Scattering of low-energy Li+ and He+ ions from clean and oxygen covered Ni(110) surfaces

Nuclear Instruments and Methods 194 (1982) 663-666 North-Holland Publishing Company

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S C A T Y E R I N G O F L O W - E N E R G Y Li + A N D H e + I O N S F R O M C L E A N A N D O X Y G E N COVERED Ni(ll0) SURFACES W. E N G L E R T , E. T A G L A U E R Max-Planck-lnstitut fiir Plasmaphysik, Euratom-Association, D-8046 Garching bei Miinchen, Fed. Rep. Germany and W. H E I L A N D UniversitiJt Osnabri~ck, Fachbereich Physik, D-4500 Osnabri~ck, Fed. Rep. Germany

Comparison of Li + and He + ion scattering allows the investigation of neutralization and multiple scattering effects. These effects contribute to a very different extent to the energy spectra of both ionic species. Results are presented for scattering from a Ni(110) surface, either clean or with a (2 x 1) oxygen overlayer. The dependence of the ion yields on crystal orientation leads to the suggestion that shadowing effects plus trajectory-dependent neutralization have to be invoked.

1. Introduction Ions with primary energies below a few keV are very well suited for surface studies [1]. Due to the large cross sections they are effectively scattered from the top atomic layers of a solid surface. This effect is enhanced in the case of rare gas ions by neutralization of virtually all ions which penetrate more than one or two atomic layers into the target. Therefore low energy ion scattering appears also to be a very useful tool for studying ordered adsorption layers. One of the first systems investigated was the (2X l) oxygen overlayer on a Ni(110) crystal surface [2]. This adsorption system was also one of the first to be studied by low energy electron diffraction [3], but the position of the atoms in the unit cell cannot be given by a LEED pattern analysis alone. The ion scattering analysis showed that it could be used to distinguish between various structure models, but no complete answer could be given. Since then the Ni(110)oxygen surface was investigated by ion scattering in several laboratories [4-7] as were other ordered adsorption structures [8]. The reason why ion scattering analysis also cannot be immediately exploited for giving unambiguous results is again the neutralization effect which ensures the specific surface sensitivity. The neutralization probability is not known in general, it depends on the particular ion-target atom combination, ion energy, and on the ion trajectory on the surface [9,10]. Therefore additional measurements or assumptions are necessary if ion scattering data are to be used for surface structure determination. In the present paper a comparison is reported between He + and Li + ion scattering from Ni(110)(2 × 1) 0029-554X/82/0000-0000/$02.75 © 1982 North-Holland

O. Li + ions were shown [11,12] to have an ion yield close to one for this Ni surface and therefore the neutralization effect is less problematic for the interpretation of the results. This advantage is obtained at the expense of increased multiple scattering contributions, so that the scattered ion energy spectra can only be fully explained using detailed calculations [13]. Application of both ionic species is intended to obtain the single scattering part from the He + spectra and quantitative numbers from the Li + results. First experimental results are given in the following.

2. Experiment The experiments were performed in an UHV ion scattering apparatus described earlier [14]. Li + and He + ions with primary energies of 600 eV were used throughout this study. Both ionic species could be extracted from the same ion source, either by helium-gas input or by heating a fl-eucryptite source for Li +. The magnetic mass separator allows to direct either one of the ionic beams on the same spot on the target surface within seconds. It is possible to operate with Li + currents in the pA range and therefore surface contamination is no problem. A collimated beam of oxygen gas could be directed onto the target surface and the development of ordered adsorption structures was observed in situ using a 4-grid LEED system. A (2 X I) structure is obtained by room temperature adsorption of oxygen on Ni(110). Subsequent heating to about 420 K results in a second adsorption phase (high XI. ION SCATTERING / SURFACE STUDIES

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w. Englert et al. / Scattering of Li " and He

temperature "H.T." phase) which shows also a (2 × 1) pattern, but with sharper spots. Both phases can be distinguished in the ion scattering results.

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shows up clearly. If we assume that the surface contains half a monolayer of O (see however refs. 4-6, 15, 16) and take into account the scattering cross sections (calculated from T h o m a s - F e r m i - M o l i 6 r e potentials: ONi/Oo =2.85) we get almost the same neutralization probability for He + scattering from O and from Ni. The azimuthal region ~p = 50 ° to 60 ° is dominated by single scattering events, and in case of He + it is single scattering only [10,13]. The He + intensity is however reduced by a factor of about 2 compared to the clean surface, whereas the Li ~ -intensity is virtually unchanged (fig. 2 and ref. 10). That is, in this azimuthal range blocking is not important. Furthermore, the Li + O intensity is independent of the azimuth, indicative of single scattering. The Li + - O intensity relative to the Li + - N i intensity ratio agrees with the assumption of ½ monolayer coverage, a cross section ratio of 3 and a single scattering contribution of about 80% to the Li + Ni intensity [13]. These observations lead to the conclusion that (1) the He + - N i intensity decrease compared to the clean surface is due to neutralization and (2) the azimuthal dependences (fig. 2) of the Ni and O signal are due to neutralization.

