Kuntze R, Salzmann M, Böwering N, Heinzmann U. Spin polarization of Auger- and of photoelectrons from barium atoms exposed to circularly polarized radiation and their cross comparison. Zeitschrift für Physik D: Atoms, Molecules and Clusters. 1994;30(2-3):235-237.New results of spin polarization of both photoelectrons and Auger electrons are reported after 5[Rho] photoionization of free Ba atoms with circularly polarized light. A substantial polarization transfer from the spin polarized photons to the spin polarized photoelectrons and via the hole state orientation to the spin polarized Auger-electrons is observed. The cross comparison of the results for photoelectrons and Auger-electrons allows a quantitative test of the assumed two step model where both electron-emission processes occur in sequence

Böwering, N.

Heinzmann, Ulrich

Kuntze, R.

Salzmann, M.

English

Publications at Bielefeld University

Z. Phys. D 30, 235-237 (1994) ZEITSCHRIFT FOR PHYSIK D © Springer-Verlag 1994 Spin polarization of Auger- and of photoelectrons from barium atoms exposed to circularly polarized radiation a d their cross comparison R. Kuntze, M. Salzmann, N. B6wering, U. Heinzmann Fakult/it ffir Physik, Universitfit B elefeld, Universitfitsstrasse 25, D-33615 Bielefeld, Germany, and Fritz-Haber-Institut der Max-Planck-Gesellschaft, D-14195 Berlin, Germany Received: 16 July 1993 / Final version: 24 August 1993 Abstract. New results of spin polarization of both pho- toelectrons and Auger electrons are reported after 5p photoionization of free Ba atoms with circularly polar- ized light. A substantial polarization transfer from the spin polarized photons to the spin polarized photoelec- trons and via the hole state orientation to the spin polar- ized Auger-electrons is observed. The cross comparison of the results for photoelectrons and Auger-electrons al- lows a quantitative test of the assumed two step model where both electron-emission processes occur in se- quence. PACS- 32.80.Fb; 32.80.Hd Creation of vacancies in inner atomic shells by photon or particle impact often gives rise to subsequent Auger decay. Recently, these Auger electrons have been meas- ured to be strongly spin polarized for the electron spec- troscopy studies with free barium atoms [1] and Rb-ad- sorbates [2], where the primary photoemission process has been performed by means of spin polarized i.e. cir- cularly polarized radiation. In their emission characteris- tics these Auger electrons reflect a non-statistical popu- lation of the magnetic substates of the intermediate ionic state produced by the spin oriented initial impact [3, 4]. Consequently, an orientation of the intermediate state can be transferred partially to the Auger electron spin polarization [5, 6]. On the other hand the data of spin- and angle resolved Auger spectroscopy can be used to determine alignment and orientation of the intermediate photoion state from the experiment [ 1 ]. Cross compari- son of these results with the corresponding dynamical parameters characterizing the primary photoelectron emission process, angular distribution and spin polari- zation parameters which form a "complete xperiment", should answer whether photoelectron and Auger electron emission are subsequent processes in a two-step model or whether they have to be seen as a dynamical combi- nation. A mutual influence of Auger- and photoelectrons would give rise to the so-called post-collision interaction, indicating a break-down of the two-step model. A ref- erence case is the 4 d ionization of xenon [7] for which both the occurrence [8] and the vanishing [9] of post- collision interaction was discussed in detail. Figure 1 shows the angle-resolved measurement [1] of the spin-polarization component A (O) (parallel to the photon beam) of Auger electrons in the 03P1Pa(1So) Auger line of Ba atoms in photoionization using circu- larly polarized synchrotron radiation from the 6.5 m N.I. Monochromator (A,~ = 0.18 nm FWHM) at BESSY [ 10]. For Auger electrons the angular distribution of the po- larization component A (O) has the same analytical form as for photoelectrons [10]: A - eP2 (cos O) A(O)=y 1 - (fl/2)-P2 (cos O) with the second Legendre polynomial P2 (cos O) and the light helicity ? but with different dynamical parameters for the angular distribution of the spin polarization. Since the photoelectrons in the example [ 1 ] of Fig. 1 were too slow ( < 1 eV) to be analyzed, we had to repeat he ex- periment at photon energies higher than 23.5 eV in order 0.5 ~J I I J L Fit-F arameter i(~ r hv=~3.5eV