# Meaning of polarization pulse in microwave (rotational) spectroscopy

In the recent rotational spectroscopy literature, "microwave polarization pulse" is often used in conjuction with pulsed Fourier transform microwave spectroscopy (Google Scholar).

Sometimes the pulse angle is also mentioned such as $$π/2$$. I gather this term is similar to FT-NMR where the net magnetization vector is tipped at a certain angle. When we talk about microwave rotational spectroscopy, what is being polarized?

In other words, why do we call the microwave pulse as a polarizing pulse? I wanted to confirm that this is related to the net electric dipole moment vector of the molecules which is being oriented after excitation?

## 1 Answer

In NMR the sum of the magnetic dipole vectors is called the magnetisation, and this is induced in the sample by the large fixed external magnetic field acting on the nuclear spins. As you mention it is influenced, i.e. moved about, by the magnetic field of the rf pulses applied to the sample and so ultimately cause the nmr signal. In optics the similar thing can occur where an intense optical field can align the electric dipoles and cause a polarisation, i.e. macroscopic dipole in the sample, this is the optical equivalent of magnetisation. The experiment is far more difficult to do in optics, however, as the dephasing time (T2 time in nmr) is several orders of magnitude faster than in nmr as is the lifetime of any level optically excited.

The field is generally called optical resonance spectroscopy and/or coherent transient spectroscopy with effects such as transient nutation, photon echos and self-induced transparency.

• Thanks. The technique I as specifically referring to is called FT-MRR, FT-Molecular Rotational Resonance Spectroscopy. You give a short chirped pulse and then monitor microwave emission by FID. I guess that also explains the word resonance. Do you recall a reference which talks about macroscopic dipole formation with reference to microwaves pulses? I could not find a definition per se that polarization = macroscopic dipole formation by an intense electric field? – M. Farooq Apr 23 at 12:15
• I'm not familiar with the particular technique you mention, but have a look at J. Steinfeld ' Molecules & Radiation' and W. Demtroder 'Laser Spectroscopy' for the background to all these types of experiment which is basically the same for all types of transitions. Conventional spectroscopy books do not generally cover the coherent optical processes you need to understand. – porphyrin Apr 23 at 12:36
• Thanks. I think the ideas must be similar to FT-NMR. – M. Farooq Apr 23 at 13:04
• Yes, thats right – porphyrin Apr 23 at 13:06