Applications of Anticrossing Spectroscopy

Abstract

During the last twenty years both the level crossing and the anticrossing methods have been used extensively to study the atomic and molecular structure and properties of atoms and molecules in external fields. The level crossing technique in fact goes back to 1924 when Hanle(1) discovered the de-polarisation of resonance fluorescence light in a weak magnetic field (Hanle effect). At the time this could be explained as the precession in the magnetic field of a damped linear (2) oscillator.(2) With progress in the development of quantum mechanics, the polarisation of resonance fluorescence light was related to the coherent superposition of the degenerate excited states and the depolarisation was readily explained by (3) the removal of the degeneracy in the magnetic field.(3) Looking at it the other way round, a resonance type signal of the polarisation or the intensity of the resonance fluorescence light is observed when the magnetic substates converge(cross) at zero magnetic field, and for this reason the Hanle effect is also referred to as zero field level crossing. The magnetic field required to remove the degeneracy is directly connected with the natural width (i.e. lifetime) of the states, and Hanle signals have, therefore, provided the basis for many lifetime measurements of excited states.