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 NH(3) sample, the main species ejected are NH(3), 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 NH(3) infrared absorption bands spectroscopy after a fluence of ~3 × 10(19) photons/cm(2). The results imply that ammonia-ice mixtures in the outer solar system are relatively stable under solar irradiation.