Sodium D line

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Yellow flame color from sodium
Platform light with high pressure sodium vapor lamps, recognizable by the orange-yellow light

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 spectrum of the sun . 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 known 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 .

Simplified term scheme for sodium. The splitting of the sodium D line is due to the fine structure splitting of the 3p level.

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-orbital 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 anti-parallel. The energy difference between the two states is 0.0021  eV , corresponding to the frequency difference of 515  GHz of the line doublet.

Sodium D line in the spectroscope. The resolution of the spectroscope is too low to show the splitting of the line.

A very close further yellow line at 587.49 nm from the Sun was found in 1868 and further referred to as D3 in the direction of lower wavelengths and was the first trace for the discovery of helium .

Individual evidence

  1. Yu. Ralchenko, AE Kramida, J. Reader: NIST Atomic Spectra Database (Version 5.5.1). National Institute of Standards and Technology, Gaithersburg MD 2017.