Sodium D line - Natrium-D-Linie
The sodium D-line is the dominant spectral line in the spectrum of sodium . It is one of the strongest Fraunhofer absorption lines in the sun's spectrum . The designation "D" is historical and comes from the naming of the Fraunhofer lines.
In emission , the sodium D line causes the typical yellow color of the flame , which can be observed by sprinkling table salt (sodium chloride) into a gas flame. It is also responsible for the yellow color of sodium vapor lamps , which are often used for street lighting.
A closer look reveals that the sodium D line is split into two closely spaced spectral lines (also referred to as doublet), the wavelengths of which are 588.9951 nm (D 2 ) and 589.5924 nm (D 1 ).  The observed splitting of the sodium D-line and similar lines of other alkali metals prompted Samuel Abraham Goudsmit and George Eugene Uhlenbeck in 1925 to postulate the existence of the electron spin .
The sodium D-line arises when the external electron changes from the excited 3p state to the 3s ground state (with emission; with absorption, the other way round). Due to the coupling of the electron spin with its orbital angular momentum ( spin-orbit coupling ), the 3p state is split into two states with total angular momentum j = 1/2 or 3/2, depending on whether the spin and orbital angular momentum are set to be parallel or antiparallel. The energy difference between the two states is 0.0021 eV , corresponding to the frequency difference of 515 GHz of the line doublet.
A very close further yellow line at 587.49 nm from the Sun was found in 1868 and referred to as D3 in the direction of lower wavelengths and was the first trace for the discovery of helium .
- Yu. Ralchenko, A.E. Kramida, J. Reader: NIST Atomic Spectra Database (Version 5.5.1). National Institute of Standards and Technology, Gaithersburg MD 2017.