How does NMR work

  1. I've read two different explanations so I'm confused. The way I thought NMR works is that the protons spins align with the external magnetic field, then the RF radiation applied causes the spins to flip from -1/2 to 1/2, if the frequency of the radio waves are equal to the energy difference between these two spin states.

    Now I read a different explanation which has me confused. Here:
    http://www.simplyphysics.com/page2_4.html
    it says that the protons actually wobble around the axis of the external magnetic field, and when you apply radio waves with energy equal to the frequency of this wobbling (Larmor frequency), then the protons absorb it. Which is it? Is it the energy difference between the two spin states, or is it the energy of the Larmor frequency? According to this animation:
    http://www.youtube.com/watch?v=0YBUSOrH0lw&list=PLC40660311CD1C353
    the absorbed RF energy causes the magnetic vector axis to drop to the XY plane, so that it is now tangent to the axis of the external magnetic field. This clearly isn't the same thing as a spin flip, so I'm confused.
     
  2. jcsd
  3. DrDu

    DrDu 4,092
    Science Advisor

    Both descriptions are correct but it is not so easy to explain why without a basic understanding of quantum mechanics. The point is that as long as you don't explicitly analyze whether a given spin point up or down, it can also be in a superposition which corresponds to the spin pointing at least partially somewhere into the x-y plane. This is not an energy eigenstate, so it will evolve in time. Actually this turns out to a revolution of the axis of the spin in the x-y plane, i.e. the wobbling with the Lamor frequency. Now if the spin interacts with radiation you change that superposition and rotate the spin into or out of the x-y plane.
     
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