High-resolution infrared spectra of carbonyl sulfide solvated with helium atoms

Abstract

Infrared spectra of HeN–OCS clusters with N up to about 20 have been studied in the 2062 cm−1 region of the O–C stretching vibration using a tunable diode laser spectrometer to probe pulsed supersonic expansions from moderately high-pressure (<35 atm) cooled (>−60 °C) jet sources. Resolved rotation-vibration transitions have been assigned for the clusters with N=2–8, and these assignments have been confirmed in detail by observations of the corresponding pure rotational transitions in the microwave region. The vibrational frequencies (band origins) were observed to move to higher frequencies (blueshift) for N=2–5, continuing the shift previously observed for the binary complex, He–OCS. Then, for N=6–8, the vibrational frequency moves back in the direction of lower frequencies (redshift), leading towards the limiting redshift previously observed in larger helium nanodroplets with N≈103–104. These vibrational shifts are consistent with a model in which the first five helium atoms fill a “ring” around the “equator” of the OCS molecule, with the subsequent heliums then taking positions closer to the ends. The cluster rotational constants decrease monotonically (increasing rotational moments of inertia) for N=1–8, falling below the value previously observed for the “free” rotation of OCS in helium nanodroplets. Strong, sharp spectral lines from clusters in the size range N≈9–20 were also observed, but not assigned. This difficulty in extending the analysis for N>8 may be related to the onset of partially free internal rotation of the OCS within the helium cluster, which in turn has interesting implications for the nature of superfluid-type behavior in this finite scale system.