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

TL;DR

This study investigates a novel alt-SABRE method that uses alternating magnetic fields to enhance and control nuclear spin polarization transfer, revealing effects of field variation on polarization amplitude, frequency, and coherence persistence.

Contribution

It introduces and analyzes a new alt-SABRE technique employing alternating magnetic fields, demonstrating effects on polarization transfer efficiency and coherence behavior.

Findings

Polarization transfer frequency increases with deviation from level anti-crossing.

Coherences persist during magnetic field variation and continue at higher fields.

Alt-SABRE can potentially improve transient complex polarization in NMR applications.

Abstract

Parahydrogen (pH2) is a convenient and cost efficient source for magnetic resonance signal enhancement. Transient interaction of pH2 with a metal organic complex in a signal amplification by reversible exchange (SABRE) experiment enabled more than 10% polarization for some 15N molecules. Here, we analyzed a variant of SABRE, consisting of an outer magnetic field alternating between a low field of ~1 \muT, where a polarization transfer takes place, and a higher field >50 \muT (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 \muT. The latter should be taken into consideration for an efficient alt-SABRE.