Hanle effect

The Hanle effect is a phenomenon that occurs when magnetic fields interact with matter. The Hanle effect was discovered in 1924 by the physicist Wilhelm Hanle. He delivered a semi-classical explanation for the effect. In the 1930s, the physicist wide presented a quantum mechanical theory, the so-called level crossing, which could explain the Hanle effect in quantum mechanics. The level crossing has been observed experimentally by physicists Cole Grave, Franken, Lewis, and Sands in 1959 and the Hanle effect, which equates to a small renaissance.

Observation / experiment

It will emit light of a mercury lamp in the x- direction and polarized in the y- direction. This light will now face one with a residual gas ( also mercury) filled glass bulb. The residual gas is then passed through the light ( primary photons ) for fluorescence ( fluorescence resonance ) stimulated. An observer observing the residual gas from the y-direction, can first detect a light. Now, a magnetic field is applied perpendicular to the plane of polarization of the light ( ie, for example in the z direction ) via the residual gas. Wherein a magnetic field around zero the observer will notice ( in the y- direction) and an intensity minimum with increasing magnetic field strength (positive and negative), an increase in intensity. This phenomenon is called the Hanle effect.

This effect works, of course, not only with mercury. It is suitable for example for measuring the lifetime of atomic and molecular states.

Semi- Classical Statement

The electrons in the residual gas are excited by the primary beam and you can consider this as a small damped oscillating dipoles. If the dipoles are excited only in the y- direction, light is not emitted due to the radiation of dipoles in the y- direction. However, applied perpendicular to the vibration direction of the electron magnetic field results in a rotation of the polarization direction by the magnetic field axis at the Larmor frequency:

The oscillation of the electrons is damped with. Then one can calculate the intensity of the light emitted by the distribution of dipole:

This results evaluated a Lorentz curve with the asymptotic value

The half-width is the Landé factor and the Bohr magneton.

If the field strength to achieve the half intensity, then it follows for the lifetime of the excited atomic state:

Quantum mechanical explanation

Hanle the effect is a special case of the level crossing, in which the lines of the fine structure splitting are crossed by the superposition of a magnetic field. The theoretical explanation for this was provided wide Gregory in 1933, the experimental observation followed but not until later.

Wide developed a formula for the rate f of the polarization of the photons re-emitted by the polarization g:

(to be completed yet)

Swell

• Wolf -Dieter Hasenclever: Construction of an apparatus for measuring lifetimes of excited atomic states using the Hanle effect. Freiburg, 1970 ( approval work, PDF file, 3.69 MB ).