We have recorded the R(0)nu(CO) = 1 <- 0 IR spectrum of CO and its isotopomers in superfluid helium nanodroplets. For droplets with average size N greater than or similar to 2000 helium atoms, the transition exhibits a Lorentzian shaped linewidth of 0.034 cm(-1), indicating a homogeneous broadening mechanism. The rotational constants could be deduced and were found to be reduced to about 60% of the corresponding gas-phase values (63% for the reference C-12 O-16 species). Accompanying calculations of the pure rotational spectra were carried out using the method of correlated basis functions in combination with diffusion Monte Carlo (CBF/DMC). These calculations show that both the reduction of the rotational B constant and the line broadening can be attributed to phonon-rotation coupling. The reduction in B is confirmed by path integral correlation function calculations for a cluster of 64 He-4 atoms, which also reveal a non-negligible effect of finite size on the collective modes. The phonon-rotation coupling strength is seen to depend strongly on the strength and anisotropy of the molecule-helium interaction potential. Comparison with other light rotors shows that this coupling is particularly high for CO. The CBF/DMC analysis shows that the J = 1 rotational state couples effectively to phonon states, which are only present in large helium droplets or bulk. In particular, they are not present in small clusters with n <= 20, thereby accounting for the much narrower linewidths and larger B constant measured for these sizes

We have measured the high-resolution infrared spectrum of the radical NO in the 2 1=2 state in superfluid helium nanodroplets. The features are attributed to the -doubling splitting and the hyperfine structure. The hyperfine interaction is found to be unaffected by the He solvation. For the -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 4 He droplets. DOI: 10.1103/PhysRevLett.95.215301 PACS numbers: 67.40.Fd, 33.15.Pw, 33.20.Ea, 33.20.Wr Spectroscopy of molecules in He nanodroplets is a very promising technique for investigating chemical reactions at ultralow temperatures. In such reactions, radicals play an important role [1]. It is, therefore, of interest to probe the influence of the He environment on a radical. While at present quite a number of closed shell molecules have been studied in He droplets [2], only little is known about the influence of the helium droplets on the spectra [3] and the electronic structure of radicals. The infrared spectra of many radicals exhibit hyperfine splitting and -type doubling. These effects are well known in the gas phase but have never been studied in solvents.In this Letter, we report the measurement of the IR spectrum of NO in He droplets. NO is an open shell molecule and has a 2 1=2 electronic ground state. The rotational levels of NO show hyperfine structure and -type doubling. While the hyperfine splitting reflects the magnetic interaction, -type doubling arises from rotational interaction of the ground state with higher electronic states. The coupling to the higher electronic or ÿ states splits the degenerate energy levels of the 2 1=2ground state. We will show that the droplets increase the magnitude of the -type doubling and discuss the implications for the electronically excited states of NO within the He solvent. The lower rotational energy levels of NO lie in the interesting region of the collective excitations of superfluid He. In particular, they are close to the roton excitations. NO is therefore a good candidate to investigate the role of these excitations in the rotational energy relaxation or in the shift of the rotational levels, as recently pointed out by theory [4,5]. For this purpose, NO as a diatomic molecule is ideal because it has only a single vibrational mode. The vibrational energy is much larger than the collective excitations of liquid He (1875 cm ÿ1 10 cm ÿ1 ). Therefore, vibrational relaxation is extremely slow.The experiments have been carried out using a new apparatus at Bochum. The setup is similar to that used by Hartmann et al. and explained in detail elsewhere [5,6]. Here we give only a brief description. Helium is expanded at 40 ...

The authors have recorded the 3 infrared spectrum of methane in helium nanodroplets using our cw infrared optical parametric oscillator. In a previous paper, Nauta and Miller ͓Chem. Phys. Lett. 350, 225 ͑2001͔͒ reported the observation of the monomer rovibrational transitions of methane in helium nanodroplets. Here, they report the observation of additional absorption bands in the frequency range between 2990 and 3070 cm −1 blueshifted compared to the monomer transitions. They attribute these absorption features to phonon wings of individual rovibrational transitions, i.e., the simultaneous excitation of collective excitation modes of the quantum fluid and the rovibrational excitation of the methane monomer in the helium nanodroplet.