Editing Dynamic nuclear polarization (section)

===Overhauser effect===
DNP was first realized using the concept of the Overhauser effect, which is the perturbation of nuclear [[Spin (physics)|spin]] level populations observed in metals and free radicals when electron spin transitions are saturated by microwave irradiation. This effect relies on stochastic interactions between an electron and a nucleus. The "dynamic" initially meant to highlight the time-dependent and random interactions in this polarization transfer process.

The DNP phenomenon was theoretically predicted by [[Albert Overhauser]] in 1953 <ref name="Overhauser">
{{cite journal
|last =Overhauser
|first = A.W.
|title = Polarization of Nuclei in Metals
|journal = [[Phys. Rev.]]
|volume = 92 |pages = 411–415
|year = 1953
|doi = 10.1103/PhysRev.92.411
|issue = 2 |bibcode = 1953PhRv...92..411O
}}</ref>
and initially drew some criticism from [[Norman Ramsey]], [[Felix Bloch]], and other renowned physicists of the time on the grounds of being "thermodynamically improbable". The experimental confirmation by Carver and [[Charles Pence Slichter|Slichter]]<ref name="Slichter">
{{cite journal
| last1       = Carver
| first1      = T.R.
| last2 = Slichter
| first2 = C.P.
| date       = 1953
| title      = Polarization of Nuclear Spins in Metals
| journal    = [[Phys. Rev.]]
| volume     = 92
| issue      = 1
| pages      = 212–213
| doi      = 10.1103/PhysRev.92.212.2
| bibcode = 1953PhRv...92..212C
}}</ref> as well as an apologetic letter from Ramsey both reached Overhauser in the same year.<ref name="ramsey">[http://www.physics.purdue.edu/about_us/history/Albert_W_Overhauser.shtml Purdue University Obituary of Albert W. Overhauser] {{webarchive|url=https://web.archive.org/web/20060109074625/http://www.physics.purdue.edu/about_us/history/Albert_W_Overhauser.shtml |date=2006-01-09 }}</ref>

The so-called electron-nucleus cross-relaxation, which is responsible for the DNP phenomenon is caused by rotational and translational modulation of the electron-nucleus [[hyperfine coupling]]. The theory of this process is based essentially on the second-order time-dependent [[perturbation theory]] solution of the [[von Neumann equation]] for the spin [[density matrix]].

While the Overhauser effect relies on time-dependent electron-nuclear interactions, the remaining polarizing mechanisms rely on time-independent electron-nuclear and electron-electron interactions.