Physicochemical Properties of α-Pinene in Water Ice Analogs under Energetic Heavy-Ion Irradiation

Studying the physicochemical properties of ice in astronomical environments is crucial to understanding the chemical processes involved in cosmic events such as comet and planet formation. The physical characteristics and chemical evolution on the surfaces of cosmic objects such as comets or interstellar grains offer key insights into these processes. This study focuses on α-pinene, a carbon- and hydrogen-rich molecule, which serves as a model for investigating radical-driven synthesis of more complex molecules under space-like conditions. It also provides a useful analogy for complex terrestrial organic molecules and sheds light on how organic matter interacts with water and radiation in extraterrestrial environments. In this work, we simulate the effects of heavy-ion cosmic ray bombardment on chiral molecules in the interstellar medium by analyzing the radiolysis of a C₁₀H₁₆/H₂O (1:1) mixture irradiated with 61.3 MeV ⁸⁴Kr¹⁵⁺ ions. Fourier Transform Infrared (FTIR) spectroscopy is employed to monitor the chemical evolution of ice samples at 10 K, both before and after irradiation. We identify 12 C_nH_m and ten C_nH_mO_k molecules, including complex products such as naphthalene (C₁₀H₈), glycolaldehyde (HCOCH₂OH), and methyl formate (HCOOCH₃). The most abundant hydrogenated product is acetylene (C₂H₂), followed by naphthalene (C₁₀H₈), while the most abundant oxygenated molecules are vinyl alcohol (CH₂CHOH) and ethanol (CH₃CH₂OH). Notably, the formation of CO₂ is minimal in this experiment. The destruction cross-sections of α-pinene and water in the (1:1) mixture are determined to be 3.5 and 6.4 × 10^–13 cm², respectively. The formation cross-sections for the products resulting from radiolysis are on average 2 × 10^–14 cm² for hydrocarbons and 0.6 × 10^–14 cm² for the oxygenated products.