@article {398,
	title = {Time reversal of cross-polarization in solid-state NMR},
	journal = {Molecular Physics},
	volume = {95},
	year = {1998},
	note = {Mol Phys146XYTimes Cited:4Cited References Count:21},
	month = {Dec 10},
	pages = {849-858},
	abstract = {<p>Cross-polarization at the Hartmann-Hahn condition in solid-state NMR frequently is described in terms of thermodynamics. Spin temperatures characterizing the canonical density operator are assigned to the Zeeman reservoirs of the two spins and the cross-polarization process brings about a state of equilibrium of the two reservoirs with a common temperature. In such a model, cross-polarization from an initially polarized spin species (I spins) to another spin species (S spins) is inherently an irreversible process accompanied by an increase in the entropy of the system. However, a cross-polarization echo can be generated whereby the polarization transferred to the S spins returns to the I spins, restoring the initial density operator. Therefore a thermodynamic description should be applied with care even in samples where the build-up and the decay of the magnetization can be approximated well by multiexponential processes. Such cross-polarization echoes are formed by the consecutive application of two pulse trains that produce effective Hamiltonians differing in sign. The \&$\#$39;time reversal\&$\#$39; of cross-polarization is consistent with both the increase in Zeeman entropy during the approach to equilibrium and with the constraint of unitary quantum evolution.</p>
},
	keywords = {echoes},
	isbn = {0026-8976},
	doi = {Doi 10.1080/002689798166477},
	url = {<Go to ISI>://WOS:000077460200014},
	author = {Ernst, M. and Meier, B. H. and Tomaselli, M. and Pines, A.}
}
@article {397,
	title = {Time-reversal of cross-polarization in nuclear magnetic resonance},
	journal = {Journal of Chemical Physics},
	volume = {108},
	year = {1998},
	note = {J Chem Phys108FUTimes Cited:11Cited References Count:17},
	month = {Jun 15},
	pages = {9611-9613},
	abstract = {<p>It is demonstrated that the time evolution of the heteronuclear polarization-transfer process in a dipolar-coupled nuclear spin system can be reversed, leading to the observation of cross polarization echoes. The cross-polarization echoes are induced by consecutive application of two pulse trains that produce effective Hamiltonians that differ only in sign, Cross-polarization echoes have been recorded for a powder sample of alanine. The time evolution of the spin;system is consistent with both unitary quantum dynamics and with spin thermodynamics of two systems approaching a common spin temperature. (C) 1998 American Institute of Physics.</p>
},
	keywords = {echoes},
	isbn = {0021-9606},
	doi = {Doi 10.1063/1.476435},
	url = {<Go to ISI>://WOS:000075256100001},
	author = {Ernst, M. and Meier, B. H. and Tomaselli, M. and Pines, A.}
}
