Tuesday, August 7, 2012

Pulsed Dynamic Nuclear Polarization

High-field, solid-state DNP experiments typically rely on a cw mechanism such as the cross-effect, thermal mixing or the solid effect. However, there are some examples in the literature about pulsed DNP experiments. Most of the experiments are performed at low magnetic field strenghts (0.35 T up to 3.3 T) but there are some rare examples of high-field pulsed DNP experiments such as this one. It is a fairly old paper but still a very interesting one:

Un, S., et al., Pulsed dynamic nuclear polarization at 5 T. Chem. Phys. Lett., 1992. 189(1): p. 54-59.

A new dynamic nuclear polarization (DNP) technique utilizing high frequency, pulsed microwaves is described and demonstrated with observation of an Overhauser enhanced, 13C magic angle spinning nuclear magnetic resonance spectrum of a powdered sample of the organic conductor, fluoranthenyl hexafluorophosphate ((FA)2PF6). The spectrum was obtained at room temperature in a 5 T magnetic field, where the conduction electron spin resonance frequency is 140 GHz. A simple theory that accounts for the salient aspects of this pulsed DNP technique is presented. In systems where Overhauser mechanisms dominate the DNP, this new pulsed method can produce enhancements which are comparable to or greater than those obtained with conventional cw irradiation techniques, with lower average microwaves powers.