Shape and Width of IR Absorption Lines of Ammonia Expanded in a Supersonic Jet

Line-width and -shape of absorption lines of the vz vibration of ammonia were measured for a series of transitions starting from vibrational groundstate levels with a different rotational energy. An increase in linewidth was found for larger rotational energies of the initial states. This effect has been attributed to the spatial rotational non-equilibrium in the supersonic jet expansion. For expansions of NH3 seeded in He and Ar a very pronounced dip in the absorption profile has been observed, due to the formation of clusters.