X-RAY AND UV PHOTOELECTRON SPECTRA FROM OUTER ELECTRONS OF SOME RARE EARTHS

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X-RAY AND UV PHOTOELECTRON SPECTRA FROM OUTER ELECTRONS OF SOME RARE

EARTHS

S. Hagström, G. Brodén, P. Héden, H. Löfgren

To cite this version:

S. Hagström, G. Brodén, P. Héden, H. Löfgren. X-RAY AND UV PHOTOELECTRON SPECTRA

FROM OUTER ELECTRONS OF SOME RARE EARTHS. Journal de Physique Colloques, 1971, 32

(C4), pp.C4-269-C4-273. �10.1051/jphyscol:1971449�. �jpa-00214650�

JOURNAL DE PHYSIQUE Colloque C4, supplkment au no 10, Tome 32, Octobre

1971,

X-RAY AND UV PHOTOELEZTRON SPECTRA FROM OUTER ELECTRONS OF SOME RARE EARTHS

Department of Physics

University of Linkoping Linkoping, Sweden

and

G. BRODEN, P. 0. HEDEN, and H. LOFGREN Department of Physics ; Chalmers University of Technology

Gothenburg, Sweden

Rksum6. - Les techniques de photokmission X (ESCA ou XPS) et de photo~mission

(UPS) ont kt6 appliqukes

I'ktude des Blectrons

f dans des kchantillons maintenus propres des terres rares suivantes

Nd, Sm, Eu, Gd, Dy, Er, Yb. Alin d'effectuer une comparaison, on a inclu Ba kcette etude. La mkthode UPS donne une rksolution sup6rieure tandis que la mkthode

a I'avantage de permettre une exploration plus profonde en dessous du niveau de Fermi et SBtude des niveaux internes.

force d'oscillateur &excitation des Blectrons 4 f est trks petite pour les photons de basse energie. Mais elle augmente rapidement avec l'knergie des photons ce qui rend la technique

Ws propice

I'Btude de ces niveaux. Le spectre enregistr6 pr6sente une forme simple pow les klkments dont la sous-couche

f est

demi-remplie (Eu, Gd) ou remplie

tandis que le spectre des autres klkments possbde une structure plus compliqu6e et Btendue. Ceci s'interprete par la s6pa- ration en multiplet du niveau final intervenant dans le processus de photo6mission.

Abstract.

-The X-ray photoemission (ESCA or XPS) and the W photoemission

techniques have been applied to the study of the 4 f electrons in clean samples of the following rare earths, Nd, Sm, Eu, Gd, Dy, Er, and

For comparison the element Ba has also been included.

The UPS method gives a higher resolution while the XPS method has the advantage of probing deeper below the Fermi level allowing also core levels

be studied. The oscillator strength for exciting

electrons is very small for low photon energies. However, it increases rapidly with photon energy which makes the XPS technique very favourable to study these levels. The recorded spectra show a simple shape for elements with a half-filled (Eu, Gd) or filled (Yb)

subshell while the spectra for the other elements exhibit a more complicated and extended structure. An interpretation of this is given in terms of multiplet splitting of the final state involved in the photoemission process.

1. Introduction. - The rare earth metals and compounds exhibit many interesting physical proper- ties which are due to the 4 f electrons. Although these are energetically located in the electronic band struc- ture part of the solid it has proven difficult to study them by means of optical or low photon energy pho- toemission techniques due to a very small excitation oscillator strength at these photon energies. We have experimentally determined the occupied 4 f energy levels in a number of rare earth metals, namely the following ones, Nd, Sm, Eu, Gd, Dy, Er, and Yb using both the X-ray photoemission (ESCA or XPS) techni- que and for some of the elements also the UV photo- emission technique (UPS). As recent band calcula- tions [I] show the element Ba has a very similar band structure to that of Eu. The difference is mainly the presence of partly filled 4 f levels in Eu. For compari- son we have therefore also included Ba in our study.

The 4 f electrons in the rare earths are shielded by the closed 5 s and 5 p shells. The fact that the 4 f electrons retain many core like properties is most strikingly shown by the chemical similarity of the rare earth metals. On the other hand it is just the 4 f electrons that are responsible for the many interesting magnetic properties characterizing the lanthanides and their compounds.

2. Experimental methods. - Before we discuss.

the experimental details pertinent to the reported experiments we shall briefly describe the applied XPS and UPS methods.

In both methods emitted electrons from a sample excited by monochromatic radiation are energy ana- lyzed. The kinetic energy distribution will reflect the energy distribution of the electrons within the sample to a binding energy limit of the exciting photon energy.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971449

c4-270

s.

