Hydrogen Fine Structure | Physics Optional Notes for UPSC PDF Download

What is the fine structure of a hydrogen atom?

The splitting of the main spectral line into two or more components with a slight variation in wavelength in the magnetic field is called fine structure in spectroscopy. These are known as fine structures of the main spectral line. The hydrogen spectrum exhibiting the fine structured lines is known as the hydrogen fine spectrum.

Hydrogen is the lightest element with a single electron in the periodic table. The hydrogen atom absorbing external energy shows the excitation of its electron. The transition of electrons between the two discrete stationary energy levels results in the emission of photons of definite wavelengths. It shows spectral lines in the hydrogen spectrum. Unlike ordinary spectrometers, a high- resolution spectrometer epitomizes the main spectral line splitting into its constituents with a slight variation in their wavelengths. The splitting of spectral emission lines of the hydrogen are known as the hydrogen fine structures.
The cleavage of the main spectral line of the hydrogen atom is due to the influence of spin-orbit coupling. The hydrogen fine structure is the splitting of spectral lines of the hydrogen atom due to the interaction of electron spin with the magnetic field generated by the electron revolution around the atomic nucleus. Hence, the interaction of spin electron magnetic moment with the magnetic field of electron's relative motion gives the hydrogen fine structures.In 1916, Arnold Sommerfeld explained the splitting of hydrogen spectral lines with elliptical stationary orbits. And he introduced a dimensionless constant a to account for the energy difference between the gross and fine structure predictions.It determines the amount of spectral splitting of the hydrogen spectrum. And it denoted by the letter α. It's formula is;

Spin-orbit coupling

The coupling of the magnetic field generated by the orbiting electron around the nucleus with quantum mechanical spin produces fine structures of the hydrogen spectrum. So, the hydrogen fine structures occur due to the coupling of spin and orbital angular momenta. 
Hence, the spin-orbit coupling is a piece of evidence for the splitting of the main spectral line into two or more ancillary lines.
The alkali metal atoms with 1S-electron in their valence shell give spectral line doublet in the presence of the magnetic field. But, the alkaline earth metals with two 1S-electrons in their valence shell give spectral line triplet due to spin-orbit interaction. In other words, the number of fine structures increases with an increase in the stable state configurations in the atom. For this reason, Lithium with atomic number three (i.e., n=3) may not be resolved by the average spectroscope. Whereas the Rubidium with atomic number 37 has widely separated spectral emissions and can be observed with a normal spectroscope.