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Physica Scripta. Vol. T115, 977–979, 2005

The Dispersive X-ray Absorption Spectroscopy beamline at LNLS H´elio C. N. Tolentinoa, Julio C. Cezara,b, Noˆemia Watanabea , C´ınthia Piamontezea,b, Narcizo M. Souza-Netoa,b, Edilson Tamuraa , Aline Y. Ramosa,c and Regis Neueschwandera a LNLS

Laborat´orio Nacional de Luz S´ıncrotron, CP6192, 13084-971 Campinas, SP, Brazil Universidade Estadual de Campinas CP6165, 13083-970 Campinas, SP, Brazil c LMCP Laboratoire de Min´ eralogie – Cristallographie, UMR 7590 CNRS, Paris, France b IFGW,

Received June 26, 2003; accepted November 4, 2003

pacs numbers: 07.85.Qe, 61.10.Ht, 61.10.Eq

Abstract The present paper describes the concept of the Dispersive X-ray Absorption Spectroscopy beamline at the Brazilian Synchrotron Light Laboratory. The present performance and some of the very first experiments are reported. Emphasis is put on experiments related to electrochemical reactions and resonant reflectivity on magnetic thin films and multilayers.

1. Introduction We describe here the concept and present performance of the Dispersive X-ray Absorption Spectroscopy (DXAS) beamline at LNLS (Brazilian Synchrotron Light Laboratory). The specificity of this beamline is that it allows collecting absorption data simultaneously over an extended range of photon energies without any mechanical motion [1, 2]. This characteristic is specially suited for measurements requiring good accuracy and high stability, for instance in XANES and XMCD experiments [3, 4]. We present in this paper some very first commissioning results and preliminary experiments related to electrochemical insertion of metals into polymers and resonant reflectivity on magnetic thin films and multilayers. 2. The conceptual design and optics The concept of a dispersive XAS beamline is based on a curved crystal monochromator that selects a bandwidth of a few hundred eV from the white synchrotron light source and focuses it at the sample position. The incident angle varies continuously along the crystal, providing a continuous change of the energy of the Bragg-reflected photons. The bending mechanism of the curved crystal monochromator has been developed at LNLS and its technical details will be published elsewhere. All rotations (Bragg, tilt and yaw angles) are accomplished by high precision Huber goniometers. The radius of curvature is imposed by two independent momenta on crystal extremities without changing the position of the central part. There is also a mechanism for twist correction and for cooling the crystal. Everything works inside a high vacuum chamber. The two independent momenta allow reducing significantly the spherical aberrations. The whole bandwidth is focused in the horizontal plane down to 150
m. The vertical focusing is provided by an 800 mm-long Rh coated mirror, working at a grazing angle of 4 mrad. Its bending mechanism allows vertical beam collimation or focusing to about 500
m at the sample position. This leads to the possibility of working with very small samples, like small single crystals or samples inside a high pressure cell. ∗ e-mail:

[email protected]

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Fig. 1. Calculated photon flux at the DXAS/LNLS beamline. The continuous line represents the emitted flux from the bending magnet; the dotted line the flux attenuated by four 125
m Be-windows, and the dashed-dot line the flux attenuated by the Be-windows and the 0.6 m-beam path in the air. Experimental points were measured with a photodiode at the focus point.

A curved Si(111) crystal monochromator selects radiation from a bending magnet source in the X-ray range from 4 keV up to 14 keV. The calculated flux throughput from LNLS storage ring working at 1.37 GeV and 100 mA is shown in figure 1. The measurements, after a total of 500
m (4 × 125
m) Bewindows and 0.6 m beam path in the air, have been performed with a photodiode at the focus point and are in good agreement with calculations. The beam path in the air can be reduced to a very small value and, for the range of interest, one can recover practically all emitted photons from the source, as shown by the curve that takes into account just the Be-windows. Taking advantage of this flux, time resolved experiments can be performed with a typical resolution of 100 ms. The acquisition of a spectrum that, in sequential mode, normally takes 20 to 40 minutes can be accomplished in less than one second in dispersive mode. The detection handles the high flux conditions with a modified CCD camera. The cryogenically cooled CCD camera system has a front-illuminated, scientific grade 1, MPP CCD of 1340 × 1300 pixels, each pixel has 20 × 20
m2 for a total image area of 26.8 × 26.0 mm2 . A GdOS phosphor screen, optimized for 8 keV X-rays, receives the incoming beam at an angle of 20◦ and converts the X-ray to visible light. A set of lenses guides the light to the CCD detector with a demagnification factor of 1.75, so that the real pixel at the phosphor screen is 35×35
m2 . In fact, the resolution measured in the dispersion direction has been about 2.5 to 3 times that pixel, i.e. 100
m, limited by the phosphor screen. To improve this resolution, which turns out to be the limiting factor, we have to work out the phosphor screen with a thinner deposited film. Physica Scripta T115

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H´elio C. N. Tolentino et al.

Fig. 2. Cu K edge spectrum at the DXAS beamline.

