Bertschat, H H

Forkel-Wirth, Doris

Haas, H

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zlwlflMcGill University, Montreal, Canada OCR Output1991GenevaContactman: H. HaasSpokesman: H. HaasG. Weyer4· and W.—D. ZeitziLuckscheiterii, A. Mayerii, B. Moorea, G. Schatzii, S. Seegerz,H.-H. Bertschatl, D. Forkelz, H. Haasz, R. Kowallikl, B.Berlinl-CERN?-Konstanz3-Arhus4 CollaborationRadioactive Ions for Surface ExperimentsProposal to the ISOLDE CommitteeBl ’*1Z"Cém #.i§£¤.;;t...9:;.13SCO0O00215 31 July 19910T~5;``{’?°'S"’*°"llllllllllllllllIllllllllllllll llliicam LIBRARIES, GENEVA CERN/ISC 91-10I \ C’ Nbeams on surfaces is being planned. OCR OutputA parasitic test of a novel concept for direct deposition of ionexperiment lS20.emission MS of ll9ln-ll9Sn and ll9Sb-ll9Sn, a technique developed inintended to study some of the systems characterized by PAC also withcan be obtained via Mossbauer spectroscopy (MS). lt is thereforeInformation about the local dynamical behaviour of surface atomsisolated 100Pd nuclei.3) Measurement of the magnetic structure of Pd-Ni interfaces usingferromagnets with 79Rb and 77Kr sources.2) Study of the magnetic nature of non-metallic impurities onemphasis on segregation phenomena.on Si and Ge surfaces via the electric field gradient with special1) Investigation of the surface sites and diffusion properties of 1**mCdsurfaces, and magnetic systems. The planned PAC experiments include:extend these measurements to other probe isotopes, semiconductorconstructed by the collaboration and successfully tested at ISOLDE-3 tobeamline and the new UHV chamber for surface physics (ASPIC) recentlycombination of the Booster ISOLDE Facility equipped with a UHVIt is now proposed to use the unique possibilities offered by thebe investigated in detail using the probe isotopes *1*mCd and lllln.isolated impurities on surfaces. Up to now only metallic systems couldpotential of perturbed angular correlation (PAC) measurements to studycomplementary experiments at Konstanz have demonstrated the greatThe results of experiment IS31 at ISOLDE and a series ofSummarytransitions in 2-dimensional systems. ln view of their catalytic OCR Outputnoble gas covered surfaces, where the primary interest are phasesemiconductor surfaces and magnetic materials as well as possiblyat ISOLDE. Clearly the next stage should be the investigation ofinterest to use other probe isotopes |ike.79Rb and 77Kr, only availablefurther systematic measurements. In this context it is also of greatof systems investigated up to now is very small, however, calling forto characterize isolated impurity sites on metal surfaces. The numberestablished that PAC measurements at least with *1*Cd are well suitedln on stepped Pd(111) surfaces could be completed. lt has thus beenwhere in particular a quantitative study of surface diffusion for Cd andKonstanz and in the pilot experiment |S31 performed at ISOLDE-2,of such experiments could be overcome in laboratory experiments aton—line isotope separator like ISOLDE. For PAC the technical problemsunique microscopic information, particularly when combined with anapplication of these methods to study surfaces therefore can givein favourable cases, much less than a monolayer (~ 1015/cm?). Thegiven detailed insight. Both techniques require as little as 10** atomscorrelation (PAC) and Mossbauer spectroscopy (MS) have in this case, of nuclei as probes. Particularly experiments with perturbed angulardefects in bulk metals has been very successfully attacked with the useIn recent years the corresponding problem for impurities andinformation on electrical and lattice structure for such systems.surfaces. It therefore would be quite important to get microscopicVery sparse data, however, exist for isolated impurities or defects oncoverages a wide variety of experimental methods has been developed.areas of physics. For the study of pure surfaces or for monolayerSurfaces and interfaces are a field of great interest in severalI. MOTIVATIONFig.1: Desorption isochrone for *11mCd from Mo(110) OCR OutputT ( K I100 200 300 L00 5000.20.1.0.60.8————————1.0Mo(110) was studied in detail /1/. We have demonstrated that theIn the first experiments performed at ISOLDE the system 11iCd onll. STATUSchannehng.surface techniques like recoil angular distribution or surfacestudies. Further progress can be expected in the development of newfurthermore open up many new possibilities for surface and interfaceThe direct deposition of radioactive ions at surfaces wouldcharacterized surfaces.defined interfaces may be produced by controlled coverage of previouslymagnetism at interfaces are a topic of great present interest. Welltechniques are not applicable at all. Both the lattice structure and thesuited for the investigation of interfaces, where the standard surfaceThe nuclear techniques PAC and MS should be particularly wellcandidates for further experiments.activity, the aIkaIi—c0vered Pd and Pt surfaces are also primaFig.2: Identified surface sites occupied by In and Cd adatoms on Pd(111) OCR Outputdl substifuiioneller Stufenplotz el substitutioneller TerrossenplotzolTerrossenplotz bl Sfufenplotz tl Eckenpiotz©!