2009ApJ...703L.178M

The 6.85 µm band observed in the spectra of young stellar objects has been analyzed recently and the most usually accepted assignment to the ν₄ bending mode of NH₄⁺ is still under debate. We present here a laboratory study of frozen solutions of NH₄⁺Cl^– in water in an astrophysical range of concentrations and temperatures. The samples are prepared by hyper-quenching of liquid droplets on a cold substrate. The ν₄ band of NH₄⁺, which is very strong in the pure crystal and in the liquid solution at ambient temperature, becomes almost blurred in IR spectra of the frozen solution. The effect of the chlorine anion is expected to be of little relevance in this study. The experimental results are supported by theoretical calculations, which predict a broad range of weak ν₄ features for amorphous samples containing different ammonium environments. The present results indicate that the ammonium ion surrounded by water molecules only cannot suffice to explain this spectral feature. This paper contributes with new evidence to the discussion on the assignment of the 6.85 µm band.