Magnetic control of coherent electron spin dynamics in room-temperature liquids
Nan Yang, Hohjai Lee
We are working on a new class of spectroscopies in which the 3-dimensional shape of the electromagnetic field is engineered to induce qualitatively new physical effects in molecules. By sculpting the electromagnetic field we hope to create photochemical reactions in which the outcome is exquisitely sensitive to an externally applied magnetic field.
This project is based on the fact that two electrons can only form a chemical bond if their spins are oppositely aligned (technically, in a spin singlet state). Magnetic fields cause electron spins to reorient, and thereby can control the outcome of some chemical processes. In some photochemical reactions, the initial photon absorption sends two electrons to distinct molecules, or to distant parts of one molecule. These electrons are decoupled from each other, but their spins are still entangled. If the electrons start in a singlet state and remain in a singlet state, they can recombine to form a bond. But if one spin flips relative to the other, the electrons enter a triplet state and can no longer recombine.
A relative spin reorientation occurs when the electrons experience different effective local magnetic fields. Well-separated electrons are each coupled to a distinct set of atomic nuclei. Nuclear hyperfine couplings lead to distinct random fields, which induce relative spin reorientation. An external field locks the electron spins to the field axis, and thereby slows the relative reorientation. We realized that magnetic nanostructures can create magnetic fields that vary strongly over molecular dimensions, so magnetic nanostructures can enhance the rate of spin reorientation.
|This image shows the trajectories of many electron-spin pairs, where each electron precesses about a random field direction (black lines).|
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- Nan Yang, Yiqiao Tang and Adam E. Cohen, Spectroscopy in sculpted fields, Nano Today, 4, 269-279, 2009.