@%2 @@9Yzzcould be studied in detail /3/.dependence of their occupation the adatom diffusion for this systemthe identified sites are shown schematically. From the temperaturegradient (efg) as observed with PAC was used as signature /2/. ln Fig.2up to their incorporation into the top surface layer. The electric fieldA model surface Pd(111) from the first landing position on the terrassesto characterize the sites taken up by isolated In and Cd atoms on theln a further series of experiments at ISOLDE-2 we have been ablesuccessfully tested.difficult to interpret. Also the case of 79Fib (20 min) has beencoverage necessary, these curves are much more complex and veryalso been recorded earlier with stable isotopes, but due to the largesimple desorption isochrone observed for this system. Such data haveobservation via well defined PAC spectra is possible. Fig.1 shows thedeposition of isolated radioactive llimCd (48 min) atoms and theirsmall handling manipulator. Surface cleaning is accomplished with the OCR Outputonto the low temperature measuring manipulator with an additionaland chemically polished, can be transferred from a 12 sample stacktakes the activity into the main chamber. The samples, mechanicallytransfer foil mounted on a special transportation manipulator thatrotatable implantation oven onto a carefully cleaned and degasedlS31: The ISOLDE isotopes are evaporated in a small chamber out of thetechnique of two—stage deposition as already developed in experimentsurface and interface measurements (see Fig.3). lt is fully based on thespecially designed for operation at ISOLDE, is a versatile system forOur new Apparatus for Surface Physics at ISOLDE CERN ("ASPIC"),proposal.and could be used in some of the parasitic tests foreseen in the presentsomewhat improvised UHV apparatus. This equipment is still availableFor the project IS31 we had installed at ISOLDE-2, beam line 2A, anuclei.of surfaces is the sample preparation and the deposition of the probeThe central problem in the use of radioactive atoms for the studylll. APPARATUSNi /6/, could be obtained.could be studied by PAC. Also first results for a magnetic system, ln on/4/ as well as isolated impurities on several metallic surfaces /5/(2.7 d) has been deposited in a similar way. Monolayer coverages on lnIn a series of off-line experiments at Konstanz the isotope llllnon a microscopic scale.at ISOLDE to investigate isolated impurities at surfaces and interfacesclearly has demonstrated the great potential of the nuclear techniquesThis complete picture of surface diffusion for the model systemexperiments proposed, the chamber can be fully operated with nonOCR Outputcharacterization. To eliminate vibrations that might impair the MSsystem also contains a LEED system for surface structuresample purity may be checked with a CMA Auger spectrometer. Thefilms, the growth of which is controlled by a quartz microbalance. Theadditional non-active materials to produce covered surfaces or thininlet system. Evaporation devices provide the option to deposituse of a sputter gun, electron beam heating, resistive heating, and a gasFig.3: Schematic layout of the new surface physics chamber ASPICm/CTO DCJICMCCquartzmed "sfcick;::1ll ll®x y z manipulatorLN — cooledspecfromereraugerfccrcxday cup -· Iso/de beamcatcher/ rotatablerafary feedfhroughmanipulatorfrcinsporfczf/onISOLDE Facility. OCR Outputexamples have been chosen for the first measurements at the Boosterfeasibility. Out of the wide spectrum of possible applications threetheoretical or technological relevance and not only their experimentalwhere the systems to be investigated may be selected due to theirThe PAC technique for surface studies has now reached a stageIV. PAC EXPERIMENTSbenefit from the versatility of ASPIC.Moessbauer spectroscopy, recoil desorption, or surface channeling shalluse of short lived PAC isotopes and other nuclear techniques likeassist in the installation of the UHV beamline there. In particular thethe