NMR - does rf cause transitions or simply rotate the magn vector?

[mrquantum]

Am I wrong in thinking that the rf radiation used in nmr is not "absorbed" by protons to cause transitions between spin states but rather it is only there to provide a magnetic field which can rotate the magnetisation vector away from the primary field?

 

[Mike H]

You can think that way. The issue is that the magnetization vector picture eventually loses the ability to really make sense of or devise the kinds of experiments people are interested in conducting.

One can get away with a fairly classical approach to magnetic resonance for spin-1/2 nuclei and imaging, to a large extent, but if, for example, one wants to work for my one former labmate - who does all kinds of stuff with quadrupolar nuclei in inorganic and biological solids, including dynamics and relaxation studies - just sticking with the vector model isn't going to cut it.

If you're interested in further reading, I'd suggest digging up the work of David Hoult on the nature of RF absorption and NMR over the last ~ 20 years. I can dig up the references, but I'll have to get back to you on that.

 

[marcusl]

mrquantum, despite your name you have confused the quantum mechanical and classical pictures of NMR. Quantum mechanically a photon is indeed absorbed by the nucleus, which makes a transition between states. In the classical view, RF radiation causes the spin to precess at the Larmor frequency about the direction of the static magnetic field such that its polar angle changes. Both views are correct within their model universes.

 

[mrquantum]

I don't understand how both can be correct. Is the RF radiation absorbed and re-emitted or does it pass straight through the sample.

I'm basically trying to understand how the magnetisation vector (along +z) gets tipped into the xy-plane, or, if you like, how the z component of each nuclei's spin angular momentum is transferred into spin angular momentum in the x and y directions.

What am I misunderstanding? Does anybody know any good articles that will elucidate the difference between the quantum mechanical and classical approaches to NMR?

 

[Ygggdrasil]

You may find the following thread to be useful: https://www.physicsforums.com/showthread.php?t=98310 especially my last post regarding the Bloch sphere.