9th June 2016

In the 19th century James Clerk Maxwell formulated the classical theory of electro-magnetism, which describes light as electric and magnetic fields oscillating perpendicular to each other. In the interaction between light and molecules, for example in an absorption process, the electric field component is usually dominating the process. The effect of the magnetic component of light only becomes evident once the electric interaction is not taking place, i. e. when it is forbidden because of symmetry reasons. Purely magnetic transitions are typically ten to a hundred thousand times smaller than comparable allowed electric transitions. This makes testing quantum mechanical predictions in our experiment by measuring the ratio between the strength of electric and magnetic transitions an enormous experimental challenge.

In the Molecular Physics department an experiment was performed in which a static electric and a static magnetic field are applied to lift molecular symmetry restrictions. The static electric field allows to mix a little bit of the otherwise forbidden electric transition together with the magnetic transition in the molecule. Thus, the molecule is interacting with both the electric and magnetic components of light. The static magnetic field enabled the observation of the so-called Stark-interference effect. This gives a deeper insight into the light-molecule interaction, because not only the electric and magnetic transition moments can be measured but also their relative sign, while in usual transitions the intensity depends only on the square of the transition moments. The experimental results agree with a quantum mechanical prediction within the accuracy of the measurements and they also confirm the predicted relative sign of the calculated electric and magnetic transition moments.

Arrangement of capacitor plates and coils

Original publication:
Stark Interference of Electric and Magnetic Dipole Transitions in the A−X Band of OH
H. Christian Schewe, Dongdong Zhang, Gerard Meijer, Robert W. Field, Boris G. Sartakov, Gerrit C. Groenenboom, Ad van der Avoird, and Nicolas Vanhaecke
Phys. Rev. Lett. 116, 153001 – Published 11 April 2016 (http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.153001)