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.
http://www.chem.ualberta.ca/~jaeger/research/ftmw/ftmw.htm
How does microwave radiation cause the dipole moments of rotating molecules to align (or polarize)? Is this because the microwave source itself is polarized?