Mechanism of spin thermalization for high-spin Na2 on helium nanodroplets

Determine the microscopic mechanism responsible for the electron spin thermalization of metastable spin-triplet sodium dimers (Na2, S=1) residing on the surface of superfluid helium nanodroplets at cryogenic temperatures, and ascertain whether molecular spin-rotation coupling is the dominant pathway for dissipating the spin Zeeman energy in this non-magnetic environment.

Background

The authors measure Stern–Gerlach deflection of sodium-doped helium nanodroplets and find that Na2 molecules on droplet surfaces exhibit one-sided deflection, indicating their spins are oriented along the external magnetic field and have thermalized with the droplet temperature (~0.37 K). This behavior aligns with previously reported sub-microsecond spin relaxation times for alkali dimers and trimers on nanodroplet surfaces.

Despite this empirical evidence of rapid spin relaxation, the specific microscopic mechanism by which the spin Zeeman energy is dissipated within the non-magnetic helium environment remains unspecified. The authors suggest molecular spin-rotation coupling as a likely pathway and note the need for theoretical guidance on the rates and pathways of spin relaxation in molecules on and within superfluid matrices and nanodroplets.