Quantum rotation of HCN and DCN in 4He -: art. no. 104517

We present calculations of rotational absorption spectra of the molecules HCN and DCN in superfluid He-4, using a combination of the diffusion Monte Carlo method for ground-state properties and an analytic many-body method (correlated basis function theory) for the excited states. Our results agree with the experimentally determined effective moment of inertia which has been obtained from the J=0-->1 spectral transition. The correlated basis function analysis shows that, unlike heavy rotors such as OCS, the J=2 and higher rotational excitations of HCN and DCN have high-enough energy to strongly couple to rotons, leading to large shifts of the lines and accordingly to anomalous large spectroscopic distortion constants, to the possibility of roton-maxon bands, and of secondary peaks in the absorption spectra for J=2 and J=3.