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SINGLE AND DOUBLE. PHOTOIONIZATION OF LANTHANIDE ELEMENTS IN THE REGION OF
THE 5p EXCITATION
C. Dzionk, W. Fiedler, C. Kortenkamp, M. V. Lucke, P. Zimmermann
To cite this version:
C. Dzionk, W. Fiedler, C. Kortenkamp, M. V. Lucke, P. Zimmermann. SINGLE AND DOUBLE.
PHOTOIONIZATION OF LANTHANIDE ELEMENTS IN THE REGION OF THE 5p EXCITA- TION. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-501-C9-504. �10.1051/jphyscol:1987981�.
�jpa-00227402�
JOURNAL D E PHYSIQUE
Colloque C9, suppl6ment au n012, Tome 48, dkcembre 1987
SINGLE AND DOUBLE. PHOTOIONIZATION OF LANTHANIDE ELEMENTS IN THE REGION OF THE 5p EXCITATION
C. DZIONK, W. FIEDLER, C. KORTENKAMP, M. v. LUCKE and P
.
Z IMMERMANNInstitut fiir Strahlungs- und Kernphysik, ~echnische Universitdt Berlin, sekr, P N 3-2, Hardenbergstrasse 36, 0-1000 Berlin 1 2 , F.R.G.
Abstract: Monochromatized synchrotron radiation, atomic beam technique and a TOF spectrometer were used to measure the cross section of single and double photoionization of the lanthanide elements in the region of the 5p excitation. A systematic difference between light and heavy elements was observed.
INTRODUCTION
Tracey /1/ observed the absorption spectra of seven lanthanide elements (Sm, Eu, Dy, Ho, Er, Tm, Yb) in the region of the 5p excitation. For Yb with the filled 4f subshell in the ground state Sp64f146s2 Iso the absorption lines could be attributed to series 5p
-
nd converging to the two spin-orbit limits Sp5 2 ~ 3 / 2 resp. 2~1/2 of Yb I1 5p54f146s2. For the other elements with unfilled 4fsubshells the coupling to the 4fq subconfiguration results in a very complex structure of accessible states. A further complication in the analysis of the spectra is caused by the fact that often initial states other than the ground state are thermally populated.
Nevertheless, the absorption spectra of Tm(q=13), Er(q=l2), Ho(q=ll) and Dy(q=lO) were quite similar to the Yb spectrum with common
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987981
C9-5 02 JOURNAL DE PHYSIQUE
features due to the 2 ~ 3 / 2 resp. 2~1/2 structure of the core 5p5. The spectra of Eu(q=7) and ~ m ( ~ = 6 ) also showed approximately these features, but were dominated by a 'giant' 2~1/2 peak. As the 2~1/2-states have additional decay channels such as
5p5(2~1/2) 4fq6s2nd
-
5p5 (2~3/2) 4fq6s2cs ,d with the subsequent Auger decay, one may assume that these states should preferentially decay to doubly charged ions. For that reason it is interesting to measure the cross section of single and double photoionization in this region. Holland and Codling /2/ and Holland et a1 /3/ performed measurements for the more easily vaporizable elements Sm, Eu, Tm and Yb.EXPERIMENTS AND RESULTS
We have started a series of measurements of the single and double photoionization cross section for all stable lanthanide elements
(except Ho). The experiments were performed at the electron storage ring BESSY in Berlin using a 1-m Seya Namioka monochromator equipped with a platinum grating with 1200 l/mm. In most cases the bandpath was 0.45 nm. with exception of the cerium measurements the photonflux was monitored by the photoelectron yield of a gold surface. We used the measurements of Gudat and Kunz /4/ to correct the variations of the gold photoelectron yield with varying photon energy. In the cerium case the spectrum was normalized with monochromator
characteristics and the decrease of the ring current. For production of the atomic beam we used an oven heated by electron bombardement.
To evaporate ytterbium resistive heating was sufficient. For detection of the photoions we used a time of flight mass
spectrometer. The ions were extracted from the interaction region by a pulse of 120 V amplitude lasting about 1 ps repeated every 40 ps. A simultaneous trigger pulse started two additional pulse generators with adjustable delay and pulse width which steered two gated
discriminators. Therefore it was possible to set appropriate windows to detect only photoions of the element under study. The voltage applied to the TOF drift tube was chosen in a way that the counting rate of the singly ionized atoms was maximal. It should be mentioned that the pulse generator providing the extraction pulse was operated under maximum conditions, so that we are not quite sure if all singly charged ions were detected. Therefore there remains an uncertainty about the reliability of the absolute values of A++/A+. On the other hand, the ~ b + + / ~ b + ratios are in good agreement with the values obtained by Holland et a1 /3/. Fig. 1 shows the experimental results
4 n—•—i—<—>—'—'—i—• • '—•—i—'—"-i 25 p—i—•—i—i—i—i—•—>->]—i—•—i—i—i—i—i—j 801—•—'—•—i—•—'—' • i—'—' • r • 4 p—'—i—i—i—>—i—i—•—i—<—r—!—i—r-j 25 p—>—•—«—i—•—«—t—• \ » • • •—r^—F—5 80 [—'—•—'—r-1—•—'—•—i—•—•—•—p 20 25 30 25 30 35 30 35
energ y \eV]
Fig. 1 Cross section for single and double photoionization of Ce, Tb and Yb o i, oC9-504 JOURNAL DE PHYSIQUE
for the elements Yb(Z=70), Tb(Z=65) and Ce(Z=58). Whereas for Yb and
~b the resonances in the cross section of o(x+) are reproduced by a(x++), although not with a constant, but with an increasing ratio in dependence of increasing photon energy, there is obviously no such correlations for the light rare earth elements like Ce. A similar behaviour was found for the 3d-elements in the region of the 3p excitation /5/.
The authors acklowledge the support of the BESSY staff. This work has been funded by the German Federal Minister of Research and Technology (BMFT) under the contract number 05 314 EX B2.
REFERENCES
/1/ D.H.Tracy, Pr0c.R.Soc.Lond.A
357,
485 (1977)/2/ D.M.P.Holland and K-Codling, J.Phys.B.: At.Mol.Phys.
14,
L359(1981)
/3/ D.M.P.Holland, K.Codling and R.N.Chamberlain, J.Phys.B:
At.Mol.Phys.
14,
839 (1981)/4/ W.Gudat and C.Kunz in: Synchrotron Radiation, Ed.: C.Kunz, 1979, Springer Verlag, p. 135
/5/ M.Schmidt and P.zimmermann in: Giant Resonances in Atoms, MolecuLes, and Solids, Ed.: J.P.Connerade, J.M.Esteva and R.C.Karnatak, 1987, Plenum Press, p. 247
