Electric-field-induced nuclear-spin flips mediated by enhanced spin–orbit coupling

Abstract

Molecular hydrogen exists in two forms, which differ in the relative orientation of their nuclear spins. Interconversion between the two isomeric forms is extremely rare, unless there is an interaction breaking the symmetry between the two nuclei. Magnetic surfaces are known to act as such a catalyst, but this study finds that electric fields can also induce the spin flips necessary for the interconversion. Molecules made of identical nuclei of non-zero spin exist in nuclear-spin modifications, and the interconversion of these spin isomers is often forbidden for isolated states1,2,3. The interconversion between the nuclear-spin modifications, however, is promoted by inhomogeneous magnetic fields, such as those present on the surfaces of magnetic materials4. Nuclear-spin conversion on diamagnetic and insulating solid substances, on the other hand, is generally considered improbable. Here we present the observation of nuclear-spin flips of H2 and D2 occurring on amorphous solid water surfaces with time constants of 370−140+340 s and 1,220−580+2,980 s, respectively. To explain these unexpected conversion processes, we propose a model of electric-field-induced nuclear-spin flips. In this model, giant and inhomogeneous electric fields present on the ice surface5 mix the electronic states of opposite parities by the Stark effect6, and significantly enhance the spin–orbit couplings between the electronic singlet–triplet spin states of the molecules. By virtue of these effects, the intramolecular hyperfine contact interaction induces the nuclear-spin conversion. This concept should have implications for controlling nuclear magnetization using external electric fields7.