- Email: [email protected]
Unoccupied electronic states of the FeS2(100) surface studied by inverse photoemission
__ B
ELSEVIER
Surface Science 377-379 (1997) 201-205
Unoccupied electronic states of the Fe&( 100) surface studied by inverse photoemission T. Ollonqvist *, R. Per%l& J. Wyrynen Materials Science, Department of Applied Physics, University of Turku, FIN-20014 Turku, Finland
Received 1 August 1996; accepted for publication 15 October 1996
Abstract Inverse photoemission spectra have been measured to study unoccupied electronic states from an Fe&( 100) single crystal along the I-X symmetry line with normal incidence electrons at energies between 20 and 40 eV. Two main features of the spectra are found, corresponding to direct bulk transitions into unoccupied iron 3d(eJ and sulphur 3p((r*) states, respectively, followed by a prominent minimum and a onset of a band with a wide structure due to mainly transitions into Fe 4sp-S 3d bands. These peaks are located at 1.4 and 3.0 eV above the Fermi level for an initial electron energy of 19.8 eV. They disperse according to band-structure calculations. The onset of the Fe 4sppS 3d bands is about 5.0 eV above the Fermi level. The experimental results obtained here are in a reasonable agreement with band-structure calculations for Fe& and with previous results of optical, BIS and XAS measurements. Keywords: Electron bombardment;
Inverse photoemission
spectroscopy; Iron d&hide;
1. Introduction FeSz is one of the semiconducting transitionmetal dichalcogenide materials having a pyrite structure and a band-gap energy of approximately 0.95 eV [ 1,2]. The electronic structures of transition-metal dichalcogenides have been the object particularly intensive research because of their wide variety of electrical, magnetic and optical properties. For this reason, a considerable number of theoretical [3-51 and experimental [6-91 examinations have been reported on transition-metal , dichalcogenides MX, (M = Fe, Co, Ni, Zn; X = S, Se) with the pyrite-type structure. Photoelectron spectroscopy has been used to * Corresponding author. Fax: +358 21 3335070; e-mail: [email protected]
Single crystal surfaces
obtain the core- and valence-band states of FeSz [2-91. Optical [ 10,111 bremsstrahlung isochromat spectroscopy (BIS) 1121 and X-ray absorption (XAS) [ 131 measurements have been carried out to study the unoccupied electronic states of FeS, above the Fermi level. Inverse photoemission studies have not been reported previously, although these provide direct information about unoccupied electronic states. In addition, FeS, has also been investigated by Auger/EELS [ 141 and by XPD [2]. Several band-structure and densityof-states calculations are available for FeS, [3-51. Even though a number of recent studies have focused on band-structure calculations for Fe&, band-structure measurements have not been considered. Here we report an inverse photoemission (IPE) study of the clean Fe&( 100) surface along the
0039-6028/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PII SOO39-6028(96)01351-9
202
T Ollonqvist et al. / Surface Science 377-379 (1997) 201-205
I-X symmetry line. k-resolved inverse photoemission (KRIPES) is powerful tool for examining the wave-vector and energy dependence of unoccupied electronic states at surfaces and in the bulk [ 151, and it is also suitable for studies of semiconductors [ 161. The experiments have been carried out with a k-resolved, tuneable inverse photoemission spectrometer [ 171. The spectra have been taken at normal incidence of the electrons with an energy between 20 and 40 eV. The experimental results obtained here are compared with band-structure calculation for FeSz [ 3-51 and with previous results of optical [lo, 111, BIS [12] and XAS [13] measurements.
