A rotationally resolved infrared spectrum has been obtained for S${\mathrm{F}}_6$ molecules captured inside of helium clusters of approximately 4000 atoms. The spectrum is interpreted in terms of a spherical top Hamiltonian, although the resulting rotational constant is considerably smaller than that of free S${\mathrm{F}}_6$. This can be attributed to the interaction between the molecule and the surrounding helium liquid. The results indicate that the S${\mathrm{F}}_6$ is located inside the helium cluster and a fit to the spectrum yields a rotational temperatures for the perturbed S${\mathrm{F}}_6$ of $0.37\pm 0.05$ K.

• Received 3 April 1995

DOI:https://doi.org/10.1103/PhysRevLett.75.1566