What is Photoelectron Spectroscopy?

Photoelectron spectroscopy (PES) is a part of experimental chemistry. It is a technique used in laboratories that involves projecting intense beams of radiation on a sample element. In response, the element ejects electrons for which the relative energies are measured.

PES is a very useful technique. Data from PES can be used in a number of ways. From studying the electronic properties and chemical state of materials to better understanding atomic structure, PES is helpful.

Angle-resolved photoemission spectroscopy (ARPES) is a powerful technique used in condensed matter physics to probe the structure of the electrons in a material, usually a crystalline solid.

X-ray photoelectron spectroscopy (XPS) was used to identify the surface characterization of carbon existing on the surfaces of various activated charcoals.

The inverse to this process is inverse photoemission. It is used to study the unoccupied electronic structure of materials and thin films.

Photoelectric Effect

To begin with, photoelectron spectroscopy is based on the photoelectric effect.

Photoelectric effect is defined as the ejection of electrons from a metal sample when it absorbs electromagnetic radiation or light.

The electron can also go under photoelectron diffraction. This means that the electrons go in and out of phase due to interference and change in photon energy.

X-ray photoelectron diffraction (XPD) has been used to investigate film structures and local sites of surface and dopant atoms in complex oxide

Experiment explanation: IN this set up, a beam of high energy radiation (commonly ultraviolet rays or X-rays) is bombarded at a sample of an element which results in the ejection of high-speed electrons termed as photoelectrons. These electrons travel from the sample towards an energy analyzer that records the kinetic energy of these particles. Kinetic energy is the energy of the particle when in motion. It further travels towards a detector. It records the number of photoelectrons for a number of kinetic energies. All of this takes place in a high vacuum environment to avoid any external radiation interference.

”The image that shows the experiment set up”
CC BY-SA 4.0 | Ponor

By the end of the process the scientists know two things:  the kinetic energy of the electrons and the energy of the radiation being projected.

Using this data, we can calculate the binding energy. Binding energy is the energy it takes to remove an electron from an atom. It is quite similar to the concept of ionization energy. We use the equation to determine the binding energy:

Binding energy (BE) = E (photon) - Kinetic energy (KE) 

Binding energy = Energy of radiation - Kinetic energy of the electron 

Energy of the radiation can be calculated as follows:

E = hv

h = plank’s constant 

v = frequency of the radiation.

The binding energy depends on its location in the atomic orbital. Electrons in the outermost shell are more shielded and farthest from the nucleus, so they have the lowest binding energies of all of the electrons in an atom.

At constant radiation energy, electrons with higher binding energies will have less kinetic energy and vice versa. A graph can then be made of the binding energy data for individual elements. Electrons are ejected from the surface of the thin film sample in UPS spectra when ultraviolet light occurs.

The electron's kinetic energy varies according to its molecular orbital. The photon energy is the summation of the absolute value of the electron kinetic energy and its orbital potential energy.

Valence band electrons are emitted by irradiation.  Angle resolved photoelectron spectroscopy detects by a sample with UV light due to photoelectric effect. The determination of the electronic structure of solids is based on the high resolution of the energy and angle distribution.

The photoemission occurring depends upon the exact binding of an electron and also at the stage in which the formal oxidation of the atom and the chemical environment and physical environment. Whenever there is an occurrence of changes in either of them, there is a rise of small shifts in the peak position of the spectrum. As the technique is of high intrinsic resolution, these shifts are easily interpretable. Also, the core levels are generally discrete and well defined and the technique is a one electron process.

A higher binding energy is exhibited due to the extra coulombic interaction between the ion core and photo emitted electron. This is the major strength of the XPS technique.

PES Spectrum

Data is graphed by plotting photoelectron (y-axis) against binding energy (x-axis). Binding energy is usually measured in the unit of electron volts. Binding energy decreases as you move away from the nucleus. This is why we can assume that the origin is the nucleus.

When observed, the graph has peaks at various binding energies. Why is it so and not a continuous graph? This is because electrons in a particular molecular orbital and subshell share the same value of binding energy. The peak depicts the energy needed to remove the electron from that subshell. The intensity of the peak gives us a count of the number of electrons in the subshell. Moreover, we can determine the electronic structure of an atom using this data. 

Difference between UV Photoelectron Spectroscopy and X-ray Photoelectron Spectroscopy

There is a difference in the uses of ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy. Some are listed below:

Ultraviolet photoelectron spectroscopyX-ray photoelectron spectroscopy.  
Measurement of molecular orbital energies.Depth profiling.
Angle dependent studies.Elemental composition.
Electron energy levels.Empirical formulas.
Electronic structure of solids.Chemical/ electronic state.

Common Mistakes

Interpret the graphs carefully and pay attention to the units.

Contexts and Applications

The topic is useful for various undergraduate and postgraduate courses, especially for Bachelors and Masters in Chemistry. 

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