NEAR THRESHOLD MEASUREMENTS OF THE K SHELL SATELLITES OF CO AND N2

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NEAR THRESHOLD MEASUREMENTS OF THE K SHELL SATELLITES OF CO AND N2

J. Feldhaus, A. Reimer, J. Schirmer, A. Bradshaw, U. Becker, H. Kerkhoff, B.

Langer, D. Szostak, R. Wehlitz, W. Braun

To cite this version:

J. Feldhaus, A. Reimer, J. Schirmer, A. Bradshaw, U. Becker, et al.. NEAR THRESHOLD MEA-

SUREMENTS OF THE K SHELL SATELLITES OF CO AND N2. Journal de Physique Colloques,

1987, 48 (C9), pp.C2-773-C2-776. �10.1051/jphyscol:19879135�. �jpa-00227245�

NEAR THRESHOLD MEASUREMENTS OF THE K SHELL SATELLITES OF CO AND N,

J. FELDHAUS, A. REIMER, J. SCHIRMER, A.M. BRADSHAW, U. BECKER* , ( )

,

^H. G. KERKHOFF*

,

B. LANGER*

,

D. SZOSTAK*

,

R. WEHLITZ* a n d W. BRAUN* *

Fritz-Haber-Institut der Max-Planck-Gesellschaft, 0-1000 Berlin 33, F.R.G.

'~nstitut fur Strahlungs- und Kernhysik, Technische Universitat Berlin, 0-1000 Berlin 12, F.R.G.

Berliner Elektronenenspeicherring-Gesellschaft fur Synchrotronstrahlung mbH (BESSY), 0-1000 Berlin 33, F.R.G.

Abstract: Core level photoemission spectra o f CO and N;! using snyhrotron radiation excitation have been measured in order t o study t h e 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 o f the second satellite towards threshold. In contrast t o this characteristic behavior there i s nothing comparable in the 0 I s satellite spectrum. The results are discussed o n the basis o f the equivalent core model, potential theory and charge transfer effects.

The core level photoemission spectra o f 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 o f the photon energy dependence o f satellite intensities. Particular interest has recently been focussed on the variation o f 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 I 1 - 31, where satellite transition strengths are directly related t o electron correlation effects associated w i t h t h e photoionization process. I n 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 t o the other associated with a simultaneous photoionization and excitation process. The simplest approach t o study such phenomena is t h e comparison between isoelectronic molecules such as CO and N2 which are prominent examples for t h e study o f K- shell excitations. The K-shell excitation spectra o f N2 and CO at the carbon edge show very similar excitation structures which may simply be explained o n the basis o f t h e equivalent core model, also known as the Z + 2

+

1 model 141. One would expect t o see this similarity n o t only i n the K-shell excitation spectra but also and even more expl~citly In t h e K-shell photoelectron spectra o f 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 o f N2 and the Cls satellite spectrum o f CO than between the t w o satellite spectra of CO originating from the I s excitation o f the carbon and oxy en atoms, respectively.

In a recent experiment we have studied the near txreshold cross sections o f the C 1s satellites o f CO 131. Surprisingly, the photon energy dependence o f the t w o C 1s r-s*

satellites is quite different. While the second satellite decreases continuously in intensity towards threshold in accordance w i t h the simple picture o f t h e 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 o f these t w o satellites is explained most likely on the basis o f differing exchange potentials i n the one-particle scattering model for the t w o differently coupled satellite states 131: The question arises as t o whether such complex details o f t h e satellite behavior can be explained w i t h i n t h e 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 t h e present paper we report the first measurements of the near threshold behavior o f the N l s 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 i m e - o f - f l i g l t photoelectron analysis. The system has already been described in some detail i n earlier publications/l, 31.

Fig. 1 shows a comparison o f the three K-shell satellite spectra o f N2 and CO. All spectra were taken at photon energies high enough t o be approximately i n the sudden limit. The first t w o spectra differ qualitatively from the third. Besides unresolved structures a t still higher binding energies both the N I s and the C I s spectra show t w o 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 i p l e t coupled and in 52 sin let coupled. In contrast, the 0 1s spectrum o f 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 w i t h a resolution o f about 2 eV.

Fig. 2 shows the energy dependence of the relative intensity o f the above mentioned satellites or groups o f satellites resolved i n our measurements. We see that f o r N 1s and C 1s n o t only the energetic positions o f the first satellites are very similar b u t also the variation o f 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 t o the adiabatic limit. This opposite behavior leads t o a characteristic cross over of satellite intensities between the first t w o satellites i n both the N I s and the C I s spectra. The 0 I s satellite or satellite group, in contrast, shows already 80 eV above i t s threshold a relative intensity .of 17 % o f 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.

Nls N ~ + ( N 1 s-')

Satellites

0.51 ^,

/k

0.0 0.0 20 40

x 1 0 0.5

- ^I

Satellites

-

0.0 20 40

Excitation energy (eV)

Fig. 1: N 1s (a) and C I s (b) and 0 1s (c) satellite spectra o f N2 and CO. The N 1s spectrum was taken at a photon energy of 470 eV, the C 1s and 0 1s spectra were taken at 390 eV and 720 ev, respect- ively, well above the corresponding I s thresholds o f 409.9 eV, 296.2 eV and 541.6 eV.

~ o I " . ' ~ . . " I ' ~

550 600 650

Photon energy (eV)

Fig. 2: Satellite peak intensities of t h e first t w o satellites (51 and S2) o f the N 1s (a) and C I s (b) satellite spectra and t h e first satellite (group o f satellites) of t h e 0 I s (c) spectrum r e l a t i v e t o t h e i r corresponding 1 s main line. The thresh- olds o f t h e individual satellites are in- dicated.

How can we explain the observed similarities and dissimilarities? One simple way t o look at the similarities is the equivalent core model. In this picture, an atom w i t h a core hole is considered equivalent t o the atom with one higher atomic number so that CO+(C Is-1) is equivalent t o NO as is N2+(N 1s-1) while CO+(O Is-1) is equivalent t o CF. These molecules.

NO and CF, are very different w ~ t h respect t o their electronegativity resulting i n localization differences o f the excited n* orbital in the presence o f a K-shell hole 191. This effect is qualitatively mirrored in the corresponding K-shell satellite spectra producing similarities and dissimilarities in the satellite structure and partial cross section behavior.

C9-776 JOURNAL

DE

PHYSIQUE

The particular structure of the 0 I s satellite spectrum can be explained in more detail by looking at the charge transfer processes associated with simultaneous photoionization and excitation 181. The occupied n orbital of

CO

is more localized on the 0 site whereas the unoccupied r* orbital is more evenly distributed. A n-n* excitation is therefore accomp- anied by a charge transfer from the oxygen to the carbon atom. This process leads t o additional screening of a C 1s hole, but t o unfavorable anti-screening of an 0 1s hole which quenches the n-n* transition. In contrast, the outer occupied a orbitals are concentrated on the C site giving rise t o a charge transfer from carbon t o oxy en in the case of a u-a* shake- up transition. The corresponding 0-0" satellites are thereibre energetically favored and interact with the unfavored 7-n* transitions in such a way that the latter disappear. This provides a reasonable qualitative description of the observed 0 lssatellite spectrum.

This work was supported by the Bundesminister fijr Forschung und Technologie (05 314 EX B2 and 05 390 FX B2).

References

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Langer, D. Szostak, R. Wehlitz and W. Braun, Phys. Rev. Lett. 57 (1986) 1707.

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