the energy dependent transmission of the analyzer. All scattered ion distributions were measured in the scattering plane defined by the incident beam direction and the surface normal. The scattering angle definitions are illustrated in Fig. 3. The incoming angle ( qi ) and the outgoing angle ( qf ) are both defined with respect to the surface normal. qi can be selected by rotating the sample about the centre of the chamber, while qf can be varied by rotating the detector. The total scattering angle ( q) is given by 180 – ( qi + qf )°.The mass, and hence the identity, of the ions emitted from the surface could be determined by time-of-flight (TOF) measurements using a pulsed ion beam. All spectra were measured at room temperature. Fig. 3 Illustration of the angles defined in the scattering experiments. Results and discussion Fig. 4 shows a typical anion distribution produced by a 320 eV Ar+ ion beam incident on PFPE and measured at i = 80° and qf = 70°. It is characterized by a peak at ~280 eV and a broad distribution of sputtered anions at lower outgoing energy (Ef). If the analyzer transmission function is considered, the intensity of the sputtered particles increases rapidly at low Ef. However, the 280 eV peak remains a prominent feature of the distribution.This energy distribution is a cross-section from a larger 3D scan measuring the outgoing anions as a function of qf and Ef. The full 3D distribution is shown in the contour plot inset on Fig. 4. It can be seen that the peak observed is part of a high intensity ridge of anions that spans the range of outgoing angles measured. The maximum scattered intensity occurs at the qi = 80° and qf = 70° geometry. TOF measurements performed at different outgoing energies showed only recoiled F–, with no negatively charged molecular ions being detected. In addition, SIMS measurements using Ar+ beam irradiation performed on the PFPE film have yielded mainly F– with only minor production of molecular anions.18 The simplest picture for the ejection of fluorine from the PFPE surface is one of direct recoil due to an Ar–F collision.Treating the fluorine as a free atom (i.e. not influenced by its bond to the PFPE), we can apply the binary elastic collision model3 to estimate, as a function of qf , the recoil energy that can be transferred due to a single collision with 320 eV Ar+. The outcome of this calculation is shown by the arrow, and by the solid line on the contour plot, in Fig. 4. The broad distribution of anions with energies below this calculation can be accounted for by additional energy losses due to inelastic processes and multiple collisions in the sputtering cascade q Fig. 4 Energy distribution of the negative ions produced by a 320 eV Ar+ ion beam incident on PFPE at qi = 80°, measured at qf = 70°.The arrow indicates the expected energy of a F atom elastically recoiled by a 320 eV Ar atom. Inset: Contour plot showing the distribution of the anions as a function of outgoing energy and outgoing angle. The dotted line indicates the crosssection shown in the main figure. The solid line shows the expected position of elastically recoiled F atoms based on the binary collision model.generated by the Ar+ impact. However, the position of the high-energy ridge requires some of the F– to gain more energy than is allowed in a single collision. Such high energy particles can be explained using a double/multiple collision model. Production of negatively charged particles with unusually high energies has previously been observed from metal surfaces bombarded by keV He+ and Ar+ ions.21 Initially these were interpreted in terms of electron emission.However, it was subsequently demonstrated that these particles were due to ejection of negatively charged surface atoms and a double collision model is required to explain the high kinetic energy attained by some of these particles.22,23 Fig. 5 Models illustrating recoiling of F– in a single and a double collision process. Fig. 5 gives an illustration of how the complexity of the energy transfer can be increased by moving from a single to a double collision model for the case of Ar recoiling F– from PFPE. In the case of a single collision, the energy gained by the F– is determined by the recoil angle ( j) and the ratio of the two colliding masses.In contrast, in the double collision model shown, the Ar is initially scattered through an unknown angle ( qscat) by an unknown mass, M. The energy it retains is given by qscat and by the mass ratio between Ar and M. The heavier M is, the more energy Ar retains. The scattered Ar can now recoil a fluorine atom as before. The two collision events occur independently and due to the PFPE structure it is unlikely that they will occur in the plane defined by the macroscopic surface (to which qi and qf are referenced). Hence, although the sum of qscat and j equals the experimentally defined q, absolute values for the two angles cannot be assigned experimentally.The final energy of the recoiled F– is now determined by two angles ( qscat and j) and three masses (Ar, M and F). By increasing M, the maximum kinetic energy that can be elastically transferred to a free F atom can be substantially greater than for a simple binary collision. It should be stressed that the double collision sequence shown in Fig. 5 is only one of a range of possible sequences that can be envisaged