Isotope effects in the infrared spectra of OCS-He complexes and clusters

Infrared spectra of the OCS-He van der Waals complex and of OCS-He(N) clusters have been studied in the region of the OCS nu1 fundamental band using a tunable diode laser to probe a pulsed supersonic slit jet. For the complex, the spectrum of the normal isotope, 16O12C32S-4He, has been considerably extended and the 34S- and 13C-substituted forms have been recorded for the first time. The data could be analyzed satisfactorily using a conventional asymmetric rotor Hamiltonian with sextic centrifugal distortion terms. For the clusters, the 34S- and 13C-substituted forms have been observed and assigned for N = 2-7, including some transitions with higher J values than previously reported for the normal isotope, e.g., R5. The observed vibrational shifts, relative to the free OCS molecule, were very similar to those of the normal isotope, and most of the difference could be explained by simple scaling. These results constitute a subtle and precise probe of intermolecular forces and dynamical effects in a system which is of current interest for cluster studies.