Rotational Study of Carbon Monoxide Solvated with Helium Atoms

High resolution microwave and millimeter-wave spectra of HeN-CO clusters with N up to 10, produced in a molecular expansion, were observed. Two series of $J=1--0$ transitions were detected, which correspond to the a-type and b-type $J=1--0$ transitions of He₁-CO. The B rotational constant initially decreases with N and reaches a minimum at $N=3$. Its subsequent rise indicates the transition from a molecular complex to a quantum solvated system already for $N=4$. For N≥6, the B value becomes larger than that of He₁-CO, indicating an almost free rotation of CO within the helium environment.