An Overview of Ionization of Organics in Water Ices: Astrophysical Implications

Details An Overview of Ionization of Organics in Water Ices: Astrophysical Implications An Overview of Ionization of Organics in Water Ices: Astrophysical Implications

May 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008aas...212.0329g&link_type=abstract

American Astronomical Society, AAS Meeting #212, #3.29; Bulletin of the American Astronomical Society, Vol. 40, p.189

Physics

Scientific paper

Water-rich ices are ubiquitous throughout our solar system and interstellar medium. While interstellar ices (DMCs, ISM, circumstellar disks etc) are mostly amorphous and exist at temperatures as low as 10 K, majority of Solar System ices (comets, icy satellites, Mars and Earth's polar regions, KBOs etc) are crystalline and occur at much higher temperatures (>100; K) with minor amorphous components. Our recent findings have shown that ionization of impurities, in the present case polycyclic aromatic hydrocarbons (PAHs), is the most prominent process in both amorphous and crystalline ices under radiation. These studies have further shown that:
• PAHs embedded in cryogenic ice are easily and efficiently ionized (>80%, i.e., near quantitative ion yields) to the cation form by VUV photons.
• PAH ionization energy is lowered by up to 2 eV compared to the gas-phase, in agreement with recent theoretical predictions.
• Some of the PAH cations are stabilized in these ices to temperatures as high as 120 K.
• Multiple ionization of aromatic molecules to generate closed-shell PAH dications occurs in water ice, generating and stabilizing PAH2+.
Spitzer observations have reaffirmed PAH abundance in our Galaxy. Water ice being equally abundant, coexistence of PAHs and water ice should be more common. Some of the implications of ionization of PAHs in ices are listed below.
• Ionization of PAH impurities in ices results in charge separation in the ice particles. Stronger Coulomb forces between the charged particles can initiate nucleation of particle accretion, which ultimately leads to the formation of large macroscopic bodies.
• PAH ionization energy reduction in low-temperature water ice substantially expands the astronomical regions in which trapped ions and electrons become important to be included in the modeling of regions of lower energy radiation fields than normally considered for ion-driven chemistry and physics.

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