Photolysis of solid NH3 and NH3-H2O mixtures at 193 nm

We have studied UV photolysis of solid ammonia and ammonia-dihydrate samples at 40 K, using infrared spectroscopy, mass spectrometry, and microgravimetry. We have shown that in the pure NH3 sample, the main species ejected are NH3, H-2, and N-2, where the hydrogen and nitrogen increase with laser fluence. This increase in N-2 ejection with laser fluence explains the increase in mass loss rate detected by a microbalance. In contrast, for the ammonia-water mixture, we see very weak signals of H-2 and N-2 in the mass spectrometer, consistent with the very small mass loss during the experiment and with a <5% decrease in the NH3 infrared absorption bands spectroscopy after a fluence of similar to 3 x 10(19) photons/cm(2). The results imply that ammonia-ice mixtures in the outer solar system are relatively stable under solar irradiation. (C) 2010 American Institute of Physics. [doi:10.1063/1.3506577]