Reversal of radio-frequency-driven spin diffusion by reorientation of the sample spinning axis

Abstract

The dipolar Hamiltonian in a rapidly rotating sample is scaled by the second Legendre polynomial of the cosine of the angle between the rotation axis and the static magnetic field. It is, therefore, possible to refocus the spatial polarization-transfer process, often termed spin diffusion, in extended spin systems by reorienting the rotor axis such that the dipolar interaction Hamiltonian changes sign. We present experimental results which demonstrate that a rapid mechanical sample reorientation leads to a time reversal of the ''radio-frequency-driven'' spin diffusion among 13 C spins.

Nuclear spin polarization transfer across an organic-semiconductor interface

“…The spin diffusion constant for 31 P in InP, D P , from the fitting routine is 3.2 ϫ10 Ϫ18 m 2 /s, in good agreement with our best estimate of 2.9ϫ10 Ϫ18 m 2 /s, which is based on D ͓111͔ from Ref. 27.…”

“…1͑b͒, 1͑c͒, and 1͑d͒. As s is increased, the intensity of the downfield peak decreases while the intensity of the upfield peak increases, its line shape approaches that of the bulk, and the overall spectral intensity remains approximately constant ͑reflecting the long 31 P T 1 of ϳ300 s).…”

“…Calculations of c based on a variety of methods have been performed, 23,[25][26][27][28] and a relatively precise experimental determination of D in CaF 2 has recently been reported. 29 The spin diffusion we wish to model occurs during Hartmann-Hahn cross polarization ͑CP͒, 30 used to connect 1 H in the surface species to 31 We model the flow of polarization from a thin 1 H rich surface layer into a semi-infinite 31 P substrate with a modified one-dimensional diffusion equation:…”

“…In that work a number of surface environments were distinguished, but no estimate of the efficiency of polarization transfer was reported. In this work, we present cross-polarization experiments demonstrating the transfer of nuclear spin-polarization between 31 P in micron-sized InP particles and 1 H and 19 F in surface-bound para-trifluoromethylbenzylic-ether ͑TFMBE͒. By modeling of the flow of nuclear spin polarization from the 1 H rich surface layer into the bulk InP, we show that spin diffusion combined with the relatively short 1 H rotating frame relaxation time (T 1 ) limits the total amount of spin polarization that may be transferred to approximately 20% of the total at room temperature.…”

Time-reversal of the evolution of a dipole-coupled, many-spin system under continuous resonant irradiation

“…Time-reversal is accomplished experimentally by manipulation of the nuclear spin Hamiltonian through the application of strong radio-frequency ͑rf͒ pulses 1-10 or a combination of rf pulses and sample rotations, 11 in accordance with the coherent averaging theory of Haeberlen and Waugh. 12,13 The earliest and simplest example of a time-reversal experiment in NMR is the spin echo experiment of Hahn, 1 in which a single rf pulse ͑optimally a pulse 14 ͒ applied at time has the effect of changing the sign of Hamiltonian terms proportional to S zk ͑the z component of spin angular momentum of spin k͒, such as resonance offset, chemical shift, and heteronuclear dipole-dipole coupling terms.…”