High resolution infrared spectra of ${\mathrm{He}}_N\mathrm{\text{−}}{\mathrm{CO}}_2$ clusters with $N$ up to 17 have been studied in the region of the ${\mathrm{CO}}_2$ ${\nu }_3$ fundamental band. The $B$ rotational constant initially drops as expected for a normal molecule, reaching a minimum for $N=5$. Its subsequent rise for $N=6$ to 11 can be interpreted as the transition from a normal (though floppy) molecule to a quantum solvation regime. For $N>13$, the $B$ value becomes approximately constant with a value about 17% larger than that measured in much larger helium nanodroplets. Quantum Monte Carlo calculations of pure rotational spectra are in excellent agreement with the measured $B$ in this size range and complement the experimental study with detailed structural information. For a larger cluster size ($N=30–50$) the simulations show a clear sign of convergence towards the nanodroplet $B$ value.

• Received 27 November 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.145503