  Rotational absorption spectra of acetylene in superfluid $^4$He are
calculated using a combined correlated basis function-diffusion Monte Carlo
method (CBF/DMC) as well as a numerically exact path integral correlation
function approach (PICF). Both approaches predict anomalously large distortion
constants in addition to reduced rotational constants, and the higher
rotational excitations are seen to be accompanied by an additional peak and
absorption band. The CBF analysis shows that these phenomena are due to strong
coupling of the higher rotational states with the roton and maxon excitations
of $^4$He, and the assocated divergence of the helium density of states in this
region.
