Rubidium D₂ Line

The diagram to the right shows the hyperfine structure for both the ground and excited states of the two main isotopes of Rubidium, ⁸⁵Rb and ⁸⁷Rb.

The splitting between the hyperfine levels is shown with the blue arrows. The allowed electric dipole transitions are shown with the black arrows. The difference in energy between the 5S_1/2 and the 5P_3/2 levels is equivalent to a wavelength of 780nm, or a frequency of ~ 3x10¹⁴Hz.

If we shine a weak probe beam (weak means that it will not excite a significant number of the atoms to the excited state) then we will see four broad (~500MHz) absorptions in the region of 780nm. these four absorptions will correspond to the four different hyperfine groundstates. We cannot resolve the hyperfine structure of the excited state as the Doppler broadening of the individual transitions, at room temperature is larger than the difference in energy between them.

There is more than one way to beat this Doppler broadening limit placed on our resolution. One of these methods is to use a stronger, pump, beam that is at the same frequency as the probe beam, but propagates in the opposite direction through the Rubidium vapour.

This pump beam excites a significant number of the atoms into the upper state. The only atooms that both the pump and probe beam interact with at the same time are those that are Doppler shifted into resonance with both beams at the same time. As the bbeams are counter propagating then they only interact with the same atoms on the same transition whe n those atoms have zero vellocity along the direction of the beams.

Each hyperfine level can be further subdivided into a number of Zeeman sub-levels. The number of Zeeman sub-levels is equal to the 2F+1, where F is the hyperfine quantum number of that level. These Zeeman sub-levels correspond to different projections of the F quantum number onto a small external magnetic field.