Albert Einstein revolutionized our understanding of light with the discovery of the photoelectric effect, demonstrating that light is not just a wave, but also a mass-less particle. This phenomenon, where light striking a solid releases an electron, is a cornerstone of modern physics. It's used in X-ray photoelectron spectroscopy (XPS), a technique that identifies atomic species at the surface of a solid. XPS measures the exact binding energy of an electron, providing insights into an element’s bonding environment. The photoelectric effect is integral to many technologies we use daily, from solar electricity generation to automatic doors and smartphone brightness adjustment.
Albert Einstein proposed the photoelectric effect. This phenomenon is where you shine light onto a solid, resulting in an electron, the photoelectron, being ejected from the solid. This event is a non-classical event. Classical mechanics cannot describe it because we have a non-classical particle called the photon, a packet of energy and a massless particle. In classical mechanics, we describe the actions of large particles moving very slowly (non-relativistic speeds). In non-classical mechanics, we have very small particles like energetic electrons (near-massless particles) and photons (massless particles). Just as in classical mechanics, photons and energetic electrons can transfer energy and momentum.
Albert Einstein took a non-classical event and used a classical mechanics equation to describe the phenomena. When the incident light with initial energy Eo strikes the solid, energy is conserved. After the reaction, the kinetic energy (KE) of the electron plus the work done (W) equals the initial energy
Eo = W + KE
In X-ray photoelectron spectroscopy (XPS), the basic process is the absorption of monochromatic light with a constant energy that results in the ejection of the photoelectron. After the bound electron absorbs the photon’s energy, it loses some of its kinetic energy in overcoming the Coulomb attraction of the nucleus; hence, the measured kinetic energy of the photoelectron is related to the binding energy of an electron in the target atom.
In this process, an incident photon transfers its entire energy to the bound electron. Element identification is provided by measuring the energy of the electrons that escape from the sample without energy loss. The binding energies in the atom are quantized, meaning the binding energies are unique to each atom.
X-ray photoelectron spectroscopy requires both a monochromatic radiation source and an electron spectrometer. Common to all the electron spectroscopies, the photoelectrons escape depth from a depth 1–2 nm. A very clean vacuum system is required. X-ray photoelectron spectroscopy is a straightforward and useful technique for the identification of atomic species at the surface of a solid. Adjacent elements throughout the periodic chart can easily be distinguished.
An element’s bonding depends on the chemical environment of that element. Another key feature of XPS is that it can measure the exact binding energy for an electron. The electron’s bonding energy is determined by the Coulomb interaction with the other electrons and the attractive potential of the nuclei. Any change in the local element’s bonding environment involves a spatial redistribution of the outer electron charges of this. This redistribution affects the potential of the core electrons and results in a change in their binding energies. For example, the chemical shift in the binding energy of the Si 2p electron in pure Si and SiO2 have clearly distinguishable energies.