2. Experimental The experiments reported here were performed in the three-chamber UHV system described elsewhere [ 171. The base pressures in the KRIPES chambers were 10pl’ mbar, and in the preparation chamber it was lo-‘mbar. The measurements were carried out using a tuneable inverse photoemission spectrometer. As an electron source, a Pierce-type electron gun with indirect heating of the cathode is used, operating with a beam energy of lo-300 eV and with a beam current of 0.01-1.0 mA. The emitted photons are collected at 45” from the sample surface by an off-Rowland grating spectrograph using microchannel plate detection. The spectra can be measured in the lo-40 eV photon energy range with parallel detection of the wavelength region of 560 A. The spherical grating is ion-etched to 1200 1nun1 and coated with Pt. The overall energy resolution (photons plus electrons) of the spectrometer is determined to be 0.26-0.6 eV, depending on the energy of the photons emitted. The natural FeSz crystals originated from Russia, and the impurity of the crystals was analysed to be below 0.1%. FeS, crystals of about 4 mm x 4 mm x 6 mm were mounted by clamps on sample holders in the forks of a linear transfer mechanism. The crystals were cleaved along the (100) surface in an ultrahigh vacuum of better than lo-’ mbar and the transported under vacuum from the preparation chamber to the x-y-z-8
manipulator in the measurement chamber. The surface geometry of the FeS, surface was determined with LEED in the preparation chamber before the samples were moved. The surface geometry of the Fe&(lOO) crystal can be described as the primitive reciprocal lattice of an fee (100) surface with an additional p(l/z x j!$R45” superstructure [ 11. Using different initial electron energies and different angles of incidence, we can probe different points of the Brillouin zone. The energy of emitted photons is measured for a chosen initial electron energy Ei, and the spectra show the measured photon intensity versus the final-state energy Ep All final-state energies Ef are measured with respect to EF, the Fermi energy of the system. The photonenergy calibration of the spectrometer and the EF position are determined experimentally with the well-defined Fermi edge of a gold sample [ 171 and from the hydrogen Lyman - a, - p and - y radiation [ 181.
3. Results and discussion In Fig. 1 the crystal structure of FeSz (pyrite) is illustrated, together with the corresponding Brillouin zone (BZ) with high symmetry lines and points. The Fe& (pyrite) structure can be regarded as NaCl-type, in which the Fe atoms occupy the Na positions and the mid-point of the anion S-S pairs occupy the Cl positions with their diatomic axes ordered equally along the (111) direction. The distance between chalcogen pairs of anions (S,)-’ is short, owing to the presence of a covalent bond. It follows that the bonding and antibonding states in S 3p have a clear separation in which the antibonding po*-state molecular orbital is unoccupied. In pyrite, the metal 3d levels split into triply degenerated t,, and doubly degenerate eg subbands due to the distorted octahedral environment of the ligand field. The crystal-field splitting of the d levels exceeds the d-d spin-exchange energy and so the lower energy subbands (t,, ) are filled with six d electrons and the upper subbands (e& are completely empty. The Fermi level falls between these two subbands. In Fig. 2 inverse photoemission spectra of the
T Ollonqvist et al. j Surface Science 377-379 (1997) 201-205
0
203
5
10
15
Energy above Fermi-level [eV]
Fig. 1. (a) Crystal geometric structure of FeSZ [ 1] and (b) Brillouin zone (BZ) for the simple cubic lattice [3].
Fig. 2. Inverse photoemission spectra recorded for different initial electron energies Ei on the Fe$( 100) surface along the T-X symmetry line. The energy of the incident electrons is given for each curve.
