Photoelectron Spectroscopy (PES) is an analytical technique used to determine the binding energies of electrons within an atom or molecule. High-energy radiation, such as X-rays or UV light, is directed at a sample, causing electrons to be ejected from their orbitals based on the photoelectric effect. By measuring the kinetic energy of these emitted electrons, scientists calculate the binding energy—the energy that originally held the electron in place. The resulting graph, the PES spectrum, provides a unique fingerprint of the element’s electronic structure.
Understanding the Spectrum Layout
The PES spectrum is a plot representing the energy required to remove electrons. The horizontal axis (x-axis) represents the binding energy, typically measured in units like electron volts (eV) or megajoules per mole (MJ/mol). The binding energy scale runs counter-intuitively: the highest energy values are plotted on the left side of the graph and decrease toward the right. This means electrons requiring the most energy to remove are found closer to the left edge. The vertical axis (y-axis) represents the relative number of electrons detected, often called intensity or photoelectron count. Each distinct peak corresponds to a group of electrons that share the same binding energy, providing evidence that electrons exist in discrete energy levels or subshells.
Relating Peak Position to Electron Shells
The position of a peak along the binding energy axis is directly related to the specific subshell from which the electrons originated. Electrons closer to the atomic nucleus experience a stronger attraction and require significantly more energy to remove. These tightly-held core electrons (like those in the 1s subshell) appear at the highest binding energy values, typically found on the far left of the spectrum.
Electrons residing in the outermost energy levels, called valence electrons, are shielded from the full nuclear charge. This greater distance and shielding result in a weaker attractive force. Consequently, valence electrons have the lowest binding energies and appear toward the right side of the PES spectrum.
For example, comparing the 1s peak of Lithium to Neon reveals how binding energy changes with atomic number. As the number of protons increases, the effective nuclear charge pulling on the 1s electrons also increases. This stronger pull means the 1s electrons in Neon will have a substantially higher binding energy than those in Lithium, causing the peak to shift further to the left. Each occupied subshell (such as 1s, 2s, 2p, 3s, and so on) generates its own unique peak at a characteristic binding energy value.
Determining Electron Count from Peak Size
While the position of a peak identifies the energy level, the relative size of the peak provides quantitative information about the number of electrons in that specific subshell. The area under a peak is directly proportional to the number of electrons ejected from that energy level. This proportionality is used to determine the electron configuration of the analyzed atom.
For instance, an s subshell can hold a maximum of two electrons, while a p subshell can hold up to six electrons. When comparing the peaks for the 2s and 2p subshells in a neutral atom where both are full, the 2p peak will have an area three times greater than the 2s peak. This 1:3 ratio (representing the 2:6 electron count) serves as a clear method to assign the correct subshell label to each peak. By calculating the relative areas of all the peaks, an analyst can determine the total number of electrons in the atom. This total count then precisely identifies the element being analyzed, as the number of electrons in a neutral atom equals its atomic number.