In X-ray spectroscopy, K-alpha emission lines result when an electron transitions to the innermost "K" shell (principal quantum number 1) from a 2p orbital of the second or "L" shell (with principal quantum number 2). The line is actually a doublet, with slightly different energies depending on spin-orbit interaction energy between the electron spin and the orbital momentum of the 2p orbital. K-alpha is typically by far the strongest X-ray spectral line for an element bombarded with energy sufficient to cause maximally intense X-ray emission.
The analogous K-alpha spectra line in hydrogen is known as Lyman alpha; however because of hydrogen's small nuclear charge, this line is in the ultraviolet, not the X-ray range. See Siegbahn notation for the newer IUPAC-recommended spectral notation system.
An example of K-alpha lines are those seen for iron as iron atoms radiating X-rays spiralling into a black hole at the center of a galaxy. For such purposes, the energy of the line is adequately calculated to 2-digit accuracy by the use of Moseley's law: , where Z is the atomic number. For example, K-alpha for iron (Z = 26) is calculated in this fashion as 10.2 eV (25)2 = 6.38 keV energy. For astrophysical purposes, Doppler and other effects (such as gravitational broadening) show the iron line to no better accuracy than 6.4 keV.
Values of Transition Energies
- Values of different kinds of transition energies like K, K, L, L and so on for different elements can be found in the NIST X-Ray Transition Energies Database and Spectr-W3 Atomic Database for Plasma Spectroscopy 
- K-alpha emission values for hydrogen-sum and helium-like ions can be found on Table 1-5 of the LNBL X-Ray Data Booklet 
- Fukumura, Keigo; Tsuruta, Sachiko (2004-10-01). "Iron Kα Fluorescent Line Profiles from Spiral Accretion Flows in Active Galactic Nuclei". The Astrophysical Journal 613 (2): 700–709. doi:10.1086/423312.
- Lee, Julia C.; Iwasawa, Kazushi; Houck, John C.; Fabian, Andrew C.; Marshall, Herman L.; Canizares, Claude R. (2002-05-10). "The Shape of the Relativistic Iron Kα Line from MCG −6-30-15 Measured with the [ITAL]Chandra[/ITAL] High Energy Transmission Grating Spectrometer and the [ITAL]Rossi X-Ray Timing Explorer[/ITAL]". The Astrophysical Journal 570 (2): L47–L50. doi:10.1086/340992.
- Spectr-W3 database
- Lawrence Berkeley National Laboratory X-Ray Data Booklet