Booster ISOLDE facility. The RISE Collaboration is prepared tothe full exploitation of its potential for on—line work can be resumed atby the new setup. After the interruption of ISOLDE operation, however,measurements will also permit us to test all the possibilities offeredsurfaces, the planned next step in our research programme. Theseallow us to perform PAC measurements of monolayers on magneticwith commercially obtained radioisotopes. The installation there willobtained. At present we are using the slightly modified system in BerlinISOLDE-3, where a first result for lllmCd on a Cu(100) surface could beinterface studies. In December 1990 ASPIC was successfully tested atallow a very systematic use of radioactive isotopes for surface andWhen installed at the new ISOLDE facility this instrument willcomputer with an XWINDOWS program.Konstanz. Sample manipulations and data taking are controlled by a UNIXplanned installation of a tunneling microscope, now being developed inmoving pumps. This feature will be also of great importance for thecancellation of conduction electron and nearest neighbour contribution. OCR Outputextremely small field may be understood as a consequence of ameasured the hyperfine fields in nickel /8/. Especially for Kr theWith the probe atoms 79Kr and 77Br we have recently at ISOLDEwill be observed for non—metaIIic adsorbates.neighbour atoms, there is evidence that more complicated behaviouron metals appear to reflect simply the magnetization of the nearestWhile the magnetic hyperfine fields at metallic impurities in and2)Nonmagnetic adatoms at Ni surfacesare of special interest and can be investigated by the nuclear methods.electronic structures (e.g. charge states) of matrix and impurity atomsmaterials which are of importance for photovoltaic applications. LocalIn future experiments we aim at investigations of semiconductorgreat advantage.Here especially the access to short-lived nuclear probes at ISOLDE is ofwetting and the growth of islands on clean semiconductor surfaces.done earlier for metals. An interesting aspect is the problem of surfacecharacterize the various surface sites via the efg in the same way as,. in the incipient phase of association. Obviously one has first toat higher coverage. Exactly the same systems can be studied with lllCdclustering phenomena have been observed by tunneling microscopy /7/elemental semiconductors Si and Ge. Very interesting diffusion andisolated atoms and clusters of simple metals like In and Cd on theon the atomic scale. As a first step in this direction we intend to studyinvestigate the geometrical and electronic structure of such interfacestechnological importance in modern electronics. PAC is a unique tool toThe contact between semiconductors and metals is of utmost1)Semiconductor surfacesusing the PAC technique in connection with ASPIC at ISOLDE. OCR Outputinvestigation of the magnetic behaviour of interfaces will be possibleproper deposition techniques. ln this way a monolayer resolvedatoms may be brought into any arbitrary layer close to an interface bySuch experiments can be further extended, since impurity probesurfaces using the PAC technique with thermally deposited Pd.100at ISOLDE the magnetic hyperfine field for isolated atoms on nickelparticular interest from the theoretical viewpoint, we intend to studyTo complete our ongoing investigation of the Ni-Pd interface, ofstrongly magnetic Fe probe /10/ first data already exist.nuclear techniques. For the non—magnetic probe atom Cd /9/ and theinterface magnetism is a very promising topic for studies with3)l00Pd in Ni-Pd interfacespossible for the noble gases themselves.parent Rb atoms may be covered with matrix atoms, which is notbehaviour of noble gases close to surfaces can be studied, because themay possibly be extended up to room temperature. Furthermore therestricted to temperatures below 50K, the desorption temperature, butbe much stronger bound, these investigations of noble gases will not betemperature. As the deposited radioactive 79Flb (decaying to 79Kr) willthe behaviour of a noble gas on metallic surfaces as function ofHaving accomplished these basic experiments, we plan to studyhyperfine field at surface sites relative to the bulk value.cancellation should be removed, leading to a strongly enhancedAs both of these will be quite different on the surface, the accidentalof new experiments. The surfaces characterized via the PAC method OCR OutputRadioactive atoms at surfaces offer the possibility for a numberVI. PARASITIC TEST EXPERIMENTSpreliminary test.this case it