HEDEN

In the UPS case this photon energy is in the 10 eV

range while in the XPS case it is of the order of 1 000 eV. This big difference in photon energies invol- ved gives the two methods very special merits and rather than competing they complement each other in many ways. With the high photon energy one can easily reach into the core-level part of the electronic structure. Since many of the inner levels are quite sharp this part of the spectrum yields information on e. g. inelastically scattered electrons, spectral broade- ning effects. The XPS technique is also capable of giving information of the chemical composition (ESCA) of the sample surface. Not only can the ele- mentary composition be monitored but by observing the chemical shift of the core levels one can obtain information about the valence states [2]. In this way we have been able to continuously monitor the growth with time of an oxygen contaminant on the sample surface.

At present the resolution obtainable with XPS is limited to about 1 eV by the natural line width of the exciting X-radiation which in this case was AlKa radiation (1 486.6 eV) from an aluminium target. In the UPS case the exciting radiation is obtained from a vacuum monochromator equipped with a discharge lamp or from a helium discharge lamp using the helium resonance line at 21.2 eV. The broadening contribu- tion in these cases from the exciting radiation is negli- ble. The UPS technique is therefore capable of consi- derably better resolution.

In the UPS case it is essential to work in the UHV region in order to secure the cleanliness of the sample

surface. The working pressure in these experiments was 7 x 10-'I torr. The sample chamber is therefore sealed from the monochromator by means of a LiF window.

This, however puts an upper limit on the transmitted photon energies at about 11 eV. When using the helium line we used a differentially pumped lamp.

The energy analysis of the photoemitted electrons is made by applying a retarding electric field between the sample and a collector can surrounding the sample and using the AC modulation technique [3]. This gives an instrumental broadening of about 0.3 eV. Recently we have designed [4] a small hemispherical analyzer with a mean diameter of 5 cm. This gives a very high resolution and allows also very weak structure to be detected by using a particle detection system.

In the XPS measurements an electrostatic, hemi- spherical analyzer with a mean radius of 36 cm was employed. The resolution obtained on a sharp core level in an Au sample was 1.6 eV. The vacuum sys- tem is an all metal gasket chamber with a working pressure of 5 x torr.

Both sets of measurements were made on in

evaporated films. All the investigated metals are very reactive and one of the main experimental difficulties was to prepare and keep a clean surface for long enough to perform reliable measurements. In the UPS case the surface stayed clean for about one hour. The dete-

rioration with time of the photo electron spectrum for a Yb film is illustrated in figure 1. In the XPS measurements the most reactive films stayed clean

N e w film The same film 12 hours old

U L L L I L- - A

-10 -8 -6 -4

-2

ENERGY O F I N I T I A L STATE (eV) FIG. 1.

-

from a Y b film.

for only about 5 minutes as observed by the appea- rance of an oxygen photoelectron line. The measure- ments and evaporations were therefore cycled with a period of less than 5 minutes.

3. Results. - A typical set of photoelectron spectra obtained from UPS measurements is shown in figure 2

I

-8 - 7 -6 - 5 -4 -3 -2 -1 0 ENERGY OF INITIAL STATE (eV)

FIG. 2. -Photoelectron spectra from Yb obtained with the UPS technique.

X-RAY AND W PHOTOELECTRON SPECTRA FROM OUTER ELECTRONS (24-27 1

for Yb. The zero of the energy scale is chosen to the

Fermi level of the sample. A number of peaks are observed which for reference are labelled A to E.

Similar results have been obtained for Eu. Froin these measurements it is very difficult to single out the contri- bution from the 4 f electrons. By comparing with measurements on Ba we notice that the structure at the points labelled A, B, C, and D on the Yb curves are genuine for Yb. However, the influence of direct transitions from other states cannot be ruled out.

By going t o higher photon energies (21.2 eV and

486.6 eV) the peaks increase in strength and due t o instrumental effects they are also broadened. In figure 3 the XPS spectrum for Yb is illustrated.

I I I ~ I I I I I I ~

YTTERBIUM - bw = 1486.6 eV (A( K a )

4 f a-exper~mental points

'B "-4%

-

ENERGY BELOW FERMl LEVEL (eV1

FIG. 3.

-

by 1.30 eV.

The large double peak is associated with the excita- tion of the 4 f electrons. The corresponding curve for Ba shows no such structure, figure 4.

BARIUM

$ 7000

-20 -15 -10 -5 0

ENERGY BELOW FERMI LEVEL 1oVI

FIG. 4. - The XPS spectrum for Ba.

The XPS spectra for the other elements that have been studied are shown in figures 5-8. A common feature is the strength of excitation of the 4 f electrons.

