Interchange-Tunneling Splitting in HCl Dimer in Helium Nanodroplets

Infrared spectra of HCl dimers have been obtained in helium nanodroplets. The splitting in the vibrationally excited state of the bonded H-Cl stretching band ($v_{2})$ in (H$^{35}$Cl - H$^{37}$Cl) dimers was obtained to be 2.7 cm$^{-1}$ as compared to 3.7 cm$^{-1}$ in free dimer. From the splitting, the strength of the interchange-tunneling interaction in liquid helium was obtained to be 0.85 cm$^{-1}$, which is about a factor of two smaller than in the free dimer. The results are compared with the previous spectroscopic study of (HF)$_{2}$ in He droplets as well as to the theoretical study of (HF)$_{2}$ and (HCl)$_{2}$ dimers in small He clusters.