Angle-resolved UV photoemission of cerium

Journal

of the Less-Common

Metals,

111 (1985)

ANGLE-RESOLVED

UV PHOTOEMISSION

G. ROSINA,

and F. P. NETZER

lnstitut

E. BERTEL

fiir Physikalische

(Received

January

Chemie,

Uniuersittit

290

285

285

OF CERIUM*

Innsbruck,

A-6020

Innsbruck

(Austria)

3, 1985)

Summary A clean, well-ordered Ce (001) single-crystal surface has been prepared and investigated in angle-resolved UV photoemission. Photon energies between 12 and 40 eV were used to discriminate between emission from 4f states and 5d6s derived conduction band states. Comparison of the photoelectron spectra with band structure calculations reveals dispersive features due to direct transitions from d states, and 4f-related emission at 0.3 eV and 2 eV. The 4f features show no dispersion within the experimental accuracy of -90 meV. The work function of the clean Ce (001) surface was determined to be 4.05 f 0.1 eV.

Introduction The electronic structure of Ce has been the subject of numerous experimental and theoretical investigations [ 1, 23. Despite the frequent preoccupation with the electronic structure, the nature of the 4f electron and its manifestation in photoemission spectra continue to be of interest [ 3 - 51. In the present paper we report on the first angle-resolved UV photoemission study from a Ce singlecrystal surface. The purpose of this work is to elucidate the band structure of y-Ce and to separate itinerant-band-state from localised-f-state emission.

Experimental

details

The experiments were carried out in a VG ADES 400 spectrometer (base pressure < 5 X 10-i’ Torr) equipped with a rare-gas resonance lamp, an electron gun and a movable electron analyser with angular resolution +l” for angle-resolved electron spectroscopy. An Ar-ion gun was used for sputter cleaning and a LEED optics for determining surface structure and orien*Paper presented land, March 4 - 8,1985. 0022-5088/85/$3.30

at the International

Rare

Earth

0 Elsevier

Conference,

Sequoia/Printed

ETH Zurich,

Switzer-

in The Netherlands

286

tation. The Ce crystal consisted of a large, single-crystal grain (-4 X 6 mm) cut to expose a (001) surface with adjacent polycrystalline zones. The crystal was clamped to a Ta holder which could be heated by electron bombardment from the back side. Surface cleaning was performed by many cycles of Ar sputtering and subsequent heating under vacuum. The following sputtering conditions proved most effective: 500 eV Ar ions, sample current -8 PA for 20 min. Annealing after ion bombardment to 400 “C!produced a reconstructed surface as evidenced by LEED. UV photoemission (UPS) showed considerable intensity at 3.2 eV and 4.5 eV binding energy (BE) due to hydrogen induced emission. After many sputtering and annealing cycles the bulk hydrogen content was reduced so that annealing to 280 “C resulted in a (1 X 1) LEED pattern and negligible H derived signals in UPS. During accumulation of the UPS spectra a weak 0 and C signal appeared at 5 - 6 eV BE. It was measurable after 30 min and grew to similar intensity as several fresh surthe conduction-band features after - 90 min. Therefore, faces were usually prepared during spectrum accumulation. The work function of Ce was determined from the width of the electron distribution curve in UPS. A value of 4.05 f 0.1 eV was measured for the clean Ce (001) surface.

Results Figure 1 shows an Auger spectrum of the clean Ce (001) surface in dN/dE mode. Auger features in the 50 - 130 eV region are all due to the decay of Ce 4d initial states holes [6]. Contaminant peak ratios were typically C/Cc (87 eV) = 0.007, O/Cc (87 eV) = 0.011, all other contaminants being below the detection limit. Careful monitoring of the structure at 188 eV during the cleaning procedure revealed that this feature is the Ce N30Z3V Auger transition rather than residual chlorine contamination [ 61. I

I

Ce (001)

kinetic

energy INI

100

I

200

1

Fig. 1. Auger spectrum

300

I

400

,

of a clean Ce (001)

500

I

surface,

y-phase.

287

binding 4 J

3

I

2

I

energy

l&l

1

I

Fig. 2. He II photoemission spectra of Ce (OOl), y-phase, as a function angle, 8. Photon angle of incidence (Y= 20”, spectrometer resolution marks indicate experimental peak positions.

of electron 180 meV.

