Infrared spectroscopy and vibrational exciton modeling of crystalline, polycrystalline and amorphous acetylene aerosol particles.

Mid-infrared spectra of acetylene aerosol particles generated under conditions relevant to Titan's atmosphere are analyzed in terms of the vibrational exciton model. The analysis reveals that acetylene aerosol particles do not form single crystals below 110 K as previously assumed. Instead, less ordered structures such as polycrystalline particles with orthorhombic crystalline domains or possibly partially amorphous particles derived from an orthorhombic crystal structure are found to be very stable. Annealing of these particles leads to the formation of crystalline particles with an orthorhombic crystal structure. Particles with a cubic crystal structure were never observed at these temperatures in agreement with the phase behavior of bulk acetylene.