Rydberg energy: The Half-Hartree Atomic Unit
What happens when an atom absorbs enough energy to release an electron? In atomic physics, the energy binding electrons to nuclei sets the terms for excitation, ionization and the light atoms emit. Rydberg energy is measured in a unit equal to exactly half a hartree, or 0.5 hartree. The rydberg is used in atomic and molecular physics, and its name honors Swedish physicist Johannes Rydberg, who lived from 1854-1919. His surname became attached to measurements of both energy and wave number, placing the name within the vocabulary of a field shaped by the study of atomic spectra.
Before atoms had a mathematical model of their internal structure, their properties could already be described and classified in bulk. Ideas about matter being composed of atoms appeared in ancient writings associated with Democritus and Kanada. Much later, British chemist and physicist John Dalton developed the modern conception of the atom as the basic unit of a chemical element. Dmitri Mendeleev's periodic system brought another major advance in the organization of the elements. At that stage, the nature of atoms remained uncertain. Their properties offered a basis for classification before the arrangement and behavior of their electrons became accessible through later physical theories.
Spectral lines opened a different route into the atom. Their discovery and the attempts to understand them, notably by Joseph von Fraunhofer, marked the beginning of atomic physics as a field. Efforts to explain atomic spectra led to the Bohr model and helped bring about quantum mechanics. Light from atoms became part of the development of a new mathematical model of matter. The atomic orbital model subsequently gave a fuller treatment of atoms and their electron shells. These developments also established a new theoretical foundation for quantum chemistry and spectroscopy, connecting the behavior of atoms with the study of their emitted light.
The name used for Rydberg energy had an older measurement role. Rydberg was formerly a name for the kayser, the CGS unit of wave number. In its energy sense, the unit has a value of approximately 2.1799 x 10-18 joule or 13.60569 electronvolts. Those decimal equivalents are approximate; the relationship to the hartree is exact. Wave number and energy are separate physical quantities, so the surrounding measurement determines the meaning of the shared name. Joules and electronvolts express the magnitude of the energy unit in other scales, while hartrees give its exact fractional relationship within the vocabulary of atomic and molecular physics.
The atoms at the center of this work consist of a nucleus and electrons. Atomic physics primarily considers them individually, including neutral atoms and ions, and concentrates on how electrons are arranged and how those arrangements change. In a gas or plasma, interactions between separate atoms can occur on timescales much longer than the atomic processes under study. Individual atoms can consequently be treated as isolated systems for those processes. This approach provides underlying theory for plasma and atmospheric physics, even though both involve enormous numbers of atoms. Atomic physics commonly sits within the broader field of atomic, molecular and optical physics.
Rydberg energy belongs to a setting in which electrons can remain bound or escape. Electrons occupy notional shells around the nucleus and normally reside in a ground state. Light, magnetic fields or collisions with particles can transfer energy to them. The energy required to remove an electron from its shell to infinity is its binding energy. When an electron absorbs more than that amount, the atom becomes ionized, and the excess energy becomes kinetic energy. If the absorbed amount is smaller, the electron can move into an excited state while remaining bound. Energy thus governs whether the electron stays with the atom or leaves it.
An excited electron can later move to a lower state. In a neutral atom, the system emits a photon carrying the difference in energy, in keeping with conservation of energy. An inner electron's removal can produce another sequence: an outer electron may move inward to fill the vacant orbital. That transition may release a characteristic X-ray. Alternatively, the released energy can pass to another bound electron and send it into the continuum. This process, called the Auger effect, allows a single photon to lead to multiple ionization. Excitation by light also follows strict selection rules for reachable electronic configurations.
Research surrounding Rydberg energy advanced rapidly after the Second World War. Improvements in computing made larger and more sophisticated models of atomic structure and collisions possible. Experimental work benefited from advances in accelerators, detectors, magnetic field generation and lasers. The field continued to encompass theoretical models alongside laboratory investigations of excitation and ionization. Nuclear physics developed a different focus, studying nuclear reactions and the special properties of nuclei rather than the whole atom as a system of nucleus and electrons. Some atomic processes also exhibit chaotic behavior that requires treatments beyond the regular quantum mechanical methods used for familiar phenomena.