More spins for protons

Protons have an intrinsic spin or angular momentum that becomes important when the proton interacts with other polarized particles or with an electromagnetic field. Particle physics experiments with polarized protons require high magnetic fields (2.5 to 5 tesla) and extremely low temperatures (0.3 to 1 K) to reach 70% polarization. In contrast, magnetic resonance imaging techniques that operate at room temperature and magnetic fields of 1 tesla only achieve 0.0003% polarization. The production of polarized protons in a modest magnetic field (0.3 tesla) at liquid-nitrogen temperatures (77 K) has now been reported by Iinuma et al. (Phys. Rev. Lett. 84, 171–174; 1999).

Iinuma et al. achieve high levels of proton polarization (up to 32%) by doping their materials with a small amount of pentacene. Electrons in pentacene can be excited to higher states (S′ and S₁ in the figure) with a laser beam. Around 2% of the electrons transfer from S₁ to the lowest triplet state T₀. Using microwaves tuned to the frequencies of the electron and proton spins, the electron polarization is transferred to nearby protons. Electrons in the triplet state decay into the ground state, where the proton spin remains polarized.

Early experiments at liquid-nitrogen temperatures polarized just 13% of the protons (the theoretical limit is 73%). Iinuma et al. trebled this using a powerful pulsed dye laser (rather than a nitrogen laser) to optically excite the electrons to the first excited state of pentacene. The authors suggest that their technique might enhance the sensitivity of NMR at higher temperatures as well as having an impact on particle physics.