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

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 < N < 10(5)). 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 (N approximately 10(5)), 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 (N > 10(4)) are well reproduced by the simulations if cuboctahedral or octahedral rather than spherical aggregate shapes are assumed.