A:-0,34±0.02 a:  -0 .2  ±0.1  8: 0.8 +0.7 0.5 -90 -45 0 45 90 O(DEGREE) Fig. 1. Measured angular dependence of the Auger electron spin polarization at hv =23.5 eV for the Auger electrons with 7.5 eV kinetic energy corresponding to a p;- 1 hole state [ 1 ] 2 236 to include the simultaneous detection of the correspond- ing photoelectrons and their spin polarization. It is the purpose of this publication to present these data obtained with circularly polarized photons at energies around 28 eV and to compare the spin polarization values of Auger- and photoelectrons measured. The ionization process of interest can be denoted by: Ba(5p66s 2) 1So+hv ~Ba + (5 pS 6 s2) 2 Ps+ ephoto (es, ed) ---~ Ba 2+ (5 p6) 1S0 -t- ep-hoto +eAuger (Sp) where J=  } or ½ is the total angular momentum of the photoion. For the J=  ½ channel the photoelectrons leave the ion either in the es, or the e& continuum state with 7 . a spin polarization transfer, characterized by the dy- namical parameter A, determined only by the ratio ). = [D,/Da] of reduced matrix elements as being [ 11 ]: A photo  _ 1 1 -- 2). 2 7 2 1+) .  2 .  In the J = 3 channel a phase shift difference between the outgoing d~ and d~ partial wave, mad different Dd~ ~ ma- T'7 trix elements have to be taken into account in the pho- toelectron spin polarization A,. In the non-relativistic • 2 approximation only, neglecting any influence of the spin orbit interaction in the continuum states [12], APhOto - 1 1--2). 2_ 1 APhOto 4 1-t-) .  2 2 is also only a function of the ratio of the matrix ele- ments ).. Assuming that the photoemission and the Auger pro- cess occur in sequence and that the transition probability of one process is not influencedeby the other, the Auger electron spin polarization AA~, g can be factorized [6] 7 '7  as being the product of the Auger decay parameter fl~ and the photoion orientation, which is proportional to the photoelectron spin polarization A ph°t° in the non- 7,7 relativistic limit. According to the theory of Kabachnik and Lee [6] this is given for the above mentioned Auger decay of Ba to be AAuger = -½ A ph°t° 2 2 AAug er  5 AP  h° t°  The second equation is only valid in the non-relativistic limit, neglecting any differences between the ed~_ and 2 e& continuum waves of photoelectrons [13] due to spin orl~it interaction. In the relativistic ase [14] the Auger electron polarization depends upon two ratios of matrix elements whereas the photoelectron polarization depends in addition on a phase shift difference and therefore no linear relation exists between A Auger and A ph°t°. Besides the spin polarization transfer from spin-po- larized photons to spin polarized photoelectrons and via the orientation of the hole states to spin polarized Auger electrons, there exists also a dynamic spin polarization of electrons (Auger [15] as well as photoelectrons [11]) due to the spin orbit interaction and phase shift dif- ferences between at least two outgoing partial con- tinuum waves. This polarization component is perpendicular to the reaction plane, given by the momenta of incoming radi- ation and of outgoing detected electrons, and is charac- terized by the dynamical spin parameter G2 sin O cos O pAuger ,  photo  __  ~¢ Auger ,  photo  A_ - -  "~ /~ Auger ,  photo  1 2 "P2 (cos O) with fl denoting the asymmetry parameter of the corre- sponding angular electron intensity distribution• For a single channel Auger decay G Auger should vanish for the above-mentioned Auger transition, since there is only one continuum state (ep) which cannot interfere with itself. Thus a deviation from G Auger- 0 would also be an indi- cation of the invalidity of the two-step-model. Figure 2 shows on the left side the experimental results of the spin polarization parameter A (full points) and (open points) measured for the photoelectrons leaving behind the ion in the 2/,, or 2p~ ionic state. The right side 2 2 of Fig. 2 gives the corresponding Auger electron polari- zations measured. The middle part of Fig. 2 shows the Auger electron polarization expected and calculated on the basis of the experimental photoelectron polarization data (left part of Fig. 2) within the model of the two-step process and within the LS-approximation (non-rela- Photoelectrons .,..,... 0.5 l:Z-"-gr--'--'T ' _  07 . . . .  -0,5 ~_~_1 , t , 28 29 "~-4  N o am o-a -0.5 0.5 ~-~ I ' -  ea/2 _{+ + 28 29 [ I [ I g8 ~9 hv(eV) Augerelectrons --I 5 p~)Z6 s-a Z8 29 , 5 p~)z 6 s-~ + 28 29 28 29 hu(eV) Fig. 2. Spin polarization parameters A (full points) and ~ (open points) measured for photoelectrons (leftpart) and Auger electrons (right part) in comparison with theoretical HF predictions [17] for A (full curves) and with Auger electron predictions (middle part) based upon the experimental values of the photoelectrons (left part). 