Radiofrequency transmitter in an NMR experiment: Is there an involvement of (electromagnetic) radio wave? A student in Chemistry StackExchange asked the effect of radio waves on matter, which led to an interesting set of arguments by some users. It was pointed out that in a proton nuclear magnetic resonance (or any other nuclei for that matter), there is no absorption of the photon corresponding to the radio waves. The following reference was cited NMR signal reception: Virtual photons and coherent spontaneous emission which says (in the abstract)


  In portions of the magnetic resonance community, there is a misunderstanding of the process of nuclear magnetic resonance (NMR) signal generation and reception, and even in accepted texts, it is frequently described in terms of absorption and emission of radio waves, or radiation, by a two‐level quantum system. While this explanation can be refuted, for those who do understand that the NMR free induction decay signal is easily explained by Faraday's law of induction, reconciling the presence of an induced electromotive force with an apparent absence of transitions between nuclear energy levels causes conceptual problems.


I had never heard of emission of (electromagnetic radio waves) by the precessing nuclei in an NMR experiment, but a Google search of NMR + "absorption of radio waves" shows the concept of radio wave absorption is very common.

This leads to my main question: What does the RF transmitter do in the NMR experiment and what is practically happening in the RF coil?

(a) My understanding was that the RF transmitter, emits (electromagnetic) radio waves. If the RF coil is being energized with AC current of MHz frequencies, isn't the coil emitting electromagnetic radiation in MHz range? The magnetic field component of the radio waves interacts with the nuclei magnetic dipole. 

(b) Another user stated "The "transmitter" powers the pulsed radio frequency B1 field. It never decouples from the probe coil to become electromagnetic radiation, i.e. actual photons. " Although this word "decouple" is more of a play of words, can an isolated oscillating B exist without a corresponding oscillating electric field?

I thought to learn from physical chemists and engineers who will have a better picture of the RF transmitters used in the NMR than us analytical chemists. My key question is related the Rf transmitter, not FID.

Thanks.
I finally asked an NMR spectroscopist who was the editor of the Journal of Magnetic Resonance and knows the inside out of NMR spectrometers. He sent a scanned copy of Hoult's paper from 1989. The title explains it all "The magnetic resonance myth of radio waves
." Surprisingly very low citations (15). It is amazing how much nonsense we have been fed with radio waves absorption. Sometimes innocent student questions on these forums can lead to introspection.

Here is the article Radio Waves Myth in NMR. Karl will like it.

The conclusion is that there is no involvement of radio waves, neither in absorption nor in emission (of course). The emission process is easier to understand but the transmission process seems to have confused even the experienced spectroscopists and more MRI people.

This is from another paper of Hoult. "The Origins and Present Status of the Radio Wave Controversy in NMR"


  The origins, history, and present status of the controversy
  surrounding a quantum description of the NMR signal as being due to
  radio waves are traced. With the Principle of Relativity and Coulomb’s
  Law as formal starting points and the minimum of mathematics needed
  for understanding, the derivation of a classical electromagnetic
  theory of signal reception is first given. The agreement between that
  classical theory and a recent NMR experiment is then presented,
  leading to proof that, except for the highest field imaging
  experiments, there is no significant contribution of radio waves to
  the signal.