The energy-direction correlation coming out from the monochromator is transformed into an energy-position correlation along one direction of the two-dimensional CCD detector. The pixels of the CCD detector are integrated along the other direction to produce the XAS spectrum. There is no mechanical motion during the acquisition of a full spectrum. Spectra with a good signal/noise ratio can be collected with a typical time resolution of 50 ms. To improve the signal/noise ratio, 20 frames compose a full spectrum with 1 s of total acquisition time (Fig. 2). 3. Preliminary results 3.1. Electrochemical cell for in situ study of metal insertion into polymers One of the most appealing applications of the dispersive XAS set-up is to follow in detail the many steps in a chemical reaction. Special attention is dedicated to the development of electrochemical cells to study the metal insertion into polymers and to the growth of small nanometric particles [5]. The aim is to understand the very first steps in the nucleation mechanism and to study the different phases of the reaction [6, 7]. In the particular case of copper insertion into polypyrrole, very different results have been presented in the literature, concerning the first steps of metal-polymer interaction. Some authors propose that the copper insertion is conducted by means of a Cu+1-PPy complex [7], while others suggest that this step is described by the formation of the [- [(C4 H2 N)3 CH3 (CH2 )11 OSO3 -]yCu + 2]n (y = 4) complex [6]. The chemical interaction between the metal and the polymer as well as the metal crystalline structure and the chemical bond at the surface are crucial issues related to small particles of magnetic and catalyst materials. First test experiments have been performed at the Cu K edge using a transmission cell [8]. The inclusion of Cu atoms into polypyrrole films has been followed (Fig. 3). The results are promising: the A feature in the spectrum, which is related to an increase of the local distortion of Cu site, is clearly seen to evolve in the very first steps of the reaction. 3.2. Resonant scattering on magnetic thin films Resonant scattering of polarized X-rays from magnetic materials represents a powerful tool to investigate interesting magnetic phenomena, because magnetic effects are strongly enhanced [9, 10] when the photon energy is swept across an absorption edge. Resonant scattering from films and multilayers have been largely used in the soft X-ray range [11, 12]. However, it is worth Physica Scripta T115

Fig. 3. In situ reaction at the Cu K edge spectrum followed at the DXAS beamline (bottom) and initial and final state measured using a step-by-step XAS scheme.

noting that, in the hard X-ray domain, very few experiments were attempted in the past [13], and none is implemented at present, in spite of the great advantage arising from the possibility of working under a variety of extreme conditions. A prototype experiment was performed in a multilayer structure formed by the alternate deposition of 1 nm of Co and 0.2 nm of Gd, repeated 40 times. Both the Co 1s (7709 eV) and the Gd 2p (7243 eV) resonances are accessible. Co is present in large amounts (about 40 nm) and carries a high magnetic moment compared to the reduced amount of Gd (about 30 atomic layers) combined with its low magnetic moment, at room temperature. On the other hand, the dichroism effect on the Co K-edge is much smaller than for Gd L3 edge. Around the Gd L3 edge, we measured the scattered intensity as a function of angle and photon energy. The critical angle for total external reflection as well as the Kiessig fringes related to the multilayer structure could be clearly identified. Around the Co K edge, we selected the radiation emitted about 0.3 mrad above the orbit and we disposed of an elliptically polarized beam with about 50% circular polarization rate. By applying a magnetic field parallel or antiparallel to the beam propagation axis we obtained the asymmetry ratio (or dichroism spectrum). The superior stability of the dispersive set up allowed us to collect dichroism spectra at a given angle and over a given energy range with a noise of the order of 0.03%.

Fig. 4. The asymmetry ratio at the Co K edge for the Gd/Co multilayer system. The dichroic signal was obtained by collecting 128 spectra of about 30 s each. The applied magnetic field was 1 kOe at room temperature.  C Physica Scripta 2005

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The Dispersive X-ray Absorption Spectroscopy beamline at LNLS The measurement obtained at the Co K edge shows that a magnetic signal as low as 0.05% could be easily identified after one hour of acquisition time (Fig. 4). Acknowledgements We wish to thank FAPESP for the grants of J. C. Cezar and C. Piamonteze and CAPES for the grant of N. M. Souza-Neto. This work is supported by LNLS.

References 1. Dartyge, E. et al., Nucl. Instrum. Meth. A246, 452 (1986). 2. Tolentino, H., Dartyge, E., Fontaine, A. and Tourillon, G., J. Appl. Cryst 21, 15 (1988).

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3. Fontaine, A. et al., Rev. Sci. Instrum. 63, 960 (1992). 4. Baudelet, F. et al., J. Synchrotron Rad. 5, 992 (1998). 5. Alves, M. C. M., Watanabe, N., Ramos, A. Y. and Tolentino, H. C. N., J. Synchrotron Rad. 8, 517 (2001). 6. Watanabe, N., Moraes, J. and Alves, M. C. M., J. Phys. Chem. B 106, 11102 (2002). 7. Liu, Y. C. and Hwang, B. J., Thin Solid Films 339, 233 (1999). 8. Kisner, A., Internal Report at LNLS, (2003). 9. Gibbs, D. et al., Phys. Rev. Lett. 61, 1241 (1988); Hannon, J. P., et al., Phys. Rev. Lett. 61, 1245 (1988). 10. Isaacs, E. D. et al., Phys. Rev. Lett. 62, 1671 (1989). 11. Kao, C. C. et al., Phys. Rev. Lett. 65, 373 (1990). 12. Sacchi, M. et al., Phys. Rev. Lett. 81, 1521 (1998). 13. Dartyge, E., Fontaine, A., Tourillon, G., Cortes, R. and Jucha, A., Phys. Lett. 113A, 384 (1986).

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