What is XAFS and XES?

What is X-ray Absorption Fine Structure (XAFS)?

X-ray absorption fine structure (XAFS) is an analytical technique that relies on the variable absorption of x-rays by matter to probe its chemical and structure. A beam of x-rays of nearly one energy are put onto a sample, and the change in x-ray absorption is measured as the energy of the beam is varied. 

For each element of the periodic table, every electron shell in an atom of that element has a unique energy required to eject it from the atom. In an XAFS measurement, the energy of the incident x-rays is first tuned just below the energy of a particular electron shell, then the energy is gradually increased until eventually the x-ray has enough energy to eject that electron and absorb an x-ray photon. Due to quantum mechanical effects, the absorption does not increase all at once, but rather has a detectable turn-on and more complicated structure of oscillations of more/less absorption.

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One of the most useful properties of XAFS measurements is that the energies of the electronic transitions of different elements are well separated in energy compared to the resolution of XAFS spectrometers. This means that by tuning the energy of a particular transition, an element-specific measurement of the sample can be performed.

What is X-ray Emission Spectroscopy (XES)?

X-ray absorption fine structure (XAFS) is an analytical technique that relies on the variable absorption of x-rays by matter to probe its chemical and structure. A beam of x-rays of nearly one energy are put onto a sample, and the change in x-ray absorption is measured as the energy of the beam is varied. 

Click here to find out how does XAFS and XES fit into the laboratory analytical landscape?

XAFS and the laboratory analytical x-ray landscape

With the x-ray spectrometers developed by easyXAFS, transmission-mode X-ray Absorption Fine Structure (XAFS) and X-ray Emission Spectroscopy (XES) are fully entering the laboratory analytical x-ray landscape. 

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With all the different X-ray analytical techniques, which one is the right one? 

The right one depends on what you are trying to find out.  

Let's look at what happens when we shine an X-ray beam into our sample.
 

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What happens when you direct an X-ray beam at your sample? 

Scattered X-rays
The scattering of X-rays emit  in two different ways, coherent scattering  (Thomas Scattering) and incoherent scattering (Compton scattering). Coherent scattering has the scattered X-rays coming into nonrandom relative to the direction that you are interested in.  Incoherent scattering is pretty much all other scattered X-rays.    This technique is used for finding out element speciation in crystalline samples.  

The X-ray beam

The beam that is transmitting through the matter will be less in significance and can be used to measure the difference in the X-ray beam that is not going through the matter with the x-rays that are going through your sample. This technique is called X-ray absorption Spectroscopy (XAS). This technique is used for finding element speciation of powers, liquids and other non crystalline samples.  

Fluorescent X-rays

The x-rays that interact with your sample above a sufficient binding energy can cause an electron from an innermost shell to be ejected and create a hole that needs to be filled.  This hole is filled with another electron from an outer shell of the same molecule.  When the outer shell electron is at a higher energy orbital going into a lower energy this can cause an energy emission that presents itself as a secondary X-ray photon that is called Fluorescent X-rays. This non-destructive analytical technique is used to determine the elemental composition of materials.

Electrons  

Electrons will also emit from matter that has been hit with an X-ray beam.  These electrons fall into two categories for different types of analytical spectroscopy.The compton recoil electrons and photoelectrons.  Both of these electrons are used for XPS and XES.  XPS is a great technique to find out the…  

Heat

There will be some heat that dissipates from the matter even at low x-ray energies.  

When should I use laboratory XAFS?

Use XAFS when you need:

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Element-specific

By tuning to the energy of an atomic absorption edge, XAFS probes only the desired element, regardless of what else is in the sample.

Bulk sensitive

The absorption length of XAFS in the energy range 5-12 keV is on the order of several microns, giving a bulk average measurement of properties.

Oxidation state sensitivity

The XANES (x-ray absorption near-edge spectroscopy) region of the spectrum gives strong sensitivity to the oxidation state of the element being measured.

Ambient or special sample environment setups

XAFS does not require samples to be in vacuum. Standard measurements are conducted at ambient temperature/pressure. Sample chambers for heating or cooling samples or for air-sensitive samples are possible.

In-situ measurements

Because of the penetrating power of x-rays at these energies, many in-situ measurements are possible from operando battery measurements to in-situ catalyst reaction cells and more.

No long-range order required

XAFS provides sensitivity to the local structure of the element being probed, regardless of long range order (e.g. works on non-crystalline or nano-sized materials).

There are several synergies between laboratory and synchrotron facilities:

  • Independence: easyXAFS instruments provide routine access for studies that don’t require synchrotron capabilities.

  • Sample validation: easyXAFS instruments can be used to validate or screen samples prior to synchrotron access to help researchers make the most out of their time.

  • Allocation: Synchrotron access is a valuable finite resource. Utilizing laboratory capabilities for suitable studies frees up synchrotron access to focus on those challenging studies that can’t be performed in the lab.

  • Special sample environments: In-situ or operando experimental hardware in use at the synchrotron can often also be used in easyXAFS systems. Additionally, this hardware can be validated in a laboratory system before being taken to the synchrotron.