Coherent evolution of signal amplification by reversible exchange in two alternating fields (alt-SABRE).

Parahydrogen (pH 2 ) is a convenient and cost efficient source for magnetic resonance signal enhancement. Transient interaction of pH 2 with a metal organic complex in a signal amplification by reversible exchange (SABRE) experiment enabled more than 10% polarization for some 15 N molecules. Here, we analyzed a variant of SABRE, consisting of an outer magnetic field alternating between a low field of ~1 µT, where a polarization transfer takes place, and a higher field >50 µT (alt-SABRE). We found effects of both of these fields on amplitude and the frequency of polarization transfer. Deviation of a lower magnetic field from a "perfect" condition of level anti-crossing increases the frequency of polarization transfer that can be exploited for polarization of short-lived transient SABRE complexes i.e. some substrates. Moreover, the coherences responsible for polarization transfer at a lower field persisted during magnetic field variation and continued their spin evolution at higher field with a frequency of 2.5 kHz at 54 µT. The latter should be taken into consideration for an efficient alt-SABRE.