Observation of coherent interference between magnetic-dipole and electric-dipole transition moments in methane

Interference between magnetic-dipole and electric-dipole transition moments has been observed in the molecularbeam rf spectrum of methane. The transition studied is between two levels in the $J=2\mathrm{}$ rotational state and ground vibronic state. This transition is both magnetic-dipole allowed and electric-dipole allowed because a static electric field mixes states of opposite parity. The transition probability was observed to depend upon the amplitudes and relative phases of the applied rf magnetic and electric fields. A theoretical explanation of this effect is given.