Theoretical and Laboratory Studies on the Interaction of Cosmic‐Ray Particles with Interstellar Ices. III. Suprathermal Chemistry–Induced Formation of Hydrocarbon Molecules in Solid Methane (CH₄), Ethylene (C₂H₄), and Acetylene (C₂H₂)

Abstract

Methane, ethylene, and acetylene ices were irradiated in a ultra-high vacuum vessel between 10 K and 50 K with 7.3 MeV protons as well as 9.0 MeV He2`nuclei to simulate the interaction of galactic cosmic-ray particles with extraterrestrial, organic ices and to elucidate a mechanistic model to synthesize experimentally detected polycyclic aromatic hydrocarbons (PAHs). Theoretical calculations center on computer simulations of ion-induced collision cascades in irradiated methane targets. MeV ions induce hydrogen and carbon knock-on particles in elastic encounters with the target atoms. Each primary knock-on triggers one collision cascade with up to 70 suprathermal carbon atoms concentrated in one to two subcascades in 0.6È5 ] 103At the end point of each single trajectory, every suprathermal Ó3. carbon atom can form an individual reaction center of hydrogen abstraction and insertion in or addition to chemical bonds of a reactant molecule. In the relaxation phase of this energized volume, overlapping reaction zones likely form observed PAHs napthalene, phenanthrene/azulene, and coronene. This multicenter mechanism establishes a versatile route to synthesize complex molecules in extraterrestrial ices even at temperatures as low as 10 K within cosmic-ray-initiated single collision cascades.

Cosmic ray impact on astrophysical ices: laboratory studies on heavy ion irradiation of methane

“…The typical virgin CH 4 FTIR spectrum (shown in Fig. 1a) is in good agreement with previous studies Moore & Hudson 1998;Kaiser & Roessler 1998;Bennett et al 2006). The initial column density of the CH 4 before irradiation, N virgin = (2.69 ± 0.50) × 10 18 molecules/cm 2 , was determined by measuring the area of the ν 1 + ν 4 vibration mode band at 4202 cm −1 .…”

“…(1) Gerakines et al (1996), (2) Kaiser et al (1998), (3) Moore et al (2003), and (4) Bennett et al (2006). atoms must stay constant during irradiation regardless of the chemical reactions induced by radiation.…”

“…For this band we adopted the A-value (25 × 10 −18 cm molecule −1 ) previously reported by Gerakines et al (1996), Moore & Hudson (2003), and Kaiser & Roessler (1998).…”

“…The occurrence of molecular synthesis in these astrophysical ices by cosmic rays, solar wind, UV radiation, and electron collision has been the object of research over the past decades (e.g. Gerakines et al 1996;Kaiser & Roessler 1998;Moore & Hudson 1998;Baratta et al 2002;Bennett et al 2006). Heavy galactic cosmic rays (HGCR) are considered as the main agent for keeping a significant fraction of gas phase material in molecular clouds (Leger et al 1985;Hasegawa & Herbst 1993;Willacy & Williams 1993;Bergin et al 1995;Shematovich et al 1997;Willacy & Millar 1998;Nguyen et al 2002).…”

UV-photoprocessing of interstellar ice analogs: New infrared spectroscopic results

“…Large organic compounds are produced either by ice photoprocessing (Agarwal et al 1985;Briggs et al 1992;Bernstein et al 1995Bernstein et al , 2002Muñoz Caro et al 2002 or by ion bombardment (e.g. Strazzulla & Baratta 1992;Kobayashi et al 1995;Kaiser & Roessler 1998;Cottin et al 2001).…”