Bruhwiler, P.A.

Golden, M.S.

Hunt, M.R.C.

Pichler, T.

Prassides, K.

Rudolf, P.

Siller, L.

Tagmatarchis, N.

English

International Migration, Integration and Social Cohesion online publications

UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl)UvA-DARE (Digital Academic Repository)Final state interference effects in valence band photoemission of (C59N)2Hunt, M.R.C.; Pichler, T.; Siller, L.; Bruhwiler, P.A.; Golden, M.S.; Tagmatarchis, N.;Prassides, K.; Rudolf, P.Publication date2002Published inPhysical Review BLink to publicationCitation for published version (APA):Hunt, M. R. C., Pichler, T., Siller, L., Bruhwiler, P. A., Golden, M. S., Tagmatarchis, N.,Prassides, K., & Rudolf, P. (2002). Final state interference effects in valence bandphotoemission of (C59N)2. Physical Review B, 66, 193404.General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s)and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an opencontent license (like Creative Commons).Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, pleaselet the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the materialinaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letterto: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. Youwill be contacted as soon as possible.Download date:09 Mar 2023mPHYSICAL REVIEW B 66, 193404 ~2002!Final-state interference effects in valence band photoemission of„C59N…2Michael R. C. Hunt,1,2 Thomas Pichler,3,4 Lidija Šiller,5 Paul A. Brühwiler,6 Mark S. Golden,3,* Nikos Tagmatarchis,7,†Kosmas Prassides,7 and Petra Rudolf11LISE, Faculte´s Universitaires Notre-Dame de la Paix, Rue de Bruxelles 61, B-5000 Namur, Belgium2School of Physics and Astronomy, The University of Nottingham, Nottingham NG7 2RD, United Kingdom3IFW Dresden, D-01171 Dresden, Germany4Institute of Materials Physics, University of Vienna, A-1090, Vienna, Austria5Department of Physics, The University of Newcastle, Newcastle upon Tyne, NE1 7RU, United Kingdom6Department of Physics, Uppsala University, Box 530, S-751 21 Uppsala, Sweden7School of Chemistry, Physics and Environmental Science, University of Sussex, Falmer, Brighton BN1 9QJ, United Kingdo~Received 1 March 2002; revised manuscript received 8 July 2002; published 11 November 2002!Oscillatory behavior of photoemission intensity with incident photon energy has been observed for severalfullerenes and fullerene derivatives. However, until now it has been unclear if these effects arise from inter-ference associated with the spatial distribution of the initial state within the molecule or are due to scatteringof the outgoing photoelectron. In order to resolve this issue we performed synchrotron radiation excitedvalence band photoemission measurements on multilayer (C59N)2 films. The highest occupied molecularorbital ~HOMO! of the (C59N)2 dimer is spatially localized, while each half of the dimer remains approxi-mately spherical. Careful normalization and fitting of the measured spectra clearly show that the intensity ofthe HOMO also displays a periodic variation of intensity with photon energy indicating that scattering of thephotoemitted electron and consequent interference effects lead to the observed intensity changes.DOI: 10.1103/PhysRevB.66.193404 PACS number~s!: 71.20.Tx, 33.60.Cv, 79.60.Dponryanvenodguetiebcfdomiereeocthceistoheinicdsor-ofsaerehefterumIRe-e.vi-stillofate--VA particularly interesting feature arising in photoemissifrom the fullerenes is the observation of an oscillatochange in photoemission intensity from the valence bstates as a function of incident photon energy. First obserin solid films of C60,1 these intensity modulations have beobserved in gas phase C60 ~Ref. 2! and C70,3 as well as infilms of (C59N)2 ~Ref. 4! and of higher fullerenes.5 An al-most antiphase oscillation in the intensity of the first twvalence band features of C60 was observed by Benning anco-workers.1 Both these states havep-orbital character anddiffer only in parity and angular momentum, and it was sugested that the photoelectron final state retained moleccharacter in the form of parity for energies as high as 120above the vacuum level. Subsequent calculations6–9 havesuggested that intramolecular interference effects may bedominant factor in determining the measured intensitSeveral similar models have been formulated which canclassified in two groups, one in which the phase differenacross the molecule of the photoemissioninitial state isdeemed to be important,6–8 the other in which scattering othe final statewave function by the molecule9 determinesintensity in a similar fashion. In both types of model a staning wavefield within the spherically symmetric ‘‘shell’’ of afullerene is created. In the initial state model this arises frinterference of a coherent spherically symmetric wave emted from the shell itself, while for a final state model scatting of a photoelectron from the quasi-spherically arrangatoms of the shell produces the standing wave—effectivphotoelectron diffraction. Consequently, in the first groupmodels increasing localization of the initial state wavefuntion should have a profound, and destructive, effect uponintensity variation with incident photon energy. For the seond type of model localization of the initial state will havlittle, if any, effect on the observed oscillations in photoemsion intensity.0163-1829/2002/66~19!