Magnetic dipole transitions in the OH $A\,^2\Sigma^+ \leftarrow   X\,^2\Pi$ system
  We report on the observation of magnetic dipole allowed transitions in the well-characterized $A\,^2\Sigma^+ - X\,^2\Pi$ band system of the OH radical. A Stark decelerator in combination with microwave Rabi spectroscopy is used to control the populations in selected hyperfine levels of both $\Lambda$-doublet components of the $X\,^2\Pi_{3/2},v=0,J=3/2$ ground state. Theoretical calculations presented in this paper predict that the magnetic dipole transitions in the $\nu'=1 \leftarrow \nu=0$ band are weaker than the electric dipole transitions by a factor of $2.58\times 10^3$ only, i.e., much less than commonly believed. Our experimental data confirm this prediction. 
Moritz Kirste, Xingan Wang, Gerard Meijer, Koos B. Gubbels, Ad van der   Avoird, Gerrit C. Groenenboom, and Sebastiaan Y. T....
physics.chem-ph quant-ph