Marco Tomaselli

Low field magnetic resonance images of polarized noble gases obtained with a dc superconducting quantum interference device

Using a low transition temperature superconducting quantum interference device as a detector, we have obtained magnetic resonance images of laser-polarized 3He gas and solid 129Xe at 4.2 K in magnetic fields as low as 0.54 mT (3He) and 1 mT (129Xe), corresponding to Larmor frequencies of 17.6 and 11.8 kHz, respectively. The experimental resolution of the images is ∼500 μm for 3He in the gas phase and ∼950 μm for 129Xe in the solid state.

NMR study of InP quantum dots: Surface structure and size effects

We report the results of 31P NMR measurements on trioctylphosphine oxide (TOPO) passivated InP quantum dots. The spectra show distinct surface-capping sites, implying a manifold of crystal–ligand bonding configurations. Two In 31P surface components are resolved and related to different electronic surroundings. With decreasing particle size the In 31P core resonance reveals an increasing upfield chemical shift related to the overall size dependence of the InP electronic structure.

TIME-REVERSAL OF CROSS-POLARIZATION IN NUCLEAR MAGNETIC RESONANCE

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.

NMR spectra with peaks at the principal values of the chemical shielding tensor

Abstract The NMR chemical shielding in a solid powder sample produces featured, but broad, lineshapes, or powder patterns, because the shielding experienced by a nuclear spin depends on the spatial orientation of its local molecular frame with respect to the external magnetic field. The lineshape, however, is fully determined by the three principal values of the shielding tensor. We present a simple approach that uses sample spinning NMR to extract peaks at the principal-value frequencies from chemical shielding powder patterns. Such techniques may simplify spectra with overlapping powder patterns without the information loss inherent in fast magic angle spinning. Experimental data and numerical simulations are presented for two 31 P model compounds.

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

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 13C spins.