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?

  • $\begingroup$ The dipole moment in this context is related to the coherence between the rotational ground state and the excited state connected by the MW transition. So when almost all the molecules are in the ground state, there is no net dipole moment, and it is created when you fire a $\pi/2$-pulse that creates a superposition of ground and excited states. $\endgroup$ – wcc Apr 22 '19 at 22:29
  • $\begingroup$ Doesn't microwave absorption require a presence of a permanent dipole moment in a ground state? $\endgroup$ – M. Farooq Apr 23 '19 at 0:17
  • $\begingroup$ @M.Farooq, why do you think that is true? $\endgroup$ – wcc Apr 23 '19 at 1:15
  • $\begingroup$ @IamAStudent Condition of non-zero dipole moment is the gross selection rule for microwave spectroscopy. Without dipole moment, the electric field component of em wave cannot interact with the molecule. $\endgroup$ – Nim Apr 23 '19 at 1:24
  • $\begingroup$ @IamAStudent, I was commenting on this part "So when almost all the molecules are in the ground state, there is no net dipole moment." Could you clarify this point? $\endgroup$ – M. Farooq Apr 23 '19 at 1:37

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