Overhauser and DNP

An interesting fact I just discovered whilst reading about dynamic nuclear polarisation on Wikipedia: apparently, when Albert Overhauser first described the process in his paper (which just talked about using DNP on copper centres in organo-metallic complexes, all the rage in the 50s and 60s), famous physicists like Norman Ramsey and Felix Bloch (and 'other renowned physicists of the time', Wikipedia) initially criticised Overhauser because they thought the process was 'thermodynamically improbable' (Wikipedia). Obviously their Force senses were clouded by the Dark Side since Overhauser essentially invented NMR and worked out its equations... :) and it works. Then when Carver and Slichter gave experimental confirmation, Overhauser apparently received an apology from Ramsey (via mail) in the same year! Fortunately, DNP most definitely does exist, I've witnessed it!

More facts: originally, the term 'dynamic' was used to represent the random and time-dependent nature of Overhauser cross-relaxation. It has been kept in the name so that we now have a cool acronym for my project! :)

What is this 'cross-relaxation' you may ask? (Ask, or else!) :).
Solution state DNP (i.e. liquid, so that molecules can move around a lot) works by coupling molecular motion to the relaxation energies of the electron spin. With a nucleus and an electron, your system has 4 levels, 2 corresponding to the nuclear spin transition and 2 corresponding to the electron spin transition. The nuclear spin transition is of much lower energy than the electron spin transition due to the smaller gyromagnetic ratio of nuclei compared with electrons. The process the works like this:
1. Nucleus and electron are in low energy state
2. Electron is excited via microwave radiation (the wavelength appropriate for the temperatures and fields of normal electron paramagnetic resonance).
3. Electron has two choices: it can decay back to its original state (with nuclear spin unchanged) or it can decay to a state where the nuclear spin changes from low to high energy ('cross-relaxation'); because the nuclear transition energy is smaller than the electron transition energy, these two choices are not much different in energy. This cross-relaxation can occur if the energy of the electron decay with the nuclear spin flip is equal to the energy of molecular motions (hence the need for solution state), and if the cross-relaxation process is faster than the other relaxation (no nuclear spin flip) the nuclei are polarised in the high-spin state.

Now you have 'hyperpolarised' nuclei! Hooray! After doing all of this work, we now have a greater nucleus spin population difference between low and high spin states, and so our NMR signal is enhanced (the Force is strong with this one). :)