K. Klementiev - XAS experiment Friday, September 11, 2009 Introduction to X-ray Absorption Spectroscopy: Experiment K. Klementiev, Alba synchrotron - CELLS Some important notes about optics Detection of x-rays Samples 13-15.09.2012 K. Klementiev - XAS experiment

K. Klementiev - XAS experiment 1. Optics Friday, September 11, 2009 What you need to know about x-ray optics 13-15.09.2012 K. Klementiev - XAS experiment

K. Klementiev - XAS experiment Main optical elements Friday, September 11, 2009 Filters (attenuators) are used as high-pass filters in order to remove low-energy photons, if not used, for lowering heat load onto the downstream optics Mirrors can be used as low-pass filters to filter out high energy photons, if these are not used The filtering (reflectivity) depends on the mirror material and the grazing angle The horizontal focusing of a toroid mirror is ideal only for a single angle. We have two working angles and therefore two toroids 13-15.09.2012 K. Klementiev - XAS experiment

Harmonics in monochromator Friday, September 11, 2009 Not only the fundamental energy is passed through a monochromator but also high harmonics. 13-15.09.2012 K. Klementiev - XAS experiment

K. Klementiev - XAS experiment Mirror reflectivity Friday, September 11, 2009 With appropriate mirrors (coatings) at appropriate incidence angles the harmonics can be suppressed due to reflectivity properties. 13-15.09.2012 K. Klementiev - XAS experiment

Importance of harmonics Friday, September 11, 2009 Consider a transmission experiment with two ionization chambers. Their signals i0 and i1 give the absorption coefficient: µd = ln[i0/i1]. i0 and i1 are strongly correlated. You can check this if you take i0 and i1 from different repetitions of the same scan; the resulted µd would be much noisier. Therefore it is important to always keep the two signals in the ratio. Let the beam be contaminated by some harmonics. The experimental absorption is then µd = ln[(i0+h0)/(i1+h1)] and the fluctuations in i0 and i1 do not cancel and thus the spectrum is noisier. In summary, in the presence of harmonics the absorption coefficient µd: 1) is distorted (typically suppressed) and 2) is noisier. Therefore you should select the right mirrors and/or detune the monochromator. 13-15.09.2012 K. Klementiev - XAS experiment

K. Klementiev - XAS experiment 2. Detection Friday, September 11, 2009 Your samples determine which detection to choose 13-15.09.2012 K. Klementiev - XAS experiment

K. Klementiev - XAS experiment Relaxation channels Friday, September 11, 2009 absorption 1. Excitation EF 2s 2p1/2 2p3/2 K absorption L3 absorption 1s Auger (KLL) fluorescence K1 K2 K 2. Relaxation Fluorescence Auger yield 2p3/2 is 4-fold, 2p1/2 is 2-fold.  K1 is as twice more intense as K2. K is more intense than K (~7-8 times for 3d metals). The core hole filling is a cascade process. The fluorescence energies are tabulated  used in x-ray fluorescence analysis. The core hole width E = ħ/. (E ~ 1eV for 1s hole in 3d elements   ~ 10-16 s) 13-15.09.2012 K. Klementiev - XAS experiment

One-electron approximation Friday, September 11, 2009 We see that many electrons are involved. Is the one-electron approximation valid? Both decay channels (fluorescence and Auger yield) keep no memory of the excitation photon  Photoionization from the chosen atomic level and excitation of the remaining system can be considered as independent processes  The measured spectrum as a probability density of two independent random processes is a convolution of a one-electron spectrum with the excitation (core-hole) spectrum  In the 1st approximation the measured spectrum is just a broadened one-electron spectrum [one-particle problems can be calculated]. 13-15.09.2012 K. Klementiev - XAS experiment

K. Klementiev - XAS experiment Ways of detection Friday, September 11, 2009 Direct intensity measurements in transmission geometry ionization chambers photodiodes with scattering foils Fluorescence yield semiconductor detectors, e.g. Ge and Si(Li) crystal analyzers Lytle detector Total electron yield current from biased collector electrode channeltron 13-15.09.2012 K. Klementiev - XAS experiment

K. Klementiev - XAS experiment Ionization chamber Friday, September 11, 2009 The principle of ionization chambers and semiconductor detectors (as reverse biased diodes) is the same: creation of electron-ion (hole) pairs in the inter-electrode medium and registering the current: bias electrode + gas in P absorption (photoionization) x-rays 2 windows (in and out) housing collector electrode guard electrode nA iI(1-e-µd) 13-15.09.2012 K. Klementiev - XAS experiment

Fluorescence detectors Friday, September 11, 2009 A typical energy resolved fluorescence spectrum: Two fluorescence detectors at Alba/CLÆSS beamline: 13-15.09.2012 K. Klementiev - XAS experiment

Fluorescence detection in high resolution Friday, September 11, 2009 Core Level Emission Analyzer and Reflectometer (CLEAR) Compatible with common in-situ cells, cryostats and a magnet, with no side window required Static acquisition, no θ–2θ scans 3 Johansson-like diced Si crystals with in-situ exchange Sagittal crystal bending, 1D detector Serves for RIXS, reflXAFS and polarimetry Energy dispersive images for sample positioned inside Rowland-circle Si (444) @ 45º (i.e. worst case!), 1.4v×2.1h mm2 facets, 100v×200h µm2 beam 13-15.09.2012 K. Klementiev - XAS experiment