/193404~4!/$20.00 66 1934ded-larVhes.ee-t--dlyf-e--Azafullerene,10 in which one carbon atom on C60 is sub-stituted by a nitrogen atom, offers a unique opportunityprobe these effects of localization of the initial state in tphotoemission process. The azafullerenyl radical C59N" canbe stabilized by reacting with hydrogen to form C59HN,11 bybonding to a semiconductor12 or a metal13 surface or byforming dimers. (C59N)2 is stable in solution as well as inthe solid phase where the dimers arrange in a monocllattice.14 It is the dimerization in the solid phase which leato the localization of the highest unoccupied molecularbital ~HOMO!.15,16We explored intensity oscillations in the valence bandsolid (C59N)2 at the I311 undulator beamline17 at the MAXII storage ring at MAX-lab in Lund, Sweden which waequipped with a modified SX-700 monochromator andhemispherical SCIENTA SES 200 analyzer. The spectra wrecorded at 96 K with a total (photons1electrons) energyresolution of 45 meV as determined from the width of tFermi edge of the graphite substrate, measured at 96 K acleaving in air and repeated heating under ultra high vacuto bright orange glow to remove contaminants. C59N wassynthesized and purified as described in Ref. 10. Frommeasurements a C60 contamination of less than 1% was dduced for this material.18 (C59N)2 multilayer films were de-positedin situ by sublimation from a boron nitride cruciblonto the clean graphite~0001! kept at room temperatureDeposition at this temperature favours island growth as edenced by the fact that the graphite Fermi edge wasvisible when more than a nominal two monolayersazafullerene were deposited. C60 multilayer films were pro-duced the same way but using commercially available mrial ~99.9% purity, MER Corporation! evaporated from a resistively heated Ta crucible. The pressure in the UHchamber rose from 2310210 mbar to the low 1028 mbar©2002 The American Physical Society04-1thiot oeigfu-aasntly,skstheOliza-oreeree inrgy.in-urest,andreor-asions-fva-into, theialns.ingentad-reetiototeCbse-BRIEF REPORTS PHYSICAL REVIEW B66, 193404 ~2002!range during fullerene deposition. The cleanliness offilms was verified using core level photoemission.Figure 1 shows a typical valence band photoemissspectrum from a film of (C59N)2 on graphite~0001!. Theoverall shape of the spectrum is somewhat similar to thaa C60 multilayer, with key differences associated with thpresence of extra states from the substitutional doping wnitrogen, and the distortion of the monomer cages brouabout by both substitution and dimerization.15,16The HOMOof the (C59N)2 dimer ~Fig. 1, peak A! has a high degree oN2p character and lies above the C2p-dominated stateswhich would form the HOMO of pure C60 ~Fig. 1, peak B!.Calculations demonstrate that the (C59N)2 HOMO is spa-tially localized on the dimer in the region of the intermoleclar dimer bond and on the adjacent nitrogen atom,15 whilethe HOMO-1~peak B! is more delocalized.Photoemission spectra as a function of photon energyshown in Fig. 2. A similar variation with photon energy hFIG. 1. Valence band spectrum of (C59N)2 obtained in normalemission geometry at a photon energy of 35 eV. The contribufrom the graphite substrate is shown by the dotted line.FIG. 2. Valence band spectra of (C59N)2 /HOPG showing oscil-latory behavior of photoemission intensity as a function of phoenergy. For a description of the normalization procedure seeAll spectra were obtained in normal emission geometry.19340enfthhtrebeen observed by Jonest al.4 However, only the first 5 eVof the valence band was reported in that work. Consequewhen the HOMO intensity was extracted by fitting it wanormalized to the intensity of either peak B or C. Both peaB and C undergo considerable intensity variations. Thus,observed variation in the normalized intensity of the HOMdepended quite strongly upon the peak chosen for normation. To eliminate these problems we have adopted a mrigorous normalization and fitting procedure. Spectra wtaken over the binding energy range 12 to21 eV. A residualsignal from the graphite substrate, close toEF was used tocalibrate energy positions. In contrast to Ref. 4, no changposition of the valence band features was found with eneSpectra were normalized to the total integrated spectraltensity over the binding energy range acquired. To ensthat the normalization procedure was sufficiently robuspectra were also normalized to the incident photon fluxthe final results of the normalization and fitting procedufound to be almost identical. The graphite background, nmalized to the intensity nearEF was then subtracted fromour spectra. Initial parameters for fitting were determinedfollows: a valence band spectrum of multilayer C60 was ob-tained during the same beamtime under the same conditas those for (C59N)2 . A difference spectrum was then generated for hn545 eV by subtracting the spectrum omultilayer C60 the from that of (C59N)2 , Fig. 3. The differ-ence spectrum should reflect the additional density oflence band states associated with substitution of nitrogenthe fullerene cage and subsequent dimerization. Indeedresulting