would be necessary to try the method for deposition in asources that may be produced at ISOLDE in the decay chain of ll9Xe. InVery favorable conditions for 119Sn MS are also found for ll9Sbcould occur due to atomic jump processes at low temperature.surface of bcc metals, where even stronger anisotropy in the planesurface is expected in most cases. Of particular interest is the (110)A strong anisotropy of the DWF parallel and perpendicular to theresulting in completely complementary information.ll 9Cd, the same sample preparation as for lllCd PAC may be employed,/12/, is the optimal candidate. As it is produced in the decay chain fromThe 119Sn state, very successfully applied for questions in solidsmany measurements feasible, however.absorber methods. The strong sources obtainable at ISOLDE should make,. demonstrating the difficulty of such experiments with conventionalreported up to now /11/. No data for isolated impurities exist,Only MS measurements on clean monolayer surfaces have beenextremely valuable.sensitive to the bonding environment. Such data for surfaces would bebut also through the Debye-Waller factor (DWF) that is particularlyinteraction (e.g. isomer shift, magnetic splitting, quadrupole coupling)microscopic information at the nucleus not only through hyperfineMossbauer spectroscopy is a very powerful tool for obtainingconfiguration; Bottom: lon trajectories into 10T magnet OCR OutputFig.4: lon optical system for ”soft landing" experiment. Top: Electrodeat l m from plot startfrom lO mm dia. focusinput beam 10 it mm—mradt mx·lTl{Tl10ieéioo vM000 Vllf200l 0 eV§ 20000 \/of a 10T magnet available at HMI Berlin and the surface chamber ASPICsuggested deceleration system is shown in Fig.4. With the combinationthe ion path. Beam—optical calculations have confirmed this idea. Theretardation is performed in a very strong magnetic field that can coil upspot size accompanying deceleration can be largely avoided if theISOLDE this should in principle be possible. The blow-up of the beamon surfaces could be accomplished. With the high beam quality fromenhanced if a direct deposition of the radioactive ion beam from ISOLDEThe possible uses of nuclei for surface studies could be greatlypossibilities we propose to test ion depositionmay be used to test such methods in a controlled way. Out of severalU. Wohrmann and G. Schatz, Phys. Rev. Lett. 66 (1991) 3199 OCR OutputJ. Voigt, X.L. Ding, R. Fink, G. Krausch, B. Luckscheiter, R. Platzer,Wohrmann, X.L. Ding and G. Schatz, Phys. Rev. Lett. 64 (1990) 2202/6/ J. Voigt, R. Fink, G. Krausch, B. Luckscheiter, R. Platzer, U.Wohrmann and G. Schatz, Surf. Sci 225 (1990) 331R. Fink, R. Wesche, T. Klas, G. Krausch, R. Platzer, J. Voigt, U.Schatz, Surf. Sci216 (1989) 270/5/ T. Klas, R. Fink, G. Krausch, R. Platzer, J. Voigt, R. Wesche and G.Hyperfine Interactions 34 (1987) 577/4/ T. Klas, J. Voigt, W. Keppner, R. Platzer, R. Wesche and G. Schatz,/3/ E.Hunger and H.Haas, Surf. Sci. 50 (90) 400/2/ E.Hunger, H.Haas, H.Grawe, Hyperfine Interactions 60 (90) 999(Kluwer Academic, 1988) p. 429Applications on Material Science, Eds. E. Recknagel, J. C. Soares/1/ E. Hunger, H. Haas, V. Marx, and M. Menningen, in: Nuclear PhysicsVlll. REFERENCESfew minutes every 1 to 3 hours.with other users, as the actual collection times will generally only be a... target/ion source units and in most cases the beamtime could be shareddays per year. All experiments could be performed with standardwould need access to the ISOLDE separator for 4 to 6 periods of 2-3To perform the experimental programme as sketched above, weVII. BEAM TIME REQUESTsurface sites from our previous experiments.deposit 1**mCd ions on Pd(111), where we know the efg signature of allFaciiity. To demonstrate that "soft landing" has occurred we intend towe are in a perfect situation t0 test this concept at the Booster ISOLDEKrolas, K. Tomala, (INP Krakow, 1988) p.145/1 2/ G. Weyer, in : Condensed Matter Studies by Nuclear Methods, Eds. K./1 1/ J. Korecki and U. Gradmann, Phys. Bev. Lett. 55 (1985) 2491Mag. Magn. Mat. 86 (1990) 71/1 0/ J. Landes, Ch. Sauer, RA. Brand, W. Zinn, S. Mantl and Zs. Kajcsos, J.2904Luckscheiter, J. Voigt and G. Schatz, Appl. Phys. Lett. 58 (1991)/9/ R. Platzer, U. Wéhrmann, X.L. Ding, R. Fink, G. Krausch, B.Fcrkel, Z. Phys. A339 (1991) and to be published/8/ H. Grawe, H.H. Bertschet, R. Kowallik, S. Seeger, H. Haas and D.(1991) 1948/7/ Y.W. M0, J. Kleiner, M.B. Webb and MG. Legally, Phys. Rev. Lett 66

Radioactive ions for surface experiments

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