X-ray emission analyzer. Application examples Friday, September 11, 2009 K and K1,3 lines: FWHM K and peak position of K1,3 give effective electron spin in the atomic d orbitals. K1,3 lines: site selective EXAFS scans K satellite lines: sensitivity to ligands (also with angular dependence for oriented samples) RIXS: 1) better resolved K pre-edge peaks; 2) soft edges with hard x-rays  Glatzel & Bergmann, Coordination Chemistry Reviews 249 (2005) 65–95 P. Glatzel et al. J. Am. Chem. Soc. 124 (2002) 9668. 13-15.09.2012 K. Klementiev - XAS experiment

K. Klementiev - XAS experiment Total electron yield Friday, September 11, 2009 A sample holder for electron (Auger) yield measurements with He gas as amplifying media Bias voltage ~100 V Current ~ 100 pA – 10 nA x-rays 13-15.09.2012 K. Klementiev - XAS experiment

Summary on detection. Applicability Friday, September 11, 2009 Which detection mode to use: concentrated samples, transparent to x-rays – transmission dilute samples – fluorescence not transparent samples (low-E or thick) – total electron yield or fluorescence (with self-absorption correction) Probing depth: transmission: the full sample thickness (bulk sensitivity) fluorescence: ~(absorption length) * sin(φ), 0.1 – 10 µm total electron yield: ~electron mean free path, 10 – 100 Å 13-15.09.2012 K. Klementiev - XAS experiment

K. Klementiev - XAS experiment 3. Samples Friday, September 11, 2009 How to prepare your samples 13-15.09.2012 K. Klementiev - XAS experiment

K. Klementiev - XAS experiment Suitable samples Friday, September 11, 2009 There are good reviews on sample preparation, for example: Matt Newville, Anatomy of an XAFS Measurement Rob Scarrow, Sample Preparation for EXAFS Spectroscopy google for them General requirements uniform on a scale of the absorption length of the material (typ. ~ 10 µm) prepared without pinholes Shape, aggregative state Solids: powders, foils etc.; single crystals and thin foils can utilize polarization properties of SR. Liquids Gases Concentrations for transmission: typ. >1 wt% for fluorescence: typ. >100 ppm and 1mM 13-15.09.2012 K. Klementiev - XAS experiment

K. Klementiev - XAS experiment Suitable amount Friday, September 11, 2009 Program XAFSmass (see its web-page for detailed description): powder: foil: gas: A typical value for total absorption is about 2. It can be as twice as smaller or bigger without significant change in spectrum quality. More important is the sample uniformity (particle size). 13-15.09.2012 K. Klementiev - XAS experiment

K. Klementiev - XAS experiment Pellets Friday, September 11, 2009 Dies for 5 mm pellets Dies for 13 mm pellets 5 mm pellets 13 mm pellets weights 220 mg, less consumption but more difficult to handle weight 10100 mg easier to remove from dies more samples in the holder 13-15.09.2012 K. Klementiev - XAS experiment

K. Klementiev - XAS experiment Mixing and pressing Friday, September 11, 2009 Dilute your powder with a supporting agent: BN (boron nitride), polyethylene, cellulose, sugar etc. Probably, the mostly used one is BN. To my experience, it gives fragile pellets strongly adhesive to dies. With PE and cellulose the pellets are more durable and easier to press. We will provide cellulose. Do not put more than 1 ton! By pressing stronger you destroy the pellets rather than make them firmer. For  5 mm dies 0.5 ton is enough. 13-15.09.2012 K. Klementiev - XAS experiment

XAS of Metallobiomolecules Friday, September 11, 2009 XAS can provide unique information about the kinds of ligands holding a particular metal in a metallobiomolecule. Symmetry information provided by XANES can help determine qualitatively the molecular geometry. For example, there is often a significant distinction between tetrahedral 4-coordinate and square planar 4-coordinate. XAS is particularly good at elucidating differences between one sample and another: e.g. active site before and after addition of substrate, or competitive inhibitor, or reductant/oxidant etc. Sample Limitations Amorphous frozen solutions with glassing agent (e.g., 20% glycerol); cryogenic T to avoid radiation damage. Concentration ~1 mM or even ~0.2 mM (for 3d metals) Volume ~0.05-0.2 ml. Homogenous metal site structure! XAS is not able to distinguish multiple site structures within a given sample: the resulting XAS-derived structure is an ”average” one. 13-15.09.2012 K. Klementiev - XAS experiment

K. Klementiev - XAS experiment Radiation damage Friday, September 11, 2009 We have ~1013 photons/s at the sample, focused into ~300h×200v µm2. In hard x-ray XAFS the radiation damage is mostly seen (if seen) in the form of photo-reduction. I had it for oxidized Au samples. In solutions the x-ray induced reduction is more common due to radiolysis: from S. Jayanetti et al, J. Chem. Phys 115 (2001) 954 from J. G. Mesu et al, J. Phys. Chem. B 109 (2005) 4042 13-15.09.2012 K. Klementiev - XAS experiment

K. Klementiev - XAS experiment Samples. Conclusions Friday, September 11, 2009 Make your samples as uniform as you can. Powders: with the finest possible grinding. Liquids: without bubbles and fast decantation. The ALBA-CLÆSS beamline will have a stack of attenuators for detecting and solving the problem of radiation damage. 13-15.09.2012 K. Klementiev - XAS experiment