Abstract
Under the conditions of very low density and weak radiation fields that are characteristic of most of the interstellar medium, the vast majority of the atoms of any particular element and ionization state reside in the ground state. The excited states of atoms are mostly populated as a result of collisions with charged species (usually electrons or protons), and the collisional excitation timescale is often long enough that the atoms are free to decay back to the ground state by radiative processes. At low enough densities, this condition will remain true even if the excited state has a radiative lifetime of several seconds. This is frequently the case for the forbidden transitions observed in ionized plasmas.
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© 2003 Springer-Verlag Berlin Heidelberg
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Dopita, M.A., Sutherland, R.S. (2003). Collisional Excitation. In: Astrophysics of the Diffuse Universe. Astronomy and Astrophysics Library. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-05866-4_3
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DOI: https://doi.org/10.1007/978-3-662-05866-4_3
Publisher Name: Springer, Berlin, Heidelberg
Print ISBN: 978-3-642-07771-5
Online ISBN: 978-3-662-05866-4
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