3. Results and discussion Energy spectra of He ÷ and Li + ions scattered from the clean N i ( l l 0 ) surface and from the H.T. Ni(110) (2 × 1) O are shown in fig. 1. The absolute numbers in relation to the primary current show that the He ~ ion yield from the clean Ni surface is only about 10% of the Li + ion yield. Thus, if the probability for ion survival is about 1 for Li + [11,12], it is of the order of 0.1 for H e + , as was discussed in more detail in a previous paper [10]. The Li + spectra exhibit more contributions from multiple scattering leading to low-energy tails and "background" as well as to asymmetric peak shapes, depending on azimuth. It is interesting to compare these spectra in the case of the "R.T." oxygen ovedayer (fig. 2). For the given experimental parameters the H e + - N i intensity decreases by a factor of about 2 and an oxygen peak

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Fig. I. Energy spectra of He + and Li + ions scattered from a clean Ni(110) surface and from the high temperature ("reconstructed"') Ni(110)(2 × 1) O. The dashes indicate the projectile energy after one single bina~ collision.

665

W. Englert et al. / Scattering of Li + and He + ions

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Fig. 3. Peak height intensities of the spectra in fig. 1 as a function of the azimuthal angle ¢p for the high temperature ("reconstructed") Ni(110)(2X 1) O.

Similar conclusions hold for the H.T.-overlayer (figs. 1 and 3). First of all the changes in both, the Li + and He + ion yield dependence show that a rearrangement of the surface takes place upon heating to 420 K although both phases show a (2 X 1) LEED pattern. This is in agreement with the results from other laboratories [3-6,16,17]. It should be noted that the L i ÷ - O scattering peak sits on a fairly high background, particularly for ~ values around 20 °, causing larger experimental errors. One of the most striking features is the change in the He ÷ - N i signal in going from room temperature to the reconstructed surface. This signal is virtually independent of ~ for the clean surface [10] and shows a shallow broad maximum for room temperature adsorption. In the H.T. case, however, the He ÷ - N i signal drops by at least a factor 5 between the (110) and the (001) azimuth. The Li + - N i signal has the more pronounced azimuthal dependence which was already found in the clean case and decreases by about a factor of two between ~ = 0 ° and 20 °. The Li ÷ - O dependence shows a tendency to the same structure as the Li + - N i dependence. This Li ÷ - N i curve could be explained by multi-

ple ("zig-zag") collisions for scattering from the clean surface [10,13] and is not found for He +, obviously due to much more effective neutralization of multiply scattered He + ions as compared to those scattered from surface atoms. The difference between the He + and Li + ¢p-dependence suggests, that shadowing is not the main reason for the intensity decrease between ~o=-0 ° and c? = 80 °. It must therefore be concluded that neutralization of He + ion depends on the path of the ion on the surface and not only on the particular scattering atom. The fairly open structure proposed by Van den Berg et al. [5] could be a starting point for an explanation. In ths model one has mainly single scattering conditions for ion trajectories in the [ll0] direction because every second [100] surface row is assumed to be missing, the O-atoms sitting close to the long bridge position in these rows. In the [100] direction, however, more O - N i multiple scattering could occur which then has to be concluded to result in lower ion yields. A more detailed discussion of scattering from the Ni(110) (2 X l) O surface including the various structure models will be given in a forthcoming paper [18]. XI. ION SCATTERING / SURFACE STUDIES

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W. Englert et al. / Scattering of Li + and He + ions

4. Conclusions The results on He + a n d Li + ion scattering demonstrate that this c o m b i n a t i o n is very useful for o b t a i n i n g more quantitative data, particularly with respect to ion neutralization. The structural changes of the investigated N i ( l l 0 ) - O surface u p o n heating clearly shows up in the ion scattering spectra. Oxygen adsorption strongly influences He + ion neutralization a n d this effect dep e n d s o n the scattered ion trajectory. Exact trajectory calculations seem to be necessary if detailed surface structure analysis is intended.

References [1] D.P. Smith, J. Appl. Phys. 38 (1967) 340. [2] W. Heiland and E. Taglauer, J. Vac. Sci. Tech. 9 (1972) 620. [3] L.A. Germer and A.U. MacRae, J. Appl. Phys. 33 (1962) 2923. [4] L.K. Verheij, J.A. Van den Berg and D.G. Armour, Surf. Sci. 84 (1979) 408. [5] J.A. Van den Berg, L.K. Verheij and D.G. Armour, Surf. Sci. 91 (1980) 218.

[6] R.G. Smeenk, R.M. Tromp, J.V. van der Veen and F.W. Saris, Surf. Sci. 95 (1980) 156. [7] C. Varelas, H.D. Carstanjen and R. Sizmann, Phys. Lett. 77A (1980) 469. [8] For a review see e.g.W. Heiland and E. Taglauer, Surf. Sci. 68 (1977) 96. [9] D.J. Godfrey and D.P. Woodruff, Surf. Sci. 105 (1981) 438. [10] E. Taglauer, W. Englert, W. Heiland and D.P Jackson, Phys. Rev. Lett. 45 (1980) 740. [1 I] E.G. Overbosch, B. Rasser, A.D. Tenner and J. Los, Surf. Sci. 92 (1980) 310. [12] A.J. Algra, Thesis, University of Groningen ( 1981 ). [13] D.P. Jackson, W. Heiland and E. Taglauer, Phys. Rev. B, in press. [14] E. Taglauer, W. Melchior, F. Schuster and W. Heiland, J. Phys. E8 (1975) 768. [15] D.F. Mitchell, P.B. Sewell and M. Cohen, Surf. Sci. 69 (1977) 310. [16] P.R. Norton, R.L. Tapping and J.W. Goodall, Surf. Sci. 65 (1977) 13. [17] C. Benndorf, B. Egert, C. N6bl, H. Seidl and F. Thieme, Surf. Sci. 92 (1980) 636. [18] W. Englert, E. Taglauer and W. Heiland, ECOSS IV (1981) Surf. Sci. 117 (1982), in press.