Electron Spectroscopy for Chemical Analysis (ESCA)

Electron Spectroscopy for Chemical Analysis (ESCA) summarizes a couple of methods, that work more or less identical, to identify the chemical make-up of compounds, specificially surfaces. (Note: sometimes ESCA is only referred to as XPS, see below)

These methods include radiating a sample with a specific kind of high energy radiation (Ultraviolett and X-ray) that will eject/emit electrons from the sample. The analysis of the kinetic energy of these electrons in relation to the energy that was put into the sample (given by the frequency of the radiation, i.e. energy of the photon) allows to draw conclusion about the exact nature of bonds and the valency of the atoms in a molecule.

4 types photoelectron spectroscopy (PS) of ESCA can be described:

Type	Abbreviation	Source	Energy (eV)
Ultraviolett PS	UPS	HeI	21
		HeII	41
(soft) X-ray PS	XPS	MgKα	1254
		AlKα	1486
Hard X-ray PS	HAXPES, HXPS		2000-15,000
Synchrotron (Radiation) PS	S(R)PES		50-15,000

XPS

Allow a quantitative conclusions regarding the chemical nature (what atom) and states (redox) of the element present, as well as their distribution. For thin films, the thickness of the film, its uniformity and chemical composition can be evaluated.

The XPS mechanism and spectrum of pure copper powder. The X-rays can only penetrate through the surface of the material (70 – 110 A, or 7-11 nm) Source: https://slideplayer.com/slide/13891604/

Let’s take a closer look on the spectra:

Typical XPS spectra, in this case from Pd, with an MgKa source with a photon energy of 1253.6 eV. The x-axis can either be representing increasing kinetic energy or decreasing binding energy.

As you can see, the measured kinetic energy of the electrons is inverse the binding energy. This simple relationship can be understood, if you one considers that electron are loosing energy, just to get out of the atom, just like rockets, that need lots of energy to get away from the earth’s gravitation.

Since the incoming energy of the radition source is fixed, no electron that is measured can have a higher kinetic energy as this incoming energy. Part of the energy is used to get the electron out of the shell. Generally the closer the electron is to the nucleus, the more energy is needed to get it away from it. In other words, the stronger the binding energy between any particular electron and the nucleus is, the more energy is used to that. In turn, from the total energy that was incoming the rest of the energy is converted to kinetic energy. In short:

• x-axis: kinetic energy from left to right, binding energy from right to left
  • the higher the kinetic, the lower the binding energy of that electron
  • the closer the electron to the nucleus, the higher the binding energy

Multiple things happen when the electrons are ejected from their orbitals:

• Auger electrons: once an electron from the outer shell of an atom has been emitted it can happen, that another atom from the inner shell is released, in order to fill this unoccupied space. Due to the energetic difference in the inner to the outer shell, the electron emitted an energetic radiation alongside its transition. This radiation can be detected. Some of this radiation can however be picked up by another electron and release it due to the increment in energy. This electron, that is released due to the radiation it experienced from another electron “falling down” an energy level, is called Auger electron, and this whole process is called the Auger effect. However, the effect was discovered by Lise Meitner, so some call it rightfully, the “Meitner effect”.

• after the electrons are ejected, some of them lose all their energy while travelling through the sample, this is why the XPS spectrum has a step-like structure, with the largest peaks (from right to left) representing the electrons that have not lost their energy and the step representing the electrons that have

Generally, elemental bonding information in addition to elemental analyses, and thereby allows for compound analyses.

Coursera Course of Surface Analysis: https://www.coursera.org/learn/methods-surface-analysis#syllabus

Introductionary video into XPS and Auger spectroscopy:

Part 1: https://www.youtube.com/watch?v=pQPVaSRN01w

Part 2: https://www.youtube.com/watch?v=SGFwsg8bhe0

For surface properties: https://www.youtube.com/watch?v=XpDqJfybma4

CalTech Lecture Slides on that topic: https://mmrc.caltech.edu/SS_XPS/XPS_PPT/XPS_Slides.pdf

Note, beside shoting your sample with high energy radiation (photons) you can also shoot it with electron. This called Auger Electron Spectroscopy (AES) and very similar to these techniques.