What is Photoelectron Spectroscopy (PES)?
Photoelectron spectroscopy (PES) is a method of determining the relative energy of electrons in
atoms and molecules.
A photoelectron count vs. binding energy graph is called a PES spectrum.
PES peaks represent electrons in various subshells of an atom. Valence electrons are
represented by the peaks with the lowest binding energies, whereas core electrons are
represented by the peaks with the highest binding energies.
Table of Contents
Introduction to Photoelectron Spectroscopy (PES)
Photoelectron Spectroscopy Fundamentals
Analyzation of PES spectra
Photoelectric Effect
PES Spectrum of Lithium
PES spectrum of Oxygen
Photoelectron Spectroscopy in Practice
Frequently Asked Questions
Introduction to Photoelectron Spectroscopy (PES)
PES (photoelectron spectroscopy) is a technique for determining the relative energy of
electrons in atoms and molecules. PES is frequently used by scientists to investigate the
elemental makeup of materials or to analyse molecular bonding.
Photoelectron spectroscopy (PES) studies the composition and electronic state of a sample’s
surface region by ionising it and analysing the kinetic energy distribution of the emitted
photoelectrons.
It analyses the electronic structure of molecules by measuring photoelectrons’ kinetic energy to
determine the binding energy, intensity, and angular distributions of these electrons. It is
distinct from standard spectroscopy in that it investigates a substance’s electrical structure by
detecting electrons rather than photons.
Photoelectron Spectroscopy Fundamentals
The photoelectric effect, first described by Albert Einstein in 1905, provides the basis for
photoelectron spectroscopy. When electrons in a metal are exposed to enough light, they are
ejected off the metal surface, which is known as the photoelectric effect. We may determine the
energy of the electrons in the solid metal if we know the kinetic energy of the expelled electrons
(also known as photoelectrons) and the energy of the incident radiation.
PES involves bombarding a sample with high-energy radiation, usually UV or X-ray, which
causes electrons to be expelled. The expelled electrons go from the sample to an energy
analyzer, where their kinetic energies are recorded, and then to a detector, where the quantity
of photoelectrons at different kinetic energies is counted. This procedure is depicted in a
simplified graphic below.
Ultraviolet Photoelectron Spectroscopy
The ionisation energy or binding energy of an electron is the amount of energy required to
expel it from a substance.
Analyzation of PES spectra
Photoelectron count vs. binding energy graphs is obtained from PES studies, with binding
energy commonly given in electron volts (eV) or megajoules (MJ) per mole.
Peaks with various binding energies can be found in a typical PES spectrum. Each of these
peaks corresponds to electrons in a distinct subshell of an atom since electrons in the same
subshell of an atom have the same binding energy. The binding energy of a peak indicates how
much energy is required to remove an electron from a subshell, while the peak’s intensity
indicates the subshell’s relative quantity of electrons.
Photoelectric Effect
A schematic illustration of the effect is shown in the diagram.
Photoelectric Effect
As Hertz observed empirically and as Einstein explained using energy quantization, electrons
are expelled from a substance with kinetic energy connected to the energy of the incident
radiation. In principle, the relationship is described. The photoelectron’s kinetic energy is Ekin,
while the electron’s binding energy is EB. A well-known work function can be found in many
materials. As is common when working with atoms, molecules, and clusters, all energies and
spectra calibrations are reported in terms of the vacuum level in this publication.
PES Spectrum of Lithium
Let’s start with the lithium, Li, idealised PES spectrum. For comparison, lithium’s ground-state
electron structure is 1s2, 2s1.
Lithium's idealized PES spectrum
The PES spectrum reveals two peaks, which correspond to electrons in lithium’s two subshells
(1s and 2s). The peak closest to the origin has twice the intensity of the peak further away.
Because the 1s subshell of lithium has twice as many electrons as the 2s subshell (2 vs. 1), the
peak closest to the origin must be the 1s subshell of lithium.
This is also consistent with binding energies: we know that electrons in the 1s subshell of lithium
are closer to the nucleus and less protected than electrons in the 2s subshell. As a result,
removing the 1s electrons requires more energy. This is supported by the fact that the PES
spectrum’s 1s peak has greater binding energy.
It’s important to note that the binding energy of lithium’s 2s peak is the same as the initial
ionisation energy of lithium–that is, the amount of energy necessary to remove a lithium atom’s
outermost or least bonded electron. The binding energy of the 1s peak, however, is not equal to
lithium’s second ionisation energy. The nucleus will hold the 1s electrons much tighter if the first
electron is withdrawn from lithium, increasing the binding energy of these electrons.
PES spectrum of Oxygen
Let’s have a look at some elements with more electrons. The idealised PES spectrum for oxygen,
O, is shown here. The ground-state electron configuration of oxygen is 1s22s22p4.
PES Spectrum of Oxygen
This spectrum has three peaks, each indicating electrons in a different subshell of oxygen
(either 1s, 2s, or 2p). Because electrons in the 1s subshell are closer to the nucleus and less
protected than those in the 2s or 2p subshells, we would anticipate the peak with the highest
binding energy (the leftmost peak) to belong to electrons in the 1s subshell. The 2s subshell
must correspond to the peak with the next highest binding energy and the 2p subshell to the
peak with the lowest binding energy (the rightmost peak).
The 2p subshell of oxygen has twice as many electrons as the 1s or 2s subshells (4 vs. 2). As a
result, we should expect the 2p peak to be twice as powerful as the 1s or 2s peak, as shown in
the spectrum.
Because electrons in the same electron shell have similar energies, we’d expect to find peaks in
a PES spectrum showing electrons in the same shell grouped together.
Photoelectron Spectroscopy in Practice
Detecting impurities, managing purification, investigating the kinetics of chemical processes,
identifying molecular weight, and determining unknown concentrations are all applications of
electronic spectroscopy.
XPS has a wider range of applications than UPS since it can probe down to core electrons.
Except for two elements, XPS is useful for detecting elements, evaluating the chemical state of
surfaces, and performing quantitative analysis. The chemical states of samples can be
distinguished using XPS. Different oxidation states of compounds can also be distinguished
using XPS.
Frequently Asked Questions on Photoelectron Spectrum
Q1 Mention the difference between XPS and UPS
UPS and XPS are both photoelectron spectroscopy techniques, with UPS standing for
ultraviolet photoelectron spectroscopy and XPS for X-ray photoelectron spectroscopy. In
the near-surface region, XPS uses high-energy X-ray photons to excite “core” electrons,
whereas UPS uses lower-energy photons in the deep UV range to excite valence electrons.
Q2 What can we learn from the photoelectron spectrum?
Ionisation can reveal the relative energies of occupied molecular orbitals in a
photoelectron spectrum. (i.e. an electron’s ejection) The energy separation between
vibrational modes of a given electronic state can also be determined using a
photoelectron spectrum.
Q3 What are the different types of photoelectron spectroscopy?
UV photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) are the
two main categories in this field. The designations are derived from the photon energies
employed in the specific spectroscopy.
Q4 Do Ca2+ and Ar have the same photoelectron spectrum?
While Ca2+ and Ar are isoelectronic and should have the same number of photoelectron
peaks, Ca2+ has more protons than Ar, resulting in a higher Coulombic force holding the
electrons. Coulombic forces are unaffected by neutrons, hence the spectra should be the
same.
Q5 In a photoelectron spectrum, what factors influence the height of each peak?
The ionisation energy defines the position of each peak, whereas the height identifies the
ratio of electrons in each level or orbital.
Test your Knowledge on Photoelectron Spectrum
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