Fe&( 100) surface are shown. The different electron beam energies Ei with respect to the Fermi energy of,the crystal are also indicated. The spectra were taken at normal electron incidence (k,, = 0) with the energy of 20.0-40.0 eV. The collection time per spectrum varied from 15 to 30 min depending on the incident electron energy, with an electron-beam current of approximately 20 PA. Normal incidence probes the perpendicular dispersion of the unoccupied states of the FeS2(100) bulk along the T-A-X symmetry line. The slowly varying background caused by inelastic electron scattering has not been subtracted from these spectra. All the measured spectra have been normalised with a response function which can be determined experimentally by bombarding the sample with high-energy electrons, usually about 100 eV. This response function was also measured for different samples and different electron energies [17]. The spectra in Fig. 2 contain two dominant
features, followed by a prominent minimum and the onset of a band with particular broad structure. Two peaks originate from direct bulk transitions into unoccupied iron 3d(e& and sulphur 3p(o*) states, respectively located at 1.4 and 3.0 eV above the Fermi level for an initial electron energy of 19.8 eV. They disperse weakly according to the band structure calculations [3-51 as a function of the incident electron energy, and they are clearly distinguished at lower incident electron energies. The wide structure is mainly caused by transitions into Fe 4sp-S 3d bands. The onset of the Fe 4sp-S 3d bands (turning point of the band edge) is about 5.7 eV above the Fermi level for an initial incident energy of 19.8 eV. At higher incident electron energies, the fine structure is smeared out due to the lower resolution of the spectrometer. The KRIPES results from this study support the previous reports of the unoccupied electronic states of FeS,. Folkerts et al. [12] obtained the first direct experimental evidence for an empty
204
T. Ollonqvist et al. / Surface Science 377-379
antibonding anion p-like state with bremsstrahlung isochromat spectroscopy (BIS). The measured BIS spectrum of Folkerts et al. agrees with the unoccupied part of the calculated DOS and XAS spectrum of FeS, [ 131. Reflectance and absorption measurements by Ferrer et al. [ 111 show two peaks at about 1.6 eV and near 4.0 eV, respectively, the first being sharper than the last. Our experimental k-resolved measurements confirm these previous reports with better resolution and give more information of the unoccupied band structure of Fe$. In the order to obtain information on bulk- or surface-band mapping, we must approximate the k, component. Usually, this is done by using the nearly-free electron approximation (NFE), which is based on the fact that the energy band is often free-electron like and locates some eVolts (i.e. a constant potential V,> above the Fermi level [ 191. This is estimated from the density-of-states calculations of Khan et al. [3] to be -7.6 eV. We compared our experimental results with band-structure calculations of the self-consistent linear combination of atomic orbitals (LCAO) method of Lauer et al. [ 51, the self-consistent band-structure calculations using an augmented spherical wave (ASW) method of Folkerts et al. [ 121, the non-self-consistent LCAO TB band-structure method of Khan et al. [3], and the partially self-consistent LCAO method of Bullet et al. [4]. The band-structure calculations of FeS,( 100) along the I-X symmetry line of Folkerts et al. (solid lines) and from Bullet et al. (dashed/dotted lines) are shown in Fig. 3, where our experimental values from Fig. 2 are depicted by filled circles. The agreement between the theoretical and experimental dispersion curves is reasonable.
4. Conclusions We have studied the inverse photoemission (IPE) spectra of a clean Fe&( 100) surface along the I-X symmetry line. Two spectral features of the spectra are found, corresponding to direct bulk transitions into unoccupied iron 3d(e.J and sulphur 3p(a*) states, respectively, followed by a prominent minimum and the onset of a band with the broad structure owing to the transition into Fe 4sp-S 3d bands. The peaks are located at 1.4
(1997) 201-205
zFr-:-__:=:~:~:~ ,
1
.I:_:_:_:_:c:_:-:-‘-:9’r
r
A
X
Fig. 3. Bulk band structure of FeS, along the T-X symmetry direction calculated by Folkerts et al. [ 121 and by Bullet [4] are shown by the solid lines and the dashed/dotted lines, respectively. The experimental values from Fig. 2 are marked by dots.
and 3.0 eV, and the onset of the Fe 4sp-S 3d bands is about 5.0 eV above the Fermi level for an initial electron energy of 19.8 eV. These three structures disperse weakly as a function of the incident electron energy, and they are clearly distinguished at lower incident electron energies. Our k-resolved measurements agree well with the examinations of unoccupied electronic states obtained by optical [lo, 111, BIS [ 121 and XAS [13] measurements, and as well with band structure calculations [ 3-51.
Acknowledgements
We would like to extend our thanks to Timo Kaurila, Matti Isokallio and Marko Punkkinen for their assistance and cooperation. References [l] A. Ennaoui, S. Fiechter, Ch. Pettenkofer, N. AlonsoVante, K. Btier, M. Bronold, Ch. Hiipfner and H. Tributsch, Sol. Energy Mater. Sol. Cells 29 (1993) 289.
