X-ray fluorescence spectroscopy is one of the most powerful methods for the understanding of the electronic structure of matter. We report here on fluorescence experiments in the 2 to 6 keV photon energy range using tunable synchrotron radiation and the resulting experimental programs on resonant inelastic scattering in atoms and on polarization measurements in resonant molecular excitations

Arp, U.

Deslattes, R. D.

Miyano, K. E.

Southworth, S. H.

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RESONANT INELASTIC X-RAY SCATTERING FROM MOLECULES AND ATOMS U. ARP. R.D. DESLATTES Physics Laboratory. National Institute of Sta&ak and Technology. Gaithersburg, MD 20899 K.E. MIYANO Physics Department, Brooktyn College, Brooklyn, 'VY I1210 S.H. SOUTHWORTH Physics Division. Argonne National Laboratory. Argonne. IL 60139 B.A. KARLM National Synchrotron Light Source. BrooWuIwn National Laboratory, Upton. NY 1 I973 X-ray fluorescence spccboscopy is one of the most powerful methods for the understanding of the electronic sbuchuc of m m r .  We report here on fluorescence cxpaiments in the 2 to 6 keV photon mrgy range using hmable synchrotron radiation and the Rsulting arpaimemal programs on resonant inelastic scattering in atom and on polarization measurements in rrsonant molecular excitations. 1 Introduction O S T I  Before synchrotron radiation was commonly applied, mainly two methods were used to ionize a deep-inner-shell: X-ray tubes and electron beams. In most experiments both were used with excitation energies far above threshold, often leaving the target in an undefined multi-vacancy state, which led to satellites in the emission spectrum. To study and understand these x-ray satellites it is necessary to excite with tunable photons close to threshold. This was achieved by applying many different anode materials in laboratory sources in a study of the KP spectra of argon, and of potassium and chlorine in KC1l2. the exciting radiation was not tunable and caused satellites. Tunable synchrotron radiation opened the door to satellite-free x-ray fluorescence spectra, because it was easily possible to continuously tune the exciting photon beam to an energy below the first double excitation threshold6. Tunability also allows studies of the creation of satellite transitions by scanning the primary photon energy over a double excitation threshold6. In addition, with tunable synchrotron radiation, resonance and threshold phenomena could be studied. With the improved performance of x-ray beamlines'-" it is possible to study the resonant inelastic Raman scattering of x-rays by atoms and molecules'2. Characteristic fluorescence can be explained using a static two-step model, whereas x-ray scattering processes like x-ray resonant Raman scattering have to be explained using one- step modelsi2, which are based on the Kramers-Heisenberg for~nalisrn'~. In many other x-ray fluorescence experiments (see ag. 1 Synchrotron x-rays are highly linearly polarized in the orbit piane of the storage ring, and monochromator crystals at a Bragg-angle close to 45" are also good polarizers". This leads to the use of x-ray spectrometers to investigate molecular structure through polarization analysis of resonant x-ray fluorescence". We will not address experiments on soIids because other authors will do that in more detail in this issue. However, we note that several results on resonant inelastic scattering From ~01 ids~I -~~  have been obtained using the same instrumentati~n'-~.'' described here, and the physical principles are the same. 2 Experiment The instrumentation is described in r e f~ .~ - ' l - ' ~  and the methods in refs.'"'