Probing phonon-rotation coupling in helium nanodroplets: Infared spectroscopy of CO and its isotopomers.

We have recorded the R(0)vco=1←0 IR spectrum of CO and its isotopomers in superfluid helium nanodroplets. For droplets with average size N>~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 12C 16O 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 4He 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 moleculehelium 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.