T. Ollonqvist [2]
[3] [4] [5] [6] [7] [8] [9]
[lo]
et al. / Surface
M. Birkholz, S. Fiechter, A. Hartmann and Tributsch, Phys. Rev. B 43 (1991) 11926. M.A. Khan, J. Phys. C: Solid State Phys. 9 (1976) 81. D.W. Bullet, J. Phys. C: Solid State Phys. 15 (1982) 6163. S. Lauer, A.X. Trautwein and F.E. Harris, Phys. Rev. B 29 (1984) 6774. A. Ohsawa, H. Yamamoto and H. Watanabe, J. Phys. Sot. Jpn. 37 (1974) 568. E.K. Li, K.H. Johnson, D.E. Eastman and J.L. Freeouf, Phys. Rev. B 32 (1974) 470. H. Van Der Heide, R. Hemmel, C.F. Bruggen and C. Haas, J. Solid State Chem. 33 (1980) 17. S. Suga, M. Taniguihi, S. Shin, M. Seki, S. Shibuya, K. Sato and T. Yamaguichi, Physica B 117/l 18 (1993) 353. W.W. Kou and M.S. Seehra, Phys. Rev. B 18 (1978) 7062.
Science
377-379
(1997)
201-205
205
[ 111 I.J. Ferrer, D.M. Nevskaia, C. de las Heras and C. Sanchez, Solid State Commun. 74 (1990) 913. [12] W. Folkerts, G.A. Sawatzky, C. Haas, R.A. de Groot and F.U. Hillebrecht, J. Phys. C: Solid State Phys. 20 (1987) 4135. [13] C. Sugiura and S. Muramatsu, Phys. Status Solidi B 129 (1985) K157. [14] Y.C. Lee and P.A. Montano, Surf. Sci. 143 (1984) 442. [ 151 V. Dose, Prog. Surf. Sci. 13 (1983) 225; J. Phys. Chem. 88 (1984) 1681; Surf. Sci. Rep. 5 (1985) 337; N.V. Smith, Rep. Prog. Phys. 51 (1988) 1227. [ 161 F.J. Himpsel, Surf. Sci. Rep. 12 (1990) 1. [ 171 T.E. Ollonqvist and I.J. Vayrynen, Vacuum 46 (1995) 1177. [ 181 T.E. Ollonqvist and I.J. Vayrynen, to be published. [19] R. Courths and S. Hiifner, Phys. Rep. 112 (1984) 53.
ELSEVIER
Surface Science 377-379 (1997) 201-205
Unoccupied electronic states of the Fe&( 100) surface studied by inverse photoemission T. Ollonqvist *, R. Per%l& J. Wyrynen Materials Science, Department of Applied Physics, University of Turku, FIN-20014 Turku, Finland
Received 1 August 1996; accepted for publication 15 October 1996
Abstract Inverse photoemission spectra have been measured to study unoccupied electronic states from an Fe&( 100) single crystal along the I-X symmetry line with normal incidence electrons at energies between 20 and 40 eV. Two main features of the spectra are found, corresponding to direct bulk transitions into unoccupied iron 3d(eJ and sulphur 3p((r*) states, respectively, followed by a prominent minimum and a onset of a band with a wide structure due to mainly transitions into Fe 4sp-S 3d bands. These peaks are located at 1.4 and 3.0 eV above the Fermi level for an initial electron energy of 19.8 eV. They disperse according to band-structure calculations. The onset of the Fe 4sppS 3d bands is about 5.0 eV above the Fermi level. The experimental results obtained here are in a reasonable agreement with band-structure calculations for Fe& and with previous results of optical, BIS and XAS measurements. Keywords: Electron bombardment;
Inverse photoemission
spectroscopy; Iron d&hide;
1. Introduction FeSz is one of the semiconducting transitionmetal dichalcogenide materials having a pyrite structure and a band-gap energy of approximately 0.95 eV [ 1,2]. The electronic structures of transition-metal dichalcogenides have been the object particularly intensive research because of their wide variety of electrical, magnetic and optical properties. For this reason, a considerable number of theoretical [3-51 and experimental [6-91 examinations have been reported on transition-metal , dichalcogenides MX, (M = Fe, Co, Ni, Zn; X = S, Se) with the pyrite-type structure. Photoelectron spectroscopy has been used to * Corresponding author. Fax: +358 21 3335070; e-mail: [email protected]
Single crystal surfaces
obtain the core- and valence-band states of FeSz [2-91. Optical [ 10,111 bremsstrahlung isochromat spectroscopy (BIS) 1121 and X-ray absorption (XAS) [ 131 measurements have been carried out to study the unoccupied electronic states of FeS, above the Fermi level. Inverse photoemission studies have not been reported previously, although these provide direct information about unoccupied electronic states. In addition, FeS, has also been investigated by Auger/EELS [ 141 and by XPD [2]. Several band-structure and densityof-states calculations are available for FeS, [3-51. Even though a number of recent studies have focused on band-structure calculations for Fe&, band-structure measurements have not been considered. Here we report an inverse photoemission (IPE) study of the clean Fe&( 100) surface along the
0039-6028/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PII SOO39-6028(96)01351-9
