I am trying to understand the concept of polarization of molecules after they are excited by a microwave pulse. The context is:
The complexes are produced in a pulsed supersonic jet expansion of a gas mixture into a vacuum chamber. The temperature of the sample in the jet expansion is on the order of a few Kelvin, and the complexes are stabilised in this nearly collision-free environment.
Interaction with a pulse of microwave radiation causes the dipole moments of the complexes to align, resulting in a macroscopic polarization of the 'ensemble' of complexes. After the pulse, relaxation occurs, and the decay of this polarization with time (free induction decay or FID) is recorded.
How does microwave radiation cause the dipole moments of rotating molecules to align (or polarize)? Is this because the microwave source itself is polarized?