High-Resolution Spectroscopy of NO in Helium Droplets: A Prototype for Open Shell Molecular Interactions in a Quantum Solvent

We have measured the high-resolution infrared spectrum of the radical NO in the ²Π_1/2 state in superfluid helium nanodroplets. The features are attributed to the Λ-doubling splitting and the hyperfine structure. The hyperfine interaction is found to be unaffected by the He solvation. For the Λ-doubling splitting, we find a considerable increase by 55% compared to the gas phase. This is explained by a confinement of the electronically excited NO states by the surrounding He. The rotational level spacing is decreased to 76% of the gas phase value. The IR transition to the J=1.5 state is found to be homogeneously broadened. We attribute both observations to the coupling between the molecular rotation and phonon/roton excitations in superfluid ⁴He droplets.