202
T Ollonqvist et al. / Surface Science 377-379 (1997) 201-205
I-X symmetry line. k-resolved inverse photoemission (KRIPES) is powerful tool for examining the wave-vector and energy dependence of unoccupied electronic states at surfaces and in the bulk [ 151, and it is also suitable for studies of semiconductors [ 161. The experiments have been carried out with a k-resolved, tuneable inverse photoemission spectrometer [ 171. The spectra have been taken at normal incidence of the electrons with an energy between 20 and 40 eV. The experimental results obtained here are compared with band-structure calculation for FeSz [ 3-51 and with previous results of optical [lo, 111, BIS [12] and XAS [13] measurements.
2. Experimental The experiments reported here were performed in the three-chamber UHV system described elsewhere [ 171. The base pressures in the KRIPES chambers were 10pl’ mbar, and in the preparation chamber it was lo-‘mbar. The measurements were carried out using a tuneable inverse photoemission spectrometer. As an electron source, a Pierce-type electron gun with indirect heating of the cathode is used, operating with a beam energy of lo-300 eV and with a beam current of 0.01-1.0 mA. The emitted photons are collected at 45” from the sample surface by an off-Rowland grating spectrograph using microchannel plate detection. The spectra can be measured in the lo-40 eV photon energy range with parallel detection of the wavelength region of 560 A. The spherical grating is ion-etched to 1200 1nun1 and coated with Pt. The overall energy resolution (photons plus electrons) of the spectrometer is determined to be 0.26-0.6 eV, depending on the energy of the photons emitted. The natural FeSz crystals originated from Russia, and the impurity of the crystals was analysed to be below 0.1%. FeS, crystals of about 4 mm x 4 mm x 6 mm were mounted by clamps on sample holders in the forks of a linear transfer mechanism. The crystals were cleaved along the (100) surface in an ultrahigh vacuum of better than lo-’ mbar and the transported under vacuum from the preparation chamber to the x-y-z-8
manipulator in the measurement chamber. The surface geometry of the FeS, surface was determined with LEED in the preparation chamber before the samples were moved. The surface geometry of the Fe&(lOO) crystal can be described as the primitive reciprocal lattice of an fee (100) surface with an additional p(l/z x j!$R45” superstructure [ 11. Using different initial electron energies and different angles of incidence, we can probe different points of the Brillouin zone. The energy of emitted photons is measured for a chosen initial electron energy Ei, and the spectra show the measured photon intensity versus the final-state energy Ep All final-state energies Ef are measured with respect to EF, the Fermi energy of the system. The photonenergy calibration of the spectrometer and the EF position are determined experimentally with the well-defined Fermi edge of a gold sample [ 171 and from the hydrogen Lyman - a, - p and - y radiation [ 181.
3. Results and discussion In Fig. 1 the crystal structure of FeSz (pyrite) is illustrated, together with the corresponding Brillouin zone (BZ) with high symmetry lines and points. The Fe& (pyrite) structure can be regarded as NaCl-type, in which the Fe atoms occupy the Na positions and the mid-point of the anion S-S pairs occupy the Cl positions with their diatomic axes ordered equally along the (111) direction. The distance between chalcogen pairs of anions (S,)-’ is short, owing to the presence of a covalent bond. It follows that the bonding and antibonding states in S 3p have a clear separation in which the antibonding po*-state molecular orbital is unoccupied. In pyrite, the metal 3d levels split into triply degenerated t,, and doubly degenerate eg subbands due to the distorted octahedral environment of the ligand field. The crystal-field splitting of the d levels exceeds the d-d spin-exchange energy and so the lower energy subbands (t,, ) are filled with six d electrons and the upper subbands (e& are completely empty. The Fermi level falls between these two subbands. In Fig. 2 inverse photoemission spectra of the
T Ollonqvist et al. j Surface Science 377-379 (1997) 201-205
0
203
5
10
15
Energy above Fermi-level [eV]
Fig. 1. (a) Crystal geometric structure of FeSZ [ 1] and (b) Brillouin zone (BZ) for the simple cubic lattice [3].