. The geometry of the x-ray spectrometer is shown in fig. 1. The focused, narrow-bandwidth, linearly polarized x-ray beam provided by beamline X24A at the National ~- Synchrotron Light Source passes through the target gas cell. Ionization chambers or thin foil photo current detectors are used to measure the F%rueI.Schcmsrticdrawingofthtxtay~~~rwdfor emission. transmitted x-ray beam, providing absorption spectra and normalization for variations in beam intensity. Scattered and fluorescent x-rays emitted normal to the propagation direction of the incident x-ray beam are dispersed by a curved crystal spectrometer" and recorded by a position-sensitive proportional counter (PSPCY'.The Brag angle of the spectrometer is adjusted to detect a particular spectral component (La, Ka, KP, or K-V), and the acceptance window of the PSPC is typically wide enough to record the entire spectrum simultaneously. Multichannel recording provided by the PSPC is very important for these experiments due to the typically low counting rates. The beamline's double-crystal m~nochrornator~-~ and the curved-crystal emission spectrometer" are designed to operate at large Bragg-angles (=20"-70") and are usually used with semiconductor crystals (Si, Ge, InSb) to work in the energy range 2-6 keV. The polarization sensitivity of these Bragg-reflections can be quite high. For example, Si( 1 1 I)  crystals have been used in both the beamliie monochromator and emission spectrometer for C1 K-edge absorption and Cl K-V and Ka emission studies of Cl-containing molecules. The x-ray energies involved are in the range 2.6 to 2.9 keV, for which the Si( 1 11) Bragg-angles are between 43" and 49", resulting in excellent polarization selectivity. intensities of the incident and thC mcaSUWllCIlt Of the aIIgUlaf distn'btlth Of Plariztd X-ray 2 As described below, this polarization selectivity has allowed observation of strongly polarized and anisotropic x-ray emission from resonantly-excited molecules ls2'. 3 Results The beamlinespectrometer system has been operated successfilly for several years, and we will give a short overview of selected results and refer to cited articles for more detailed information. 3. I Multivacancy Efjects in Atomic Argon Exciting an atom using an x-ray source far above threshold causes additional muitivacancy structures to appear in the fluorescence spectrum. Deslattes et U I ! ~  studied the origins of multivacancy processes by measuring argon KP spectra below the 1s - 4p resonance, above the Is threshold but below the ls3p double ionization threshold, above the double ionization threshold and far above thresholds. Fig. 2 illustrates the results. Spectrum 2a) was obtained at high excitation energy and shows three prominent features, labeled KP,,, KV, and KP". .. -!.VI Fxurc 2. Sample Ar KP emission spectra obtained with incident photon energy at a) 3281.4 eV, b) 3245.9 cV, c) 21 13.1 eV, and d) 31993 eV. 3 At excitation energies between -20 and 4 0  eV above threshold the KP" features disappear Fig. Zb].'Between the Is threshold and +20 eV both satellites are absent, leaving only the KP,., diagram line [Fig. 2c)l. Below threshold, several features dominate the emission specbum due to Compton (A), resonant Raman (B), and Rayleigh 0 scattering [Fig. 2d)]. The argon fluorescence spectrum changes dramatically as the energy of the incident x- ray beam is changed, evolving from a typical Raman scattering structure below threshold, over a normal satellite-free x-ray fluorescence spectrum into a spectrum containing many additional structures created through multivacancy processes. 3.2 Inelastic X-Ray Scattering in Atoms There are relatively few experiments done in the field of resonant x-ray Raman scattering or resonant inelastic scattering. The first reported observation of x-ray resonant Raman scattering was done by Sparks" using x-ray anodes. Eisenberger et aLZ4 used tunable synchrotron radiation and observed linear dispersion and linewidth narrowing for resonant inelastic x-ray scattering near the Cu K edge of copper metal. They also presented a theodcal formulation. Briand et ai.= observed resonant Raman scattering from both bound and continuum intermediate states. Aberg and Tulkki have developed a detailed theoretical Aberg and T u l l d c i ' ~ ~ ~ ~ - ~ - ~ ~ ~ o ~ u t i o n  of inel-dc sca tering from the Ar 1s -. np Rydberg-resonances and the continuum threshold into "normal" fluorescence. C0war1''~ and applied their theory to model observed  result^^^. A Figure 3. Variation of the Xe La,, emission spectrum as the incident x-ray energy is scanned across the b edge region to illustrate resonance inelastic scattering and the onset of fluomccncc. 