This is the article as published in Physical Review Letters. http://prl.aps.org/\ud \ud It is available at the publishers website at http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PRLTAO000095000021215301000001&idtype=cvipsWe have measured the high-resolution infrared spectrum of the radical NO in the 2Π1/2 state in superfluid helium nanodroplets. The features are attributed to the Λ doubling splitting and the hyperfine\ud 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\ud and phonon/roton excitations in superfluid 4He droplets
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