While gas-phase and surface reactions on bare carbonaceous or silicaceous dust grains contribute to cosmic chemistry, energetic processing via photochemistry and radiation chemistry of cosmic ices is thought to be the dominant mechanism for the cosmic synthesis of prebiotic molecules. Radiation chemistry is defined as the “study of the chemical changes produced by the absorption of radiation of sufficiently high energy to produce ionization.” Ionizing radiation in cosmic chemistry includes high-energy particles (e.g., cosmic rays consisting mostly of protons) and high-energy photons (e.g., extreme-UV (12.4 –124 eV), X-rays and g- rays). In contrast, photochemistry is defined as photon-induced electronic excitation not involving ionization. In addition to photochemistry, vacuum-UV (6.2 –12.4 eV) light may initiate radiation chemistry because the condensed matter work function is lower than the gas-phase ionization energy for a given molecule. For example, the work function of amorphous ice (the main constituent of cosmic ices) is ~ 8.7 eV, which is smaller than the gas-phase ionization energy of 12.6 eV for water. Because most previous astrochemical studies have used light sources that produce &gt; 8 eV photons, discerning the role of photochemistry vs. radiation chemistry in astrochemistry is challenging. By using a source whose photon energy does not exceed 7 eV, we have studied ammonia cosmic ice reactions attributable solely to photochemistry. We will compare and contrast these results to those obtained in the same ultrahigh vacuum chamber with 1 keV electrons which initiate radiation chemistry in condensed ammonia.

Non-Ionizing UV (< 7 eV) photochemistry of cosmic ice analogs of ammonia

253rd National Meeting (2017)

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