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, H₂, and N₂, 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 H₂ and N₂ 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 ∼3 × 10¹⁹ photons/cm². The results imply that ammonia-ice mixtures in the outer solar system are relatively stable under solar irradiation.