Infrared spectroscopy of solid mixed ammonia-water and acetylene-water aerosol particles

Mixed water aerosols are important components of planetary and lunar atmospheres. In this work, we use rapid-scan Fourier transform infrared (IR) spectroscopy to study solid ammonia-water and acetylene-water aerosol particles formed in a bath gas cooling cell at 78K. With this set-up, we record time-dependent extinction spectra of particle ensembles in the mid-IR to monitor changes to the internal structure of the aerosol particles. Both ammonia-water and acetylene-water were found to form molecularly mixed structures. The mixing is observed by monitoring the profile for the ammonia nu(2) band and the acetylene nu(5) band, both of which are sensitive to particle properties. Depending on the injection conditions, the mixed particles form either immediately after sample injection or after a short mixing period of several tens of minutes. We confirm the formation of mixed particles by comparing the experimental spectra with spectra calculated with the vibrational exciton model.