Near Threshold Measurements of the K Shell Satellites of Co and N2

نویسندگان

  • J. Feldhaus
  • A. Reimer
  • J. Schirmer
  • A. Bradshaw
  • U. Becker
  • H. Kerkhoff
  • B. Langer
  • D. Szostak
  • R. Wehlitz
  • W. Braun
چکیده

Core level photoemission spectra of CO and N;! using snyhrotron radiation excitation have been measured in order t o study the neat threshold photon energy range of the K-shell satellites. The C I s and N 1s r-r* satellites show very similar behavior characterized by increasing intensity of the first satellite and decreasing intensity of the second satellite towards threshold. In contrast t o this characteristic behavior there is nothing comparable in the 0 I s satellite spectrum. The results are discussed on the basis of the equivalent core model, potential theory and charge transfer effects. The core level photoemission spectra of atoms and molecules exhibit complex satellite structures which originate from different ionic final states. These structures are attracting rapidly growing interest, expecially since monochromatized synchrotron radiation has made possible the systematic exploration of the photon energy dependence o f satellite intensities. Particular interest has recently been focussed on the variation of intensities and ("present address : Physikalisches Institut, Universitlt WUrzburg, Rm Hubland. D-8700 WUrzburg, F.R.G. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879135 C9-774 JOURNAL DE PHYSIQUE angular distributions near threshold I1 31, where satellite transition strengths are directly related t o electron correlation effects associated with the photoionization process. In addition t o correlation effects, which are also known from atoms, molecular photoelectron spectra also display information on the charge distribution and the transfer o f charge from one atomic site to the other associated with a simultaneous photoionization and excitation process. The simplest approach to study such phenomena is the comparison between isoelectronic molecules such as CO and N2 which are prominent examples for the study of Kshell excitations. The K-shell excitation spectra of N2 and CO at the carbon edge show very similar excitation structures which may simply be explained on the basis o f the equivalent core model, also known as the Z + 2 + 1 model 141. One would expect t o see this similarity not only in the K-shell excitation spectra but also and even more expl~citly In the K-shell photoelectron spectra of isoelectronic molecules because these spectra should mirror the excitational structure of these molecules through their satellite intensities. Following the equivalent core model there should be more similarity between the N 1s satellite spectrum of N2 and the Cls satellite spectrum of CO than between the two satellite spectra of CO originating from the Is excitation of the carbon and oxy en atoms, respectively. In a recent experiment we have studied the near txreshold cross sections of the C 1s satellites o f CO 131. Surprisingly, the photon energy dependence of the t w o C 1s r-s* satellites is quite different. While the second satellite decreases continuously in intensity towards threshold in accordance wi th the simple picture of the sudden t o adiabatic transition 151, the first satellite exhibits a strong enhancement towards threshold due t o a shape resonance 161 in this satellite channel. The different behavior of these two satellites is explained most likely on the basis of differing exchange potentials in the one-particle scattering model for the two differently coupled satellite states 131: The question arises as t o whether such complex details o f the satellite behavior can be explained within the equivalent core model. If so, i t would then be possible t o draw further conclusions regarding the satellite structure both here and in other more complex molecules. In the present paper we report the first measurements of the near threshold behavior o f the Nls satellites in N2 and the 0 Issatellites in CO. The experiments were performed at the electrori storage ring BESSY using a h i h energy toroidal grating monochromator (HE-TGM-1) in conjunction with t ime-off l ig l t photoelectron analysis. The system has already been described in some detail in earlier publications/l, 31. Fig. 1 shows a comparison of the three K-shell satellite spectra of N2 and CO. All spectra were taken at photon energies high enough to be approximately in the sudden limit. The first two spectra differ qualitatively from the third. Besides unresolved structures at still higher binding energies both the N I s and the C I s spectra show two well separated satellite lines around 9 eV (S1) and around 15 eV (S2) above the I s main line 171. Both 51 and 52 are due t o r-s* excitations, whereby in S1 the n-l-s*l electron-hole pair is t r ip let coupled and in 52 sin let coupled. In contrast, the 0 1s spectrum of CO has a completely different structure wit% a dominant satellite around 17 eV above the main line. A new high resolution measurement I81 with Al K,-radiation reveals that this satellite consists o f several sharp, closely spaced lines which are not resolved in our measurement at the HE-TGM wi th a resolution of about 2 eV. Fig. 2 shows the energy dependence of the relative intensity o f the above mentioned satellites or groups of satellites resolved in our measurements. We see that for N 1s and C 1s not only the energetic positions of the first satellites are very similar but also the variation of their cross sections with photon energy near threshold. Whereas the triplet coupled satellites exhibit a broad shape resonance almost at threshold, the singlet coupled satellites decrease continuously in intensity towards threshold following the predictions of the shake model for the transition from the sudden to the adiabatic limit. This opposite behavior leads t o a characteristic cross over of satellite intensities between the first two satellites in both the N I s and the C Is spectra. The 0 Is satellite or satellite group, in contrast, shows already 80 eV above i t s threshold a relative intensity .of 17 % of the main line staying nearly constant at higher energies. Preliminary results of near threshold data suggest that the satellite intensity falls t o lower values towards threshold. . I ' . . I . . ' I . Nls N ~ + ( N 1 s-') Satellites 0.51 , /k

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تاریخ انتشار 2016