exit Tick

Figure 2 shows spectra taken with He II radiation (hv = 40.8 eV) at (Y= 20” photon angle of incidence and four different electron exit angles between 8 = 0” (normal emission) and 19= 30”, in the F XW crystal azimuth. There are two pronounced maxima at 0.3 eV and 1.95 eV BE in all four spectra, which relate to 4f emission also seen in XPS at 0.27 eV and 1.92 eV [ 71. On polycrystalline Ce, a recent high-resolution study (overall resolution 100 meV) using synchrotron radiation [S] places 4f emissions at 0.2 eV and 2.0 eV. This is in good agreement with the present results, if the resolution of our He II spectra (180 meV) is taken into account. Another feature in the He II spectra appears at 1.05 eV BE at normal emission, which shows only slight dispersion but a strong change in intensity as the exit angle is varied. Its origin at normal emission is shown to be consistent with a direct transition from the second band (d character) close to the X point (see Fig. 4). Normal emission from this band is possible if relativistic selection rules are considered [9]. The 1 eV feature at higher angles (0 = 10 - 30”) may also be due to direct transitions involving the first and the second band near the K and the W point. A structure showing some variation with exit angle is seen at -3.5 eV BE in the He II spectra (see Fig. 2). This corresponds to the energy position of the bottom of the s band at the F point [lo]. A direct transition from the F point in He II, however, is excluded by h conservation. Also, at normal

288

emission, s band contribution should be weak for the present photon angle of incidence, where X, y polarisation is dominant. On the other hand, we note that on the hydrogen-covered surface H-induced emission is apparent at 3.2 eV and 4.5 eV. We therefore attribute the structures around -3.5 eV to emission induced by residual hydrogen. Figure 3 shows a set of Ne I spectra (hv = 16.8 eV, resolution 90 meV) at o = 20” and 8 = 0 - 40”. Again, features are observed at 0.25 eV and -2.1 eV BE (0 = 0”) which obviously relate to 4f emission. At exit angles near 27” slight dispersion could be inferred from the data, but interference with underlying s,d-derived states cannot be excluded. A distinct feature is seen at 1.2 eV in normal emission, which cannot be related to a direct transition with free-electron-like final states (Fig. 4). It could originate from a surface state, a surface resonance or from density-of-states emission [ 11, 121. For a surface state the feature has to appear in a band gap. According to the calculations of Pickett et al. [lo] (Fig. 4) a band gap is apparent only

-

1

1

\

Ne I

.IIJL z

e ” e

e

EF ; Y

-

-1

-

ii

WJ

h”

”

-2

binding energy 4

I

3

I

2

1

1

1

-

.

r

,

x

Fig. 3. Ne I photoelectron spectra of y-Ce (001). (Y = 20”, meV. Tick marks indicate experimental peak positions.

spectrometer

resolution

90

Fig. 4. Band structure of y-Ce in the rA X direction as calculated by Pickett et al. [lo]. Experimental points from normal emission spectra are included, assuming free-electronlike final states. He II: A; He I: A; Ne I: 0; Ar I: 0. The insert shows the geometry of the experiment.

289

below 1.3 eV. Possible explanations of the 1.2 eV feature would then be a surface resonance or density-of-states emission. Alternatively, free-electron final states may not be applicable at Ne I energies. Similar to the He II spectra we notice some intensity in Ne I at -3.5 eV, which is presumably hydrogen induced. An interesting observation is made close to the Fermi level at 8 = 25” and beyond. A remarkable increase in intensity at 0.4 eV BE is followed by the splitting-off of a spectral band, which disperses downward with increasing 6’ to 1.1 eV (see Fig. 3). This is tentatively assigned to direct transitions from d-derived states, increasing in binding energy as the zone boundary is approached. A more detailed assignment requires an interpolated band structure, and this will be attempted elsewhere [ 131. The spectra recorded at Ar I (11.8 eV) and He I (21.2 eV) photon energies show essentially similar features. A somewhat peculiar behaviour is observed for the higher binding energy 4f emission: this feature appears at -2.3 eV in the Ar I spectra, but as a broad band and eventually a split peak at -2.1 - 2.3 eV in Ne I (Fig. 3). In He I the 4f emission is found at 2.0 eV. These spectral changes cannot be attributed to a surface core-level shift: firstly, they occur in the wrong energy direction (see, e.g., ref. 14), and secondly, the binding energy as measured in He II, which results in the highest surface sensitivity, is very similar to that found in XPS [7]. It is possible that this behaviour is related to collective effects in the manner of Zangwill and Soven [15] near the 5p threshold of Ce. The 5p3,* channel opens at - 17 eV [ 161, and it is in this photon-energy region that we observe the change in the 4f peak position.

Conclusion

The angle-resolved UPS spectra of the Ce valence region reveal two 4frelated features at 0.3 eV and 2 eV, the latter showing slight binding energy variations at photon energies below the 5p threshold; no hi, dispersion is apparent. Dispersing features are observed between E, and 1.5 eV BE. They are attributed to direct transitions, with the possibility of a surface resonance or density-of-states emission. A more detailed discussion comparing the photoemission spectra with calculations of an interpolated band structure for y-Ce is underway [ 131.

Acknowledgment

This work Wissenschaftlichen

has been supported by the Forschung of Austria.

Fonds

zur

Fijrderung

der

290

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