Coherent Rotational and Vibrational Raman Spectroscopy of CO₂ Clusters

Abstract

Coherent Stokes Raman scattering (CSRS) has been employed to study the rotational and vibrational spectroscopy of carbon dioxide clusters in the expansion of a supersonic jet. In the low-frequency range, around 72 and 88 cm⁻¹, the cluster spectrum features two peaks which can be assigned to librational modes of symmetry E_g and F_g−. This observation indicates that the gas-phase CO₂ clusters adopt the structure of crystalline CO₂. Comparison with the data obtained in solid CO₂ shows that the cluster librations are in thermal near-equilibrium with the monomer rotations. The vibrational spectroscopy of CO₂ clusters has been studied in the (ν₁, 2ν₂) Fermi dyad. In the ν₁ component a strong cluster peak, red-shifted by 9.7 cm⁻¹ from the monomer line, is observed which can be assigned to large CO₂ clusters with crystalline structure. Weaker and less red-shifted spectral features, significant at milder expansion conditions, are attributed to small CO₂ clusters, i.e. dimers, trimers, and tetramers.