in order to yield high-energy recoils. For instance, the recoiling of the F– may occur first, followed by scattering of the F– from a component of the PFPE liquid. Alternatively, it is conceivable that the high-energy recoils are produced in a single concerted collision of an Ar+ with a CF3 group. In this case, the double collision model would involve the F– scattering off the functional group to which it was initially attached.Moving to a multiple collision model will further complicate matters by increasing the number of variables in the collision sequence. For keV noble gas atoms colliding with a single crystal metal surface, application of the binary collision model is relatively straightforward. Collisions occur between atoms of well-defined mass, and the position and spacing of the scattering centers in the crystal lattice is known. The situation for PFPE is more complex. The surface is no longer composed of well-ordered, single-mass units, and the sub-units of the polymer chain have a high mobility at room temperature compared to metal atoms. If one accepts the description of the scattering in binary collision terms, then an effective surface mass (ESM) can be ascribed to the surface in order to explain the observed energy of the scattered particles.Previous studies have demonstrated that the ESM of the PFPE for scattering of incident particles is dependent on the energy and mass of the particles in question16 and on the total scattering angle (i.e. no single ESM is universally applicable to the surface). In view of this, making quantitative assignments of a precise collision mechanism for the production of high energy F– is difficult. For metal surfaces under the incidence conditions studied,23 high-energy recoils arise from collision processes that occur with a low frequency relative to low energy sputtering and single collision recoiling events.Hence the recoil processes leading to high-energy particles are a minority channel accounting for only a small fraction of the total recoiled ion flux. In contrast, in the case of the PFPE high-energy recoils are a prominent feature in the measured distribution. High-energy recoils from a metal surface can be produced if the incident ions undergo an initial pre-scattering followed by recoil sputtering. This is only possible at high incident energies, where the scattering cross-section of the individual atoms is very small relative to the surface unit cell. For ions incident in the keV regime, the small crosssection of the individual atoms translates to an open surface composed of widely-spaced scattering centers (e.g.ref. 24). Hence successive collisions between the incident particle and adjacent surface atoms are feasible. Scattering crosssections increase at lower incident energies and eventually (<50 eV), the collision changes from scattering between isolated atoms to scattering from a corrugated surface potential.25 MARLOWE calculations, which utilize the binary collision approximation, have been successful in simulating high-energy recoils for 5 keV Xe+ incident on Cu(110).23,26 Similar high-energy recoils are not produced by the MARLOWE code for 350 eV Ar incident on single crystal Cu.26 High-energy recoils can be produced at this energy if a polycrystalline surface is used (which permits incident trajectories along very open surface planes). These calculations generate recoil distributions that are comparable to the F– distributions shown in this paper, suggesting that the PFPE surface also has an open structure.In the case of the PFPE surface, the high-energy recoils form a peak that is clearly resolved and significantly more intense than the ions that can be attributed to single collision recoiling. Simply having an open structure is not sufficient to explain the large intensity of high-energy recoils measured from the PFPE, since this also favours the production of single collision and low-energy sputtered recoils to an equal or greater degree. In order to explain the intensity distribution in Fig. 4, we must account for the enhancement of the multiple collision sequences that lead to high energy recoils.One factor that will tend to favour such multiple collisions is the absence of high atomic mass (z) atoms on the PFPE surface.The heaviest atom in the polymer, F (z = 19), is less than half the Ar mass (z = 40). This means that the individual constituent atoms of the PFPE are incapable of deflecting Ar through a large q in a single collision (e.g. for a binary elastic collision of an Ar atom with a F atom: q £ Ar, the maximum q through which f variationf max » 28°). This is illustrated in Fig. 6, which shows a 3D and contour representation of the distribution of positive ions obtained for 320 eV Ar+ incident on PFPE at qi = 80°. The distribution shows a sharp, high energy (~300 eV) peak at grazing outgoing angles, which decreases rapidly both in intensity and energy as qf decreases. In addition, there is a broader distribution of ions at lower energy and smaller outgoing angle.TOF measurements demonstrate that the high energy peak is composed exclusively of Ar+, whereas the low energy distribution is due to C+. The Ar+ distribution is clearly peaked along grazing exit angles. Also shown on the contour plot are three calculations assuming binary elastic scattering of 320 eV Ar by a mass 40 (MAr), mass 31 (MCF) and mass 19 (MF). These models illustrate that, for a single elastic scattering of Ar by a given mass M, where M < M the Ar can be scattered decreases with M.The E of the scattered Ar+ as a function of qf is typical of scattering by an ESM equal to or less than the mass of Ar. The trend in the Ar+ distribution suggests that as q decreases (increasing q) the ESM responsible for scattering Ar decreases. q Fig. 6 Contour and 3D representation of the positive ion distribution as a function of outgoing energy and outgoing angle produced by a 320 eV Ar+ ion beam incident on PFPE at qi = 80°. The solid, dashed and dotted lines show the expected position of elastically scattered Ar atoms on the basis of scattering by mass 40, mass 31 and mass 19, respectively. Unlike F–, the Ar+ intensity distribution shown in Fig. 6 does not have a clear intensity maximum as a function of f. The scattered intensity continues to rise until the physical cut-off due to the sample at qf = 90°.The Ar+ peak qq is only observed for grazing incident angles (large qi ). When qi is reduced the peak maximum remains at grazing f but diminishes rapidly in intensity. The peak is already absent for qi = 70°. This is indicative of scattering between isolated particles, where the cross-section for scattering is strongly peaked in the forward direction (i.e. scattering into or parallel to the surface is favoured). The Ar+ distribution seen in Fig. 6 can be regarded as the edge of a peak due to single scattered Ar+, where the peak maximum is directed into the sample surface. Thus, changing q results in the ions measured at q i from 80 to 70° f = 85° in Fig. 6 being shifted to qf = 95° (i.e.more of the Ar+ ions undergo multiple collisions with the PFPE). The Ar+ ions in Fig. 6 must be scattered from the outermost layers of the PFPE surface. Ar+ ions that enter the PFPE film will undergo multiple collisions and will be scattered through a broad angular and energy distribution resulting in a low scattered flux along any given direction. In contrast to the Ar+ distribution, the maximum intensity of the scattered F– appears at a sub-specular angle suggesting that these ions emerge from somewhat deeper in the PFPE film. The intensity drop at large qf can be attributed to efficient blocking and deflection of ions emitted along grazing trajectories. Classical trajectory calculations of H2 scattering from Ag(111)27,28 have successfully reproduced experimentally measured grazing angle scattering of the type demonstrated by the Ar+ in Fig.6. These calculations show that double collisions become more important at grazing incident angles, and that a large impact parameter collision is more likely to lead to further collisions. Under grazing incidence conditions, large-impact parameter interactions of the incident ions with the functional groups of the polymer can result in steering toward a trajectory more parallel to the surface (planar scattering). A rough surface comprised of isolated protruding functional groups would facilitate this mechanism. Deflecting Ar by means of several largeimpact parameter (weak interaction) collisions allows its trajectory to be changed with a minimum of energy loss.Such a mechanism is necessary to explain the highest energy F– ions observed at the most grazing exit angles on Fig. 4. Moving the Ar to a planar trajectory increases the probability of colliding with a protruding functional group rather than with a random point on the polymer chain. This should increase the similarity between collision events that yield high-energy F– ejection and result in some uniformity in the energy transfer during ‘hard� collisions. Such uniformity is suggested by the fact that the high-energy F– recoils exhibit a peak structure rather than a simple extension of the broad low energy distribution. F– production arising from a collision involving a specific functional group is consistent with the picture of a surface comprised mainly of CF3 groups that has been presented by previous studies.+ x The sharp peak at ~12 eV seen in Fig. 6 is due to sputtered ions produced in a traditional collisional cascade (as opposed to directly recoiled). TOF measurements on this peak reveal that it is mainly composed of CF (x = 0 to 3). SIMS measurements performed on the PFPE also demonstrate that these low energy sputtered particles consist of several different fluorocarbon cations.18 These measurements show that there is no electronic impediment to the production of fluorocarbon cations. Hence, it is noteworthy that C+ is the only cation recoiled with a high energy (>20 eV), and also that this ion does not attain as high a maximum Ef as the F–.In order to gain an appreciable kinetic energy from the incident ion, an emitted fragment should be directly involved in the collision rather than simply arising from the collision cascade or from fragmentation due to a local deposition of energy. Theabsence of high energy fluorocarbon recoils from both the positive and negative ion distributions suggests that the hard collisions that are required to yield these energetic particles result in complete atomization of the local molecular structure. The failure of the C+ recoils to attain upper energies comparable to the F– can be attributed to steric hindrance by the F atoms. A direct collision between Ar and C is prevented by the F atoms which will always act as shields (cf.space-filled model of Fig. 1). Hence, the energy transferred in the production of C+ will be distributed among several different atoms. In contrast, direct recoiling of F– with relatively little energy transfer to the functional group to which it is attached is feasible. + 2 3 2 The production of F– by incident Ar+ is remarkable given the high ionization potential of the surface (>12 eV). It is most probably formed during the hard collision with the incident Ar+. Once formed, it does not have a readily available neutralization path on the insulating PFPE surface. Charge fraction measurements of incident cation beams have demonstrated a high survival probability for ions interacting with the PFPE surface.For scattering of H and C+ from the PFPE ion survival probabilities of ~50 and >90%, respectively, have been measured.29 If in addition to F– production, the Ar+ is neutralized in the collision then the surface must accommodate the loss of two electrons for the production of a single F–. In reality neutralization of the Ar+ may not be a very important factor due to the insulating nature of PFPE. However, the production of F– still raises the prospect of beam induced charging of the surface. Cation distribution measurements do indeed show evidence of a beam-dependent local charging of the PFPE by the incident beam. Measuring the cation distribution normal to the surface ( qf = 0°) showed a sharp peak with a beam-dependent energy of typically between 50 and 80 eV.In terms of width, this peak resembled the low energy sputter peak seen on Fig. 6. TOF measurements reveal that the peak consisted of several fluorocarbon ions, predominantly C+, CF+, CF + and CF +, which is also consistent with the SIMS measurements performed on low energy sputtered ions from this surface.18 The unusually high and identicalnergy of these ions can be attributed to coulombic repulsion from charged regions of the sample. This normal emission peak was not always observed (it is absent from Fig. 6); its appearance was flux dependent. It was not observed for low incident beam currents (<10 nA), as used to measure Fig. 4 and 6. It emerged for higher currents (>20 nA), although the reproducibility in terms of energy position and shape was poor as a function of time.In contrast, the measured anion distributions were very reproducible as a function of exposure time and beam current. There is no evidence of a progressive accumulation of charge on the surface, or for the development of a uniform global charge. Hence, the high-energy normal emission cations appear to originate from local highly-charged regions created by the Ar impact. Polymer rearrangement, coupled with ejection of fluorocarbon cation fragments by coulombic repulsion, must allow these regions to discharge on the time-scale of the experiment, thus preventing charge build-up. Conclusions Collision of 320 eV Ar+ with PFPE results in the production of substantial amounts of high-energy F– recoils, which are attributed to a double/multiple collision process.The high flux of these high-energy recoils relative to the ‘normal� recoils is direct evidence of a very open and rough structure to the PFPE surface. Despite this, the peak structure of the high-energy recoils points to the presence of preferred scattering events. This is consistent with the picture of the PFPE surface being comprised of mainly CF3 groups that provide the site for the bulk of the collision events. The scattering cross-section for Ar+ on the PFPE surface is strongly peaked in the forward direction, which leads to the observation of a maximum in scattered intensity along the surface. The results are consistent with the incident ions being deflected into a more planar trajectory due to scattering by surface atoms.Subsequent `hard� collision on a protruding molecular group can then lead to high energy recoils. Acknowledgements The authors would like to thank F. G. Giskes, R. Schaafsma and D. Conceiço for technical support. This work is part of the research program of the `Stichting voor Fundamenteel Onderzoek der Materie (FOM)�, which is financially supported by the `Nederlandse organisatie voor Wetenschappelijke Onderzoek (NWO)�. Footnote † Current address: Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, P.O. Box 9502, 2300 RA Leiden, The Netherlands. E-mail: a.kleyn@chem. leidenuniv.nl; Fax: +31 (71) 5274451. References 1 J.Los and J. J. C. Geerlings, Phys. Rep., 1990, 190, 133. 2 H. Niehus, W. Heiland and E. Taglauer, Surf. Sci. Rep., 1993, 17, 213. 3 Low energy ion-surface interactions, ed. J. W. Rabalais, Wiley, Chichester, 1994. 4 J. G. 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Niehaus, Surf. Sci., 1999, 423, 276. 24 E. J. J. Kirchner, A. W. Kleyn and E. J. Baerends, J. Chem. Phys., 1994, 101, 9155. 25 R. J. W. E. Lahaye, A. W. Kleyn, S. Stolte and S. Holloway, Surf. Sci., 1995, 338, 169. 26 T. T. Nuver, unpublished data. The incoming polar angle was 70° and the incoming azimuthal angle coincided with the close packed surface direction, [110]. 27 U. van Slooten, E. J. J. Kirchner and A. W. Kleyn, Surf. Sci., 1993, 283, 27. 28 U. van Slooten and A. W. Kleyn, Chem. Phys., 1993, 177, 509. 29 M. A. Gleeson and A. W. Kleyn, unpublished data. Paper a907932g PhysChemComm © The Royal Society of Chemis

ISSN:1460-2733    

DOI:10.1039/a907932g

出版商:RSC    

年代:1999

数据来源: RSC