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 (2)Pi(1/2) state in superfluid helium nanodroplets. The features are attributed to the Lambda-doubling splitting and the hyperfine structure. The hyperfine interaction is found to be unaffected by the He solvation. For the Lambda-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 He-4 droplets.