Roton-rotation coupling of acetylene in 4He

Rotational absorption spectra of acetylene in superfluid He-4 calculated using a path-integral correlation function approach are seen to result in an anomalously large distortion constant in addition to a reduced rotational constant, with values in excellent agreement with recent experiments. Semianalytic treatment of the dynamics with a combined correlated basis function-diffusion Monte Carlo method reveals that this anomalous behavior is due to strong coupling of the higher rotational states of the molecule with the roton and maxon excitations of He-4, and the associated divergence of the He-4 density of states in this region.