Energy dependent photoemission (UPS/XPS) for USb single crystal

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Energy dependent photoemission (UPS/XPS) for USb single crystal

R. Baptist, J. Naegele, Y. Baer

To cite this version:

R. Baptist, J. Naegele, Y. Baer. Energy dependent photoemission (UPS/XPS) for USb single crystal.

Journal de Physique Colloques, 1979, 40 (C4), pp.C4-40-C4-41. �10.1051/jphyscol:1979415�. �jpa-

00218811�

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JOURNAL DE PHYSIQUE Colloque C4, supplkment au no 4, Tome 40, avn'l 1979, page C4-40

Energy dependent photoemission (UPSIXPS) for USb single crystal

R. Baptist (*), J. Naegele and Y. Baer ( t )

Commission of the European Communities, Joint Research Centre, European Institute for Transuranium Elements, Postfach 2266, D-7500 Karlsruhe 1, F.R.G.

(') Laboratorium fiir Festkorperphysik, ETH-Honggerberg, CH-8093 Ziirich, Switzerland

RBsumB.

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Des mesures de photokmission sur des monocristaux de USb sont prksentees pour diffCrentes valeurs de l'tnergie d'excitation (He1 : 21,2 eV, HeII : 40,8 eV et AI-Ka monochromatisCe : 1 486,6 eV). Un pic relativement intense est observe ii 1'6nergie de Fermi et sa croissance en fonction de 1'6nergie d'excitation permet de lui attribuer un caractbre essentiellement de type f . L'Btude de l'oxydation de la surface n'a pas permis d'bter le doute quant ii I'identification des pics situks B - 2,2 eV et - 5,5 eV. Cette oxydation semble conduire B la formation d'un film mince oh les atomes d'oxygbne occupent les vacances ou interstices du rkseau cristallin.

Abstract. - Photoemission measurements on USb single crystals are reported for different excitation energies (He1 : 21.2 eV, HeII : 40.8 eV and monochromatized AI-Ka : 1 486.6 eV). A relatively strong emission at the Fermi energy is observed. From the increasing emission with growing photon energy, substantial f-character is found at the Fermi edge. The study of surface oxidation did not permit unambiguously the identification of the two emission bands at - 2.2 eV and - 5.5 eV. The surface oxidation is considered to produce a thin film with oxygen in the vacancy or interstitial places of the USb lattice.

1. Introduction.

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To date energy dependent photoemission measurements for metallic NaC1-type uranium compounds have been published only for US 113.

The use of different excitation energies permitted the determination of the angular momentum character of occupied electronic orbitals.

Taking into consideration only the different elec- tronegativity of the components of USb, the electronic configuration could be described in a simple ionic picture as Sb-5p6, U-5f3, with probably an admixture of U-6d and U-7s states. Lander et al. [2]

have suggested that a strong mixing of the U-Sf levels with the Sb-Sp would explain their neutron results.

In the present study the energy dependent photoemission from clean USb surfaces has been measu- red in order to obtain information about the electronic structure, in particular the character of the Sf electrons (localized versus itinerant).

2. Experiments. - The XPS-valence spectrum was obtained with an instrument [3] equipped with an X-ray monochromator allowing high energy reso- lution measurements in extremely clean vacuum conditions. The UPS experiments have been per- formed using a Leybold instrument. The base pressure was about 1 X lo-'" mb, the He partial pressure about 1 x low8 mb. Two crystals of dimensions 4 x 3 x 3 mm3 glued side by side were used in UPS.

In both cases the samples were cleaned by in situ

(*) Universite scientifique et mLdicale, Grenoble, France.

mechanical scraping. Oxygen contamination was checked by the observation of the 0-1s line and the S b 3 d doublet.

3. Results and discussion.

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^<<CLEAN >> SURFACE.

Photoemission intensity down to about

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4 eV binding energy is attributed to a conduction band showing high density of states at the Fermi level and a weak maximum at about ^-2.2 eV (Fig. I). When going from 21.2 eV to 40.8 eV excitation energy the first peak indicates a substantial f-character. In comparison to the XPS spectra the UPS spectra show an additional strong and broad (about 4 eV FWHM) emission at - 5.5 eV. Non self-consistent

,

f:..

I I I I 1 .

-8 -6 -L -2 EO;

BINDING ENERGY l e v ) Fig. 1. - Photoemission spectra of USb single crystals.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979415

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ENERGY DEPENDENT PHOTOEMISSION (UPSIXPS) FOR USb SINGLE CRYSTAL C4-41 band calculations [4] predict a Sb-Sp band at about

- 3 eV. Thus the peak at - 5.5 eV can probably not be attributed to Sb-Sp electrons which more likely are responsible for the - 2.2 eV peak. Due to the high surf ace sensitivity of UPS, oxygen contamination effects must be taken into consideration.

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OXYGEN CONTAMINATED SURFACE. Figure 2 shows the influence of increasing oxygen contamination on the valence band emission recorded with high surface sensitive UPS (curves a to c) and on Sb-3d core level emission recorded with XPS (curves a, e and f). The UPS curves show a change in shape and peak position with increasing contamination (curve a : clean, curves b and c : growing oxidation. A typical recording time for clean surface was about 15 minutes). Curve c is similar to the spectrum of clean UO, [S] (curve d). The XPS spectrum of the Sb3d core levels show an additional doublet emerging at higher binding energy, after a characteristic time of some hours. This effect is probably due to oxidation. If the XPS spectrum corresponds to curve f, the UPS one is similar to curve d. The chemical shift is evaluated to be 3.3 eV.

Since the intensity of the Sb-3d doublet is the same for clean and contaminated surfaces we suppo- se that the oxygen atoms in the USb lattice occupy vacancy or interstitial places which extend to a thickness of the order of the electron escape depth.

Therefore we interpret the change of the curves for

I

^- ^-10 ^-8^-6B ~ N D I N G E N E R G Y ^-4 ^-2 ⁰ ^-547l e V ) ^-537 ^-527

i

Fig. 2. - UPS and XPS results for clean and oxidized surface of USb single crystals ; a : clean sample ; b, c : oxidized sample ; d : UO, spectrum ; e , f : oxidized sample.

UPS as being due to a gradual transfer of Sb-Sp electrons to the 0-2p band ; and we regard the shift of the Sf-electron energies as being'due to a more pronounced localization. This is indicated by the shift of the sharp peak at the Fermi level to an energy position as found in UO,.

Acknowledgments. - We wish to thank Dr.

0 . Vogt for providing the USb single crystals. The excellent technical assistance of Mr. H. Winkel- mann is gratefully acknowledged.

References

-[I] EASTMAN, D. E. and KUZNIETZ, M., Phys. Rev. Lett. 26 (1971) [4] ALLEN, R. and BROOKS, M. S . S . , J. Physique Colloq. 40

846. (1979) C4-19.

[2] LANDER, G. H., SINHA, S . K . , SPARLIN, D. M. and VOGT, O., [5] NAEGELE, J . , MANES, L. and CHAUVET, G., to be published.

Phys. Rev. Lett. 40 (1978) 523.

[3] BAER, Y., BUSCH, G. and COHN, P., Rev. Sci. Instrum. 46 (1975) 466.