Knowledge Bank

The rotational spectra of He-cyanoacetylene clusters ($N$ = 1, 2,3, 4, 5..) have been measured in the 3-27 GHz range using a high-resolution Fourier transform microwave spectrometer. The weakly bound clusters were generated in a pulsed free jet expansion. Manipulating the sample and nozzle conditions allowed some degree of control over the successive solvation of the molecule with He-atoms. Both the HCCCN and DCCCN isotopomers have been studied, allowing information about cluster geometry and dynamics to be determined. The presence of $\mathrm{<mrow\; data-mjx-texclass="ORD">14</mrow>}$N and D nuclei in the clusters causes nuclear quadrupole hyperfine splitting of the He-HCCCN and He-DCCCN rotational transitions. This hyperfine structure was resolved, assigned, and analysed for low-$J$ transitions. The value of the rotational constants decreases towards the helium nanodroplet value with increasing cluster size for small clusters. A "turn-around" in rotational constant value is expected for $N$ $>$6.HyperfinefittingparametersandrotationalconstantswillbeinterpretedintermsofthestructureanddynamicsofHe$_N$-HCCCN clusters.