Laudatio 2004

Awarded Contribution:

E. L. Hahn, Spin Echoes, Bull. Am. Phys. Soc. 24, No. 7, 13 (1949), reprinted in Phys. Rev. 77, 746 (1950).
(This is the abstract for a ten minutes presentation to be given at the Chicago meeting of the American Physical Society on November 25, 1949.)

The Prize Winner:

Erwin L. Hahn, Professor Emeritus, University of California, Berkeley, USA.

The Technologies:

The awarded contribution contains several original ideas and results that have had a strong impact on modern NMR technology, notably

• the two pulse spin echo that still is the method of choice for e.g. refocusing chemical shift dephasings in  pulse sequences, not to mention widespread applications in MRI;
• the interpretation of spin echoes, where time (rather than frequency) is used as the essential variable beyond the initial stage of Bloch's theory of CW spectroscopy and of relaxation measurements: this spin dynamics method was immediately essential for the development of spin echo applications, and it is still today the theoretical approach used for most NMR techniques;
• the experimental demonstration that the observation of NMR pulse responses is a viable technology that can provide higher sensitivity than CW spectroscopy.

The awarded contribution clearly was the foundation for the more extensive description of spin echoes in E. L. Hahn, Spin Echoes, Phys. Rev. 80, 580-594 (1950), that was submitted six months after the lecture at the Chicago meeting, where further high-impact ideas related to spin echoes were presented:

• the study of molecular diffusion and bulk motion by observation of their effects on the spin echoes: with minor modifications, this is still the method of choice for accurate measurements of molecular diffusion coefficients in liquids and for flow measurements in general;
• the study of "secondary" spin echoes after three pulses, another step towards multiple-pulse techniques;
• the observation of a modulation of the peak spin echo amplitudes in some homonuclear spin systems and the conclusion that the modulation cannot be explained by differences in chemical shifts, hence that it indicates a new spin-spin coupling not averaged out by molecular motion. This proved later to be J couplings. It also showed that multiple-pulse spectroscopy provides important qualitative information that was not directly available by CW techniques;
• the description and use of a coherent pulse spectrometer including a CW reference oscillator at the NMR frequency, hence control of the phase of the pulses and observation of the phase of the spin responses: the basic elements of modern pulse spectrometers are presented here for the first time.