Fig. 2. Inverse photoemission spectra recorded for different initial electron energies Ei on the Fe$( 100) surface along the T-X symmetry line. The energy of the incident electrons is given for each curve.
Fe&( 100) surface are shown. The different electron beam energies Ei with respect to the Fermi energy of,the crystal are also indicated. The spectra were taken at normal electron incidence (k,, = 0) with the energy of 20.0-40.0 eV. The collection time per spectrum varied from 15 to 30 min depending on the incident electron energy, with an electron-beam current of approximately 20 PA. Normal incidence probes the perpendicular dispersion of the unoccupied states of the FeS2(100) bulk along the T-A-X symmetry line. The slowly varying background caused by inelastic electron scattering has not been subtracted from these spectra. All the measured spectra have been normalised with a response function which can be determined experimentally by bombarding the sample with high-energy electrons, usually about 100 eV. This response function was also measured for different samples and different electron energies [17]. The spectra in Fig. 2 contain two dominant
features, followed by a prominent minimum and the onset of a band with particular broad structure. Two peaks originate from direct bulk transitions into unoccupied iron 3d(e& and sulphur 3p(o*) states, respectively located at 1.4 and 3.0 eV above the Fermi level for an initial electron energy of 19.8 eV. They disperse weakly according to the band structure calculations [3-51 as a function of the incident electron energy, and they are clearly distinguished at lower incident electron energies. The wide structure is mainly caused by transitions into Fe 4sp-S 3d bands. The onset of the Fe 4sp-S 3d bands (turning point of the band edge) is about 5.7 eV above the Fermi level for an initial incident energy of 19.8 eV. At higher incident electron energies, the fine structure is smeared out due to the lower resolution of the spectrometer. The KRIPES results from this study support the previous reports of the unoccupied electronic states of FeS,. Folkerts et al. [12] obtained the first direct experimental evidence for an empty
204
T. Ollonqvist et al. / Surface Science 377-379
antibonding anion p-like state with bremsstrahlung isochromat spectroscopy (BIS). The measured BIS spectrum of Folkerts et al. agrees with the unoccupied part of the calculated DOS and XAS spectrum of FeS, [ 131. Reflectance and absorption measurements by Ferrer et al. [ 111 show two peaks at about 1.6 eV and near 4.0 eV, respectively, the first being sharper than the last. Our experimental k-resolved measurements confirm these previous reports with better resolution and give more information of the unoccupied band structure of Fe$. In the order to obtain information on bulk- or surface-band mapping, we must approximate the k, component. Usually, this is done by using the nearly-free electron approximation (NFE), which is based on the fact that the energy band is often free-electron like and locates some eVolts (i.e. a constant potential V,> above the Fermi level [ 191. This is estimated from the density-of-states calculations of Khan et al. [3] to be -7.6 eV. We compared our experimental results with band-structure calculations of the self-consistent linear combination of atomic orbitals (LCAO) method of Lauer et al. [ 51, the self-consistent band-structure calculations using an augmented spherical wave (ASW) method of Folkerts et al. [ 121, the non-self-consistent LCAO TB band-structure method of Khan et al. [3], and the partially self-consistent LCAO method of Bullet et al. [4]. The band-structure calculations of FeS,( 100) along the I-X symmetry line of Folkerts et al. (solid lines) and from Bullet et al. (dashed/dotted lines) are shown in Fig. 3, where our experimental values from Fig. 2 are depicted by filled circles. The agreement between the theoretical and experimental dispersion curves is reasonable.