2p~ and 2p~ denote the intermediate ionic states 7 2 237 tivistic model) for the channel of the 2P! ion. It is worth 2 noting that this last LS restriction does not apply for the 2p± ionic channel since AAuWr and APh°t° are proportional to2each other. Finally, Fig. 2 left and right parts show a comparison with theoretical predictions based upon ma- trix elements obtained by means of Hartree-Fock calcu- lations [17] (full curves)• Since for both ionic channels 2p± and 2P3 the Auger electron spin parameter ~has been measured 2to be - 0.06__ 0.08 and 0•02_+ 0.06, respec- tively, (close to zero within the experimental uncertainty), there is no indication of the invalidity of the two-step • photo  photo  model used. The ratio of A± /A~ as well as of photo  photo  z z ~_~ /~z is not --2 as predicted for the non-rela- • 2•  • 2 tlvlstlc treatment. This could be due to a strong influence of the spin-orbit interaction onto the ed~ and ed5 conti- nuum states in the 5p~ed photoionization process or due to the strong 5 d -  6s configuration i teraction for the Ba + state with the 5pff-hole [18]. Thus the correspond- ing Auger electron polarizations calculated by means of the photoelectron polarizations and given in the middle part of Fig. 2 do not need to agree with the directly measured A A~g~ value given in the right part. 2 This cross comparison can, however, be made for the 2p± channel, where the photoelectron polarization is not 2 influenced by any phase shift differences and thus directly connected to the Auger electron polarization via the hole orientation• Since this comparison between the calculated (middle part) and directly measured (right part) A Aug~ 2 results shows agreement within the experimental uncer- tainties, it might be summarized that the results reported in this paper do not disagree with the two-step model assuming the emissions of photoelectron and Auger elec- tron to occur in sequence without significant post-colli- sion interaction. Financial support by the BMFT (055 PBAXI) is gratefully acknowledged. References 1. Kuntze, R., Salzmann, M., B6wering, N., Heinzmann, U.: Phys. Rev. Lett• 70, 3716 (1993) 2. Stoppmanns, P., Schmiedeskamp, B., Vogt, B., Miiller, N., Heinzmann, U.: Phys. Scr. T41, 190 (1992) 3. F1/igge, S•, Mehlhorn, W•, Schmidt, V.: Phys. Rev• Lett. 29, 7 (1972) 4. Mehlhorn, W•: In: Atomic inner shell physics. Crasemann, B. (ed.), p. 119. New York: Plenum 1985 5. Klar, H.: J. Phys. B13, 4741 (1980) 6. Kabachnik, N.M., Lee, O.V.:J. Phys. B22, 2705 (1989) 7. Schmidt, V.: Z• Phys. D2, 275 (1986); J. Phys. (Paris) Colloq. 48, C9-401 (1987) 8. Schmidt, V., Krmrtmacher, S. Wuilleumier, F.J•, Dhez, P. : Phys. Rev. A24, 1803 (1981) 9. Borst, M., Schmidt, V.: Phys. Rev. A33, 4456 (1986) 10. Heckenkamp, Ch., Sch/ifers, F., Sch6nhense, G., Heinzmann, U.: Phys. Rev. Lett. 52, 42t (1984) 11. Heinzmann, U.: J. Phys. B13, 4353, 4367 (1980) 12. Cherepkov, N.A•: Sov. Phys.-JETP 38, 463 (1974) 13. Heckenkamp, Ch., Sch/ifers, F•, Sch6nhense, G•, Heinzmann, U.: Z. Phys. D2, 257 (1986) 14. BuBert, W., Klar, H.: Z. Phys. A312, 315 (1983) 15. Hahn, U., Semke, J., Merz, H., Kessler, J.: J. Phys. BIg, L417 (1985) 16. Kabachnik, N.M., Sazhina, I.P.: J. hys. B23, L353 (1990) 17. Berezhko, E.G., Ivanov, V.K., Kabachnik, N.M.: Phys. Lett. A66, 474 (1978) 18. Bizau, J.M., Cubaynes, D., Grrard, P., Wuilleumier, F. : Phys. Rev. A40, 3002 (t989) 

Spin polarization of Auger- and of photoelectrons from barium atoms exposed to circularly polarized radiation and their cross comparison

New results of spin polarization of both photoelectrons and Auger electrons are reported after 5p photoionization of free Ba atoms with circularly polarized light. A substantial polarization transfer from the spin polarized photons to the spin polarized photoelectrons and via the hole state orientation to the spin polarized Auger-electrons is observed. The cross comparison of the results for photoelectrons and Auger-electrons allows a quantitative test of the assumed two step model where both electron-emission processes occur in sequence

Heinzmann, U.

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Spin polarization of Auger- and of photoelectrons from barium atoms exposed to circularly polarized radiation and their cross comparison

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