# Online Encyclopedia

# Hyperfine structure

**Hyperfine structure** is a small perturbation in the energy levels (or spectral) of atoms due to the proton-electron dipole moment interaction. The amount of correction to the Bohr energy levels due to "hyperfine splitting" is on the order of:

where *m* is the mass of an electron, *m _{p}* is the mass of a proton,

*α*is the fine structure constant (

*1/137.036*), and

*c*is the speed of light. This is a much smaller perturbation than the fine structure or Lamb shift.

Also hyperfine structure gives the 21 cm line observed in HI region in interstellar medium.

## Use in defining the SI second

The hyperfine structure transition can be used to make a microwave notch filter with very high stability, repeatability and Q factor, which can thus be used as a basis for very precise atomic clocks. Typically, the hyperfine structure transition frequency of a particular isotope of caesium or rubidium atoms is used as a basis for these clocks.

Due to the accuracy of hyperfine structure transition-based atomic clocks, they are now used as the basis for the definition of the second. One second is now *defined* to be *exactly* 9,192,631,770 cycles of the hyperfine structure transition frequency of caesium-133 atoms.