I decided to peek into the NMR section of my spectroscopy book, and I found out lots of interesting stuff. And along with the interesting stuff, I did come along some stuff that I didn't understand.

My book tells me this (not copied word-for-word):

Under the presence of a magnetic field, the two spin states of the nuclei split up, and if a correct wavelength of light is incident upon the sample, the nuclei get excited to a higher state.

In the above scenario, I believe light behaves as particles, wherein a photon of light collides with a nuclei, and push it to higher spin states.

There's another thing my book says:

Under the presence of a magnetic field, the nuclei begin to precess (wobble like a spinning top). If an incoming electromagnetic radiation has the same frequency as the nuclear precession, resonance occurs and the light is absorbed.

In this case, light behaves as a wave. It's the oscillating electromagnetic fields that are resonating the nuclei, and causing them to absorb the energy.

Out of the two cases that I mentioned, which is the one that actually happens? Is it resonance or spin excitation?

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    $\begingroup$ Both... Excitation depends on resonance, a wave-like property. But the photon is absorbed all at once, particle-like behavior. Such is the nature of quantum phenomena. "We are the watching of our lives." $\endgroup$ Jul 11, 2017 at 3:08

1 Answer 1


The nuclear spin states in an atom are excited by the radio frequency radiation and one spin state moves to the other when a photon is absorbed.

The precession part is not so much about individual spins but the magnetisation of the sample which is the vectorial addition of all the nuclear magnetic dipoles; precession is motion about the permanent magnetic field. (Think of the magnetisation rather like a vertical rotating pendulum if you want.) As circular polarised rf is used the spins are excited coherently and all add up 'in phase', hence a large magnetisation is produced that then undergoes precession. Loosing this phase in the magnetisation corresponds the the T2 decay time and loosing population excess above equilibrium to the T1 time. It happens that the precession frequency is the same as the transition frequency.

(An individual spin angular momentum is a complicated nutation type motion as it experiences a force from the rf field which is at right-angles to he permanent magnetic field.)

  • $\begingroup$ Thank you for your answer, but I have lots of questions. What are the T1 and T2 times? What is nutation? And what is an rf field. I'm really sorry to bug you, but I'm just a beginner in spectroscopy. $\endgroup$ Jul 11, 2017 at 15:06
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    $\begingroup$ Ok :) These are things that are in the textbooks and too long to answer here except rf field which just means the radio frequency radiation used to stimulate the nmr signal. $\endgroup$
    – porphyrin
    Jul 11, 2017 at 16:49

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