May 19, 2015
Prof. Louis Bourchard
Paper by Professor Louis Bouchard & PhD student Nanette Jarenwattananon selected by the journal Physical Review Letters as PRL Editors’ Suggestion. 
 
Titled “Motional Averaging of Nuclear Resonance in a Field Gradient” the paper was published in May 12, 2015 issue of Physical Review Letters (PRL), one of the most prestigious journals in the field of physics.
 
According to the journal’s website, papers that are judged to be particularly important, interesting, and well written are chosen as Editors’ Suggestions. These Suggestions are intended as a way to direct readers to Letters that lead them beyond their usual interests, into another area of research.
 
Co-authors graduate student Nanette Jarenwattananon and Professor Louis Bouchard.
 
The paper presents nuclear magnetic resonance (NMR) experiments that demonstrate the fundamental difference between liquids and gases, and amend the six-decades old theoretical description of self-diffusion effects in a magnetic-field gradient to include a correct description of gases. The decay of NMR signal in a magnetic field gradient is a flagship experiment to measure molecular self-diffusion. The theory, which was developed in the early 1950s by Hahn, Carr and Purcell, overlooked an important detail that manifests itself when examining the temperature dependence of the signal decay in gases.
 
A molecule within the bulk of a fluid undergoes a random motion during a process known as self-diffusion. After a certain amount of time, the molecule moves along a random direction. However, the nature and history of the path matters. In gases (lower image), the molecules move more quickly and take larger steps than the molecules in liquids (higher image). This back-and-forth motion leads to efficient averaging of the molecule’s position and longer nuclear spin decoherence times in magnetic field gradients at higher temperatures.
 
Because diffusion-weighted NMR has been the flagship experiment to measure molecular self-diffusion phenomena (as pioneered by Stejskal and Tanner in 1965), current methods that read out signals in the presence of magnetic field gradients will be impacted.  One such example is hyperpolarized lung MRI imaging.  Other impacted experiments include fundamental studies of fluids in porous media, single-sided portable NMR and low-field MRI. This phenomenon has already produced a useful technique for thermal mapping of gases during catalytic reactions [Nature 502, 537-540 (2013)].  The work is also projected to have impacts on the design of dynamic decoupling schemes for quantum computing.
 

The conventional theory of NMR linewidth in the presence of a magnetic-field gradient predicted that linewidth would increase with temperature.  However, the opposite behavior is observed experimentally in gases.  The revised theory correctly predicts the narrowing of nuclear resonance with temperature. The dependence on gradient strength (g2) remains the same.

To learn more, visit the Bouchard lab website here.

Top photo by Penny Jennings, UCLA Department of Chemistry and Biochemistry.