Magnetic dipole transitions in the OH $A{}^2{\mathrm{\Sigma }}^{+}\leftarrow X{}^2\mathrm{\Pi }$ system

We report on the observation of magnetic dipole allowed transitions in the well-characterized $A{}^2{\mathrm{\Sigma }}^{+}-X{}^2\mathrm{\Pi }$ 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 $\mathrm{\Lambda }$-doublet components of the $X{}^2{\mathrm{\Pi }}_{3\mathrm{/}2},v=0,J=3\mathrm{/}2$ ground state. Theoretical calculations presented in this paper predict that the magnetic dipole transitions in the ${\nu }^{\mathrm{\prime }}=1\leftarrow \nu =0$ 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.