4. Conclusions We have studied the inverse photoemission (IPE) spectra of a clean Fe&( 100) surface along the I-X symmetry line. Two spectral features of the spectra are found, corresponding to direct bulk transitions into unoccupied iron 3d(e.J and sulphur 3p(a*) states, respectively, followed by a prominent minimum and the onset of a band with the broad structure owing to the transition into Fe 4sp-S 3d bands. The peaks are located at 1.4
(1997) 201-205
zFr-:-__:=:~:~:~ ,
1
.I:_:_:_:_:c:_:-:-‘-:9’r
r
A
X
Fig. 3. Bulk band structure of FeS, along the T-X symmetry direction calculated by Folkerts et al. [ 121 and by Bullet [4] are shown by the solid lines and the dashed/dotted lines, respectively. The experimental values from Fig. 2 are marked by dots.
and 3.0 eV, and the onset of the Fe 4sp-S 3d bands is about 5.0 eV above the Fermi level for an initial electron energy of 19.8 eV. These three structures disperse weakly as a function of the incident electron energy, and they are clearly distinguished at lower incident electron energies. Our k-resolved measurements agree well with the examinations of unoccupied electronic states obtained by optical [lo, 111, BIS [ 121 and XAS [13] measurements, and as well with band structure calculations [ 3-51.
Acknowledgements
We would like to extend our thanks to Timo Kaurila, Matti Isokallio and Marko Punkkinen for their assistance and cooperation. References [l] A. Ennaoui, S. Fiechter, Ch. Pettenkofer, N. AlonsoVante, K. Btier, M. Bronold, Ch. Hiipfner and H. Tributsch, Sol. Energy Mater. Sol. Cells 29 (1993) 289.
T. Ollonqvist [2]
[3] [4] [5] [6] [7] [8] [9]
[lo]
et al. / Surface
M. Birkholz, S. Fiechter, A. Hartmann and Tributsch, Phys. Rev. B 43 (1991) 11926. M.A. Khan, J. Phys. C: Solid State Phys. 9 (1976) 81. D.W. Bullet, J. Phys. C: Solid State Phys. 15 (1982) 6163. S. Lauer, A.X. Trautwein and F.E. Harris, Phys. Rev. B 29 (1984) 6774. A. Ohsawa, H. Yamamoto and H. Watanabe, J. Phys. Sot. Jpn. 37 (1974) 568. E.K. Li, K.H. Johnson, D.E. Eastman and J.L. Freeouf, Phys. Rev. B 32 (1974) 470. H. Van Der Heide, R. Hemmel, C.F. Bruggen and C. Haas, J. Solid State Chem. 33 (1980) 17. S. Suga, M. Taniguihi, S. Shin, M. Seki, S. Shibuya, K. Sato and T. Yamaguichi, Physica B 117/l 18 (1993) 353. W.W. Kou and M.S. Seehra, Phys. Rev. B 18 (1978) 7062.
Science
377-379
(1997)
201-205
205
[ 111 I.J. Ferrer, D.M. Nevskaia, C. de las Heras and C. Sanchez, Solid State Commun. 74 (1990) 913. [12] W. Folkerts, G.A. Sawatzky, C. Haas, R.A. de Groot and F.U. Hillebrecht, J. Phys. C: Solid State Phys. 20 (1987) 4135. [13] C. Sugiura and S. Muramatsu, Phys. Status Solidi B 129 (1985) K157. [14] Y.C. Lee and P.A. Montano, Surf. Sci. 143 (1984) 442. [ 151 V. Dose, Prog. Surf. Sci. 13 (1983) 225; J. Phys. Chem. 88 (1984) 1681; Surf. Sci. Rep. 5 (1985) 337; N.V. Smith, Rep. Prog. Phys. 51 (1988) 1227. [ 161 F.J. Himpsel, Surf. Sci. Rep. 12 (1990) 1. [ 171 T.E. Ollonqvist and I.J. Vayrynen, Vacuum 46 (1995) 1177. [ 181 T.E. Ollonqvist and I.J. Vayrynen, to be published. [19] R. Courths and S. Hiifner, Phys. Rep. 112 (1984) 53.