Photoionization, fluorescence, and inner-shell processes

Article By:

McLaughlin, Brendan M. Centre for Theoretical Atomic, Molecular, and Optical Physics, Queens University, Belfast, United Kingdom.

Ballance, Connor P. Department of Physics, Auburn University, Auburn, Alabama.

Last reviewed:2013

DOI:https://doi.org/10.1036/1097-8542.YB130136

Much of the light (photons) from stars ultimately originates from the photon emission from a particular atom or molecule at an exact wavelength (color). These wavelengths allow us to uniquely identify the atoms and molecules that distant stars are composed of without the considerable effort required to travel there. For example, closer to home, the vivid green and blue colors observed in the aurora borealis (the northern lights) correspond to emissions from oxygen and nitrogen, respectively. Most of the known matter in the universe is in a plasma state (the state of matter similar to gas in which a certain portion of the particles are ionized) and our information about the universe is carried by photons (light), which are dispersed and detected for example by the orbiting Chandra X-ray Observatory. When photons travel through stellar atmospheres and nebulae (interstellar clouds of gas and dust), they are likely to interact with matter and therefore with ions. This makes the study of photoionization (the physical process in which an incident photon ejects one or more electrons from an atomic or molecular system) of atoms, molecules, and their positive ions very important for astrophysicists, helping them to interpret stellar data (spectroscopy). To measure the chemical evolution of the universe and then understand its ramifications for the formation and evolution of galaxies and other structures is a major goal of astrophysics today. The answers to these questions ultimately address human and cosmic origins. Our ability to infer chemical abundances relies extensively on spectroscopic observations of a variety of low-density cosmic plasmas including the diffuse interstellar and intergalactic media, gas in the vicinity of stars, gas in supernova remnants, and gas in the nuclei of active galaxies. Cosmic chemical evolution is revealed through emission or absorption lines from cosmically abundant elements.

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