  Purely rotational transitions of a molecule, such as HCCCN, embedded in large
$^4$He clusters have been detected for the first time. The saturation behavior
shows that, in contrast to previous expectations, the microwave line profiles
are dominated by inhomogeneous broadening mechanisms. Spectral holes and peaks
produced by microwave-microwave double resonance have widths comparable to
those of single resonance lines, indicating that relaxation occurs among
quantum states of the inhomogeneous distribution of each rotational level, at a
rate approximately 10 times faster than rotationally inelastic relaxation.