The spectra from elements with a half-filled (Eu, Gd) or filled (Yb) 4 f shell show great sirniIarity. The

spectra from the other elements on the other hand show a different structure.

-20 -15 -I0

-

ENERGY BELOW FERMl LEVEL (eV)

FIG. 5.

-

-20 -15 -10 -5 0

ENERGY BELOW FERMl LEVEL l e v )

FIG. 6 . -The W S spectrum for Gd. The weak structure at the Fermi level could possibly be due to excitation of 5 d elec-

trons.

4. Discussion. - The strength of excitation of the 4 f electrons when using soft X-rays can be explained by the high l-quantum number involved. Calculations of oscillator strength [5] for atomic elements imme- diately following the rare earths in the periodic table show that the cross-section for 4 f electrons has a maximum several hundred eV above the excitation threshold. The XPS technique is therefore parti- cularly suited to study these electrons.

Yb has a filled 4 f shell with 14 electrons, and conse- quently the photoelectron spectrum should show ordi- nary spin orbit splitting. The shape of the recorded Yb spectrum is fully consistent with this simple picture.

The intensity ratio of the two peaks is 6

8 [6],

In the other rare earth metals the spectrum will

be more complicated due to the presence of the unfilled

4 f shell. Even the light rare earths show a large split-

ting and complicated structure. Gd and Eu which

I I I I I I I

-

-

FIG. 7. - The XPS spectra of Dy and Er.

both have a half-filled 4 f shell show on the other hand a structure with a symmetric peak which has a width of 2 eV. The spectra of the elements Nd, Sm, Dy and Er all exhibit the same type of structure. The width is of the order of 10 eV which excludes possible crystal effects. A slight oxidation of the sample surface would give rise to an extended structure. However, the close control of the experimental conditions makes us feel sure to rule out that explanation.

In interpreting photoelectron spectra it should be born in mind that the outgoing photoelectrons also reflect the final state of the system. This is quite obvious from work on gaseous samples. Recently one has observed splittings of core levels due to the coupling of a hole in a metal-atom subshell to an unfilled valence sub-shell [8]. A similar interpretation may be valid here. The final state is not a single state but can be one of several excited states which are separated by multiplet splittings. We would therefore interpret the observed structure as being due to multiplet splittings of these excited states.

A particular simple case is the half-filled shells in Gd and Eu. After photoexcitation of one electron the hole state 4 f 6 6 s1 will give rise to a 4F,, J

...

term (Russel-Saunders coupling). The overall splitting of this term is 0.6 eV which compares well to our measured value.

-20 - 15 - 10 - 5 0

Energy below Ferrni level ( e V )

FIG. 8. - The XPS spectra of Nd and Sm. The peak to the far left in the Nd spectrum is due to 5 p states.

A summary of the energy locations of the observed 4 f peaks is given in figure 9, where preliminary data have been used for the element Pr.

FIG. 9.

-

Acknowledgements.

Dr. C. Norris contributed

much to the UPS work in its early stage. This work

has been supported by grants from Statens Natur-

vetenskapliga Forskningsrgd and Styrelsen for Tek-

nisk Utveckling.

X-RAY AND UV PHOTOELECTRON SPECTRA FROM OUTER ELECTRONS

DISCUSSION

M. FABIAN. - 1. HOW much is the fine structure 2. The UPS measurements are extremely sensitive in UPS spectra attributable to indirect transitions to the sample surface condition due to the small which do not interfere in the XPS spectra

escape depth.

2. You told us that your vaccum in the UPS

measurements was 5 x 10-l1 torr. and in the XPS M. S~IBOWSKI, Universitat, Miinchen.

By which measurements 5 x lo-', but that in the former method could you reach a pressure of 5 x lo-" torr.

case the surface remained clean for only one hour during the experiments using the He-resonance line while in the second case it remained clean for 5 min. at 21.2eV

To what do you attribute the rapid contamination M. HAGSmSM. - We used a pumped in the first case, where the vacuum is so good

He lamp. After about one hour the pressure in the M. HAGSTROM. - 1. Part of the fine structure in the sample chamber rose to about lo-* torr. However, UPS Yb spectrum could possibly be due to direct tran- the pressure rise was mainly due to inflowing He gas sitions from other states than 4 f

s. which did not disturb the sample surface.

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[s]

COMBET FARNOUX

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CREMO-

SIEGBAHN

and ONORI

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and NoRRrs

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131

SPICER

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1964,35, 1664. [71

HAGSTROM

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0.) and LOFGREN

[4] LINDAU (I.) and HAGSTROM

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