Direct FTIR high resolution probe of small and medium size Arn(CO2)m van der Waals complexes formed in a slit supersonic expansion

A slit nozzle continuous expansion of argon seeded with carbon dioxide is probed using a Bruker IFS 120 HR FTIR spectrometer operating at 0.05 cm⁻¹ or 0.01 cm⁻¹ spectral resolution. About 250 individual rovibrational transitions are assigned which belong to the asymmetric stretch of the CO₂ moiety in Ar-CO₂ and (CO₂)₂ complexes. This made it possible to refine the set of spectroscopic constants characterizing these van der Waals species. Analysis of the observed spectral features allowed for evaluation of the number densities of complexes formed in a jet. The manifold of rovibrational lines of van der Waals complexes (along with the monomer lines) sits on an unresolved pedestal, the shape of which varies dramatically as a function of reservoir pressure and initial CO₂-Ar mixing ratios. Thorough examination of these variations allows for the observed features to be assigned to Ar_n(CO₂)_m clusters formed in CO₂ seeded Ar expansion.