spectrum shows strong similarity to the N partdensity of states~N-PDOS! of Ref. 15 formed by projectingthe one electron states onto the nitrogen atomic functioAlthough the difference spectrum contains intensity arisfrom the reduction in molecular symmetry and consequbroadening of other states, to a first approximation the leing feature is representative of the (C59N)2 HOMO. The dif-ference spectrum was then fitted by a combination of thGaussian functions~two singlets, one doublet! consistentwith the theoretical N-PDOS~Ref. 15! as shown in Fig. 3.nnxt.FIG. 3. Difference spectrum between the valence bands of60and (C59N)2 obtained at a photon energy of 45 eV~solid circles!.The fit to the difference spectrum, shown as solid lines, was suquently used as the basis of fits to the (C59N)2 valence band spectraas a function of photon energy, as described in the text.4-2edstthxegtoigdinizdrrunssonitythitthnthnob-tednes-elec-hehatsityr aisried-u-l-anipyhiv.ey,litz,ys..omexdeV,yrilysBRIEF REPORTS PHYSICAL REVIEW B66, 193404 ~2002!The normalized (C59N)2 spectra were then fitted with thsum of these functions and a pair of Gaussians obtainefitting the leading features of the C60 valence band. Positionand intensities~including relative intensities in the double!were allowed to relax, but the relative positions and widof the peaks generated by the difference spectrum were fiFigure 4 illustrates the high quality of typical fits resultinfrom this approach. It was found that extending the fitinclude higher binding energy peaks did not lead to any snificant changes in the intensities associated with the leaedge features. From these fits one can extract normalareas associated with each of the peaks, which are plotteFig. 5. It is clearly seen that the areas of the peaks cosponding to the maxima A, B, and C~Fig. 1! display oscil-latory intensity superposed on a decreasing backgrowhich arises from a reduction in the photoemission crosection ofp-orbital derived states with respect to thes-orbitalderived states which play an important role in normalizatiAlthough some small contribution to the oscillatory intensobserved in peak A may arise from overlap with peak B,difference in the variation of the intensity of these peaks wphoton energy is sufficient to draw the conclusion thatintensity of peak A varies nonmonotonically with photon eergy. The presence of oscillations in the cross section of(C59N)2 HOMO contradicts the assertion in Ref. 4 thatsuch oscillation could be detected.If initial state effects are dominant in inducing the oserved intensity oscillations6–8 then it can be expected thathese would vanish if the initial state were spatially localizrather than distributed over the entire cage of a fullere*Present address: Van der Waals-Zeeman Institute, UniversitAmsterdam, Vackenierstraat 65, NL-1018 XE Amsterdam, TNetherlands.†Present address: Dipartimento di Scienze Farmaceutiche, Unsità degli Studi di Trieste, P.le Europa 1, I-34127 Trieste, Italy.1P. J. Benning, D. M. Poirier, N. Troullier, J. L. Martins, J. HFIG. 4. Typical fit to a valence band photoemission spectrum(C59N)2 . The points correspond to the experimental spectrusmooth lines to the fit and its components. For fit details: see t19340bysd.-gedine-d-.ehe-eemonomer or over the complete dimer. The continued prence of intensity oscillations for the (C59N)2 HOMO, whichis rather localized within the dimer,15,16 indicates that inter-ference effects associated with the passage of the phototron out of the molecules is primarily responsible for tintensity oscillations. Thus we can draw the conclusion tit is the scattering of theoutgoing photoelectron from thequasispherical monomer components of the (C59N)2 dimer,and resulting interference effects which lead to the intenoscillations observed as a function of photon energy fonumber of fullerenes and fullerene derivatives.1–5,9The authors would like to thank J. N. Andersen for hvaluable assistance at beamline I 3.11. This work was carout within the EU TMR network ‘‘FULPROP’’ ~Contract No.ERB FMRX-CT 970155! and supported also by the EUfunded ‘‘Access to Research Infrastructure action of the Hman Potential Program.’’ T.P. and N.T. gratefully acknowedge funding under the APART program of the AustriAcademy of Sciences and the TMR Marie-Curie Fellowshprogram, respectively.ofeer-Weaver, R. E. Haufler, L. P. F. Chibante, and R. E. SmallPhys. Rev. B44, 1962~1991!.2T. Liebsch, O. Plotzke, U. Hergenhahn, O. Hemmers, R. WehJ. Viefhaus, B. Langer, S. B. Whitfield, and U. Becker, PhRev. A52, 457 ~1995!.3T. Liebsch, R. Hentgens, A. Ru¨del, J. Viefhaus, U. Becker, and Rf,t.FIG. 5. Intensity oscillations of valence band features~inte-grated area! from (C59N)2 as a function of photon energy. Fillecircles~d! correspond to the area of the fit peak centered at 1.8and open squares~h! to that at 3.1 eV, both of which are primarilC2p derived ~peaks B and C in Fig. 1!. 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Final state interference effects in valence band photoemission of (C59N)2

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Final state interference effects in valence band photoemission of (C59N)2

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