Molecular dynamics simulations of CO2 formation in interstellar ices

Area of Science:

• Astrochemistry
• Computational Chemistry
• Interstellar Ice Formation

Background:

• Carbon dioxide (CO2) ice is abundant in interstellar ices, alongside water (H2O) and carbon monoxide (CO).
• The precise formation pathways for CO2 ice in interstellar environments remain incompletely understood.
• Ultraviolet (UV) photodissociation of H2O is a known process in space that can generate reactive species.

Purpose of the Study:

• To investigate the formation routes of CO2 ice via UV photodissociation of CO-H2O ice mixtures.
• To determine the relative probabilities of CO2 formation versus other products like the HOCO complex.
• To compare theoretical findings with experimental data on CO2 ice formation pathways.

Main Methods:

• Molecular dynamics simulations were performed on CO-H2O ice systems at 10 K.
• Simulations focused on the ultraviolet photodissociation of H2O and subsequent reactions with CO.
• Different ice surface models were employed to assess the influence of the ice environment.

Main Results:

• The reaction between CO and OH radicals (from H2O photodissociation) can form CO2 ice.
• The HOCO complex is found to be a more probable product than CO2, with a formation probability of (3.00 ± 0.07) × 10(-2) compared to (3.6 ± 0.7) × 10(-4) for CO2.
• CO2 formation is highly dependent on the initial position of CO within the ice; deeper locations favor CO2.
• The HOCO complex