IR signature of (CO2)N clusters: size, shape and structural effects

Abstract

The structure of carbon dioxide aggregates is investigated by means of direct absorption IR specroscopy in the region of the antisymmetric stretching vibration v₃. The (CO₂)_N particles are generated under dynamic (supersonic cooling in Laval nozzles) and static (collisional cooling cells) conditions over a broad mean size range (20 < < 10⁵). The vibrational exciton approach is used to interpret the observed spectral features. The particles generated by supersonic cooling remain globular in shape even for the largest explored aggregate sizes ( ≈ 10⁵), thus highlighting the absence of agglomeration between primary clusters under our jet conditions. This is in contrast to collisional cooling where the primary particles strongly agglomerate after a few seconds. The spectra for the larger particles ( > 10⁴) are well reproduced by the simulations if cuboctahedral or octahedral rather than spherical aggregate shapes are assumed.