4 c and coworkers observed linewidth narrowing and lineshape variation in the Ar Ka x-ray spectrum recorded at several energies below, on and above the Is - 4p resonance. Southworth et d.'O reported a small anisotropy effea in the Ar Ka emission recorded on the 1s - 4p resonance. edge. They investigated the evolution of x-ray resonance Raman scattering into x-ray fluorescence while scanning the excitation energy through the L, edge region. Fig. 3 shows the resulting spectrum as a surface plot over the energy of the exciting x-rays and the emission energy. The evolution from a strongly asymmetric profile into the normal fluorescence is clearly shown, in good qualitative agreement with  the^$&'^. Recently MacDonald et reported a resonant scattering experiment at the xenon 3.3 X-ray scattering in molecules Cowanm and So~thworth'~ summarized the work on the polarizations and anisotropies in moiecular x-ray fluorescence. Large polarizations and anisotropies have been observed in x-ray scattering from molecules which were selectively excited at strong subthreshold r e s o n a n c e ~ ~ * ~ ~ J ~ ~ ~ ~ .  Resonant excitation can select a spatially aligned set of molecules from a randomly oriented gas phase target, resulting in strongly polarized and anisotropic x-ray scattering1szo, The classical version of the two-step model shows that large anisotropies and polarizations are expected for x-ray absorptiodemission processes involving molecular states of well defmed ~yrn rne t r i e s~~~~~ .  In this m0de1~~J' the absorption and emission dipole moments are replaced by classical dipole oscillators treated as rigidly attached to the molecular 440 - - 5 8 220 0 2800 zoos 2 1 0  2 1 s  a x - r a y  energy bV) F i r e  4. Parallel polarized CI K-V emission specpd from CF,CI recorded on resonance at emission angles 8 = 0" (top) and 90" @onom). The solid curves arc the results of apcak fitting procedure used to obtain dative intensities for transitions involving the loa, and 7e molecular o r b i s .  framework. In fig. 4 we show the parallel polarized C1 K-V emission spectra from CFJl recorded on resonance at emission angles 8 = 0" and 90" (from rep?. The variation of the x-ray emission spectrum with angle of observation is in good qualitative agreement with the classic two-step modelz0. Alternatively, the effects of alignment by resonant excitation can be observed in the polarization dependence of x-rays emitted at futed (90") emission ang~e18.19.3J,34 5 4 Conclusions We have briefly deqribed several results obtained using tunable synchrotron radiation and a curved-crystal spectrometer for resonance and threshold studies of atoms and molecules. Special emphasis was given to studies of multi-vacancy effects, resonant Raman scattering, and the polarization and anisotropy of emitted x-rays. Significant advances in this field are expected using the improved flux and resolution available at third-generation synchrotron beamlines. For example, the investigation of vibrationallydependent effects should provide further insight into resonant x-ray scattering from molecules. Acknowledgments We would like to thank all the coworkers who contributed to the results described here. This review is dedicated to the memory of Paul L. Cowan, our friend and colleague who made outstanding contributions to the field of x-ray scattering and was the driving force for many of the experiments reported here. UA is indebted to the Alexander von Humboldt-foundation for the fellowship within the Feodor-Lynen program. This work was supported in part by the Department of Energy Office of Basic Sciences under Contract W-3 1 - 1 09-ENG-3 8. References 1. 2. 3. 4. 5.  6. 7. 8. 9. 10. R.D. Deslattes, Phys. Rev. 133, A390 (1964). R.D. Deslattes, Phys. Rev. 133, A399 (1964). R.E. Lavilla, J. Chem. Phys. 37,899 (1972). R.E. Lavilla, J. Chem. Phys. 62,2209 (1975). R.C.C. Perera and R.E. LaVilla, J. Chern. Phys. 81,3375 (1984). R.D. Deslattes, R.E. LaVilla, P.L. Cowan, and A. Henins, Phys. Rev. A27, 923 (1 983). P.L. Cowan, J.B. Hasting, T. Jach, and J.P. Kirkland, Nucl. Instrum. Methods 208, 349 (1983). P.L. Cowan, S. Brennan, RD. Deslattes, A. Henins, T. Jach, and E.G. Kessler, Nucl. Instrum. Methods Phys. Res. A246, 154 (1986). P.L. Cowan, S. Brennan, T. Jach, D.W. Lindle, and B.A. Karlin, Rev. Sci. Instrum. 60, 1603 (1989). S.H. Southwoxth, M.A. MacDonald, T. LeBrun, and RD. Deslattes, Nucl. Instrum Methods A347,499 ( 1994). 6 t 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. S. Brennan, P.L. Cowan, RD. Deslattes, A. Henins, D.W. Lindle, and B.A. Karlin, Rev. Sci. Instrum. 60,2243 (1989). T. Aberg, Phys. Scripta 21,495 (1980); T. A-berg and B- Ctasemann, in R e s o m t  Anomalous X-hy Scattering: lko ty  andApplications, edited by G. Materlik, C.J. Sparks, and K. Fischer (North-Holland, Amsterdam, 1994). p. 43 I ; T. Aberg, Phys. Scripta T41,71 (1992). See e.g.: W. Heitler, n e  Quantum Theoy of Radiarian (Clarendon, Oxford, 1954). AS. Schlachter, in New Directions in Research with Third-Generation Soft X-Ray Synchrotron Radiation Sources, edited by A.S. Schlachter and F.J. Wuilleumier (Kluwer, Dordrecht, 1994), p. 1; D.L. Ederer, K.E. Mijano, W.L. O'Brien, T.A. Callcot, Q.-Y. Dong, J.J. Jia, D.R. Mueller, J.-E. Rubensson. R.C.C. Perera, and R. Shuker, in New Directions in Research with ThudGeneration So) X-Ray Synchrotron Radiation Sources, edited by A.S. Schlachter and F.J. Wuilleumier (Kluwer, Dordrecht, 1994), p. 28 1. K.E. Miyano, Y. Ma, S.H. Southworth, P.L. Cowan, and BA. Karlin (unpubIished). Y. Ma, K.E. Miyano, P.L. Cowan, Y. Aglitzkiy, and P.L. Cowan, Phys. Rev. Lett. 74,478 (1995). P.L. Cowan, Phys. Scripta T3 1, 1 12 ( 1990). B.P. Duval, J. Barth, R.D. Deslattes, A. Henins, and G.G. Luther, Nucl. Instrum. Methods. 222,274 (1984). D.W. Lindle, P.L. Cowan, R.E. LaVilla, T. Jach, RD. k la t t e s ,  B. Karlin, J.A. Sheely, T.J. Gil, and P.W. Langhoff, Phys. Rev. Lett 60, 1010 (1988). R. Mayer. D.W. Lindle, S.H. Southworth, and P.L. Couq Phys. Rev. A43,235 (1991). S.H. Southworth, D.W. Lindle, R Mayer, and P.L. Co~an, Phys. Rev. Lett. 67, 1098 (1991). R.D. Deslattes, Aust. J. Phys. 39,845 (1986). C.J. Sparks, Phys. Rev. Lett. 33,262 (1974). P. Eisenberger, P.M. Platzmann, and H. Winick, Phys. Rev. Lett. 36,623 (1976). J.P. Briand, D. Girard, V.O. Kostroun, P. Chvalier, K. Xoherer, and J.P. Moss& Phys. Rev. Lett. 46,1625 (1981). T. Aberg and J. Tukki, in Atomic Inner-Shell Physics, edited by €3. Crasemann (Plenum, New York, 1985), p. 419. J. Tukki, Phys. Rev. A 27,3375 (1 983). J. Tukki and T. Aberg, J. Phys. B 15, L435 (1982). P.L. Cowan, in Resonant Anomalous X-Ray Scanering: Theory and Applications, edited by G. Materlik, C.J. Sparks, and K. Fjscher (Sortl~-Holland, Amsterdam, 1994), p. 449. S.H. Southworth, D.W. Lindle, R Mayer, and P.L. Cowan. Nucl. Instrum. Methods B56/57,304 (1991). DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor my of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of my information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- en= herein to any specific commercial product, process, or service by trade name, trademark, ream- manufacturer, or otherwise does not necessarily constitute or imply its endorsement, mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. . .  L i:, . ,- . 31. M.A. MacDonald, S.H. Southworth, J.C. Levin, A. Henins. RD. Deslattes, T. LeBrun, Y, Amma, P.L. Cowan, and B.A. Karlin, Phys. Rev. A51,3598 (1994). 32. S.H. Southworth, Nucii Instrum. Methods B87,247 (1994). 33. - D.W. Lmdle, P.L. Cowan, T. Jach, RE. Lavilla, R.D. Deslattes. and R.C.C. Perera, and B. Karlin, J. Phys. C 9,761 (1987). 34. D.W. Lindle, P.L. Cowan, T. Jach, RE. Lavilla, R.D. Deslattes, and R.C.C. Perera, Phys. Rev. A 43,2353 (1991). 35. RC.C. Perera, P.L. Cowan, D.W. Lindle, R.E. Lavilla, T. Jach, and R.D. Deslattes, Phys. Rev. A43,3609 (1991). 36. P.P. Feofilov, 2’2e Physical Basis of Polarized Emission (Consultants Bureau, New Yo&, 1961). 37. R.N. Zare, AnguIar Momentum (Wiley, New York, 1988). 8 

Resonant inelastic x-ray scattering from molecules and atoms

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Resonant inelastic x-ray scattering from molecules and atoms

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