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Measurements of Zn L2,3 satellites using X-ray

emission spectroscopy

Martin Magnuson and Joseph Nordgren

Linköping University Post Print

N.B.: When citing this work, cite the original article.

Original Publication:

Martin Magnuson and Joseph Nordgren, Measurements of Zn L2,3 satellites using X-ray

emission spectroscopy, 1997.

Preprint available at: Linköping University Electronic Press

(2)

Hamburger Synchrotronstrahlungslabor HASYLAB, Deutschen Electronen-Synchr., DESY, Jahresbericht (1997), I.

Measurements of Zn L

2,3

satellites using x-ray emission spectroscopy

Martin Magnuson and Joseph Nordgren

Department of Physics, Uppsala University, P. O. Box 530, S-751 21 Uppsala, Sweden

PACS numbers:

X-ray emission spectroscopy is a powerful experimental technique for studying the electronic structure of solids. Presently, the existence of X-ray emission satellite fea-tures originating from the decay of L2,3core holes in the

presence of additional 3d vacancies have been studied. The satellites of the L2,3 x-ray emission spectra of metals

can be produced either directly in the ionization process or as a result of Coster-Kronig processes preceeding the core-hole decay. The vacancy satellites are traditionally referred to as Wentzel-Druyvesteyn satellites [1] and end up on the high energy side of the main line.

In a recent work [2], we presented X-ray emission spec-tra of Cu metal. For excitation energies passing the L2

threshold, a sharp step of increasing satellite intensity was found at the Cu L3 emission line, proving the

im-portance of Coster-Kronig decay to the satellite contri-bution. Due to the grazing-incidence sample orientation, the spectra were found to be almost free of self-absorption effects. From a quantitative analysis of the spectra, the L3/L2intensity ratio and the Auger contribution to the

life-time broadening were extracted. In this contribution we have extracted the corresponding values for Zn metal using spectra excited at high resolution.

The experiments were carried out at the undula-tor beamline BW3 at Hasylab in Hamburg. A high-resolution, grazing-incidence spectrometer with a two-dimensional detector was used [3]. The Zn spectra were recorded using second order of diffraction of a 1200 lines/mm grating (radius 5 m). The base pressure in the experimental chamber was 5 × 10−9Torr. In order to determine the excitation energies, x-ray absorption spec-tra in the threshold region were obtained by measuring the total electron yield from the sample with 0.5 eV res-olution of the monochromator of the beamline. During the X-ray emission measurements the resolution of the monochromator of the beamline and the X-ray fluores-cence spectrometer were 0.9 eV and 0.8 eV, respectively. The sample was of high purity (99.99 %) and oriented so that the photons were incident at an angle of 7o and with the polarization vector perpendicular to the surface plane. The emitted photons were recorded at an angle near normal to the sample surface, perpendicular to the incoming photons. The grazing-in normal-out setup was chosen to minimize self-absorption.

The Zn spectra were measured from excitation ener-gies from 1021.8 eV, at the L3 threshold, up to energies

as high as 1200.8 eV, above both the L2(EB=1044.9 eV)

and the L1(EB=1196.2 eV) thresholds. The X-ray

emis-sion process above the L3threshold is normally described

as a “two-step” process. In the first step a photoelectron

25 20 15 10 5 0 IS /(I R +I S )% 1080 1070 1060 1050 1040 1030 1020

Excitation Photon Energy (eV)

L2

L3

Zn satellite intensity

FIG. 1: (Color online) The relative x-ray emission satellite vs. the main line intensity ratio (in %) shown close to the L3

and L2 thresholds.

is excited from a core-orbital. In the decay step the core hole is filled by a valence electron and a photon is emit-ted. With separated excitation and emission steps, the satellite contribution can be separated from the main line by a proper subtraction procedure [4].

From a statistical point of view, the L3 and L2

core-level ionization ratio is 2:1 for Zn. With excitation ener-gies above the L1threshold, the L1 Coster-Kronig decay

will also affect the initial core-hole population for X-ray emission of the L2and L3core levels. Just below the L1

threshold, the L3/ L2 emission intensity ratio was found

to be 3.1, using the threshold spectra to subtract the satellite contribution. From this ratio, it is possible to obtain the Auger contribution to the life-time broaden-ing ΓA, using an experimental value of the Coster-Kronig

width (0.39 eV) obtained by Nyholm et al. [5]. The Auger width ΓA, for pure Zn metal is then found to be

0.71 eV by using the formula ΓA = 2ΓCK/(Iratio− 2)

[5, 6], where Iratio is the L3/L2main-line intensity ratio

(3.1). These values are in fairly good agreement with cal-culations by Yin et al.[7], where ΓCK and ΓAwere found

to be 0.75 eV and 0.65 eV, respectively.

Figure 1 shows the relative IS/(IR+IS) satellite

inten-sity at the L3 emission line (in percent), normalized to

the satellite-free L3threshold spectrum excited at 1021.8

eV. The intensities were extracted after normalization by

(3)

subtracting the satellite-free threshold spectrum so that the intensities of the difference spectra were always pos-itive. The error bars were obtained by var ying the fit parameters.

For excitation energies below the L2 threshold, the

satellite has a slowly increasing intensity to less than 5 % of the total intensity, whereas for excitation energies closer to the L2 threshold, a step of very rapid intensity

increase is observed up to a new plateau at about 21 %. To summarize, X-ray emission spectra of Zn metal have been measured close to the L3, L2, and L1

exci-tation thresholds with monochromatic synchrotron

radi-ation. From the quantitative analysis of the spectra, it is possible to extract the L3/L2 intensity ratio and the

L2,3Auger-width. For excitation energies passing the L2

threshold, a sharp step of increasing satellite intensity is found at the L3emission line, proving the importance of

Coster-Kronig decay to the satellite contribution also for Zn metal.

This work was supported by the Swedish Natural Sci-ence Research Council (NFR) and the G¨oran Gustavs-son Foundation for Research in Natural Sciences and Medicine.

[1] G. Wentzel, Z. Phys., 31, 445 (1925), M. J. Druyvesteyn, Z. Phys. 43, 707 (1927).

[2] M. Magnuson, N. Wassdahl and J. Nordgren, Phys. Rev. B, 56, 12238 (1997).

[3] J. Nordgren and R. Nyholm, Nucl. Instr. Methods A246, 242 (1986); J. Nordgren, G. Bray, S. Cramm, R. Nyholm, J.-E. Rubensson, and N. Wassdahl, Rev. Sci. Instrum. 60, 1690 (1989).

[4] N. Wassdahl, J.-E. Rubensson, G. Bray, P. Glans, P.

Bleckert, R. Nyholm, S. Cramm, N. M˚artensson, and J. Nordgren, Phys. Rev. Lett. 64, 2807 (1990).

[5] R. Nyholm, N. M˚artensson, , A. Lebugle and U. Axelsson, J. Phys. F 11, 1727 (1981).

[6] E. Antonides, E. C. Janse, and G. A. Sawatsky, Phys. Rev. B 15, 4596 (1977); 15 1669 (1977).

[7] L. I. Yin, I. Adler, M. H. Chen and B. Crasemann, Phys. Rev. A 7, 897 (1973).

References

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