Classical control of quantum-state relaxation in confined molecular hydrogen

Nathan McLane; Richard C. Remsing; Leah G. Dodson

paper

Classical control of quantum-state relaxation in confined molecular hydrogen

arXiv:2611.09850

Abstract

Quantum-state conversion between the ortho and para nuclear-spin states of H $_{2}$ is forbidden in the isolated molecule but these states can become connected through interactions with a surrounding environment. How that same environment governs the dynamics of the newly connected states remains poorly understood. Here, we investigate this problem using H $_{2}$ confined in crystalline molecular solids, where the host environment selectively enables conversion between a pair of otherwise disconnected ortho-H $_{2}$ and para-H $_{2}$ states. Rotationally resolved infrared spectroscopy directly determines the state-specific conversion dynamics in crystalline CO $_{2}$ and N $_{2}$ O at 10 K, revealing that increasing H $_{2}$ concentration suppresses nuclear-spin conversion by nearly an order of magnitude, reaching complete suppression by 20% H $_{2}$ loading. Molecular dynamics and Monte Carlo simulations identify two complementary origins of this concentration dependence: changes in the thermally populated phonon modes available to mediate relaxation, and strain-driven H $_{2}$ clustering that progressively removes molecules from the host crystal field responsible for enabling conversion. The resulting picture connects a classical collective response of the host to microscopic quantum-state dynamics: H $_{2}$ loading generates strain that reorganizes the local molecular environment, thereby modifying both the phonon bath and the molecular coupling to that bath. These results establish a multiscale mechanism by which condensed-phase environments can govern the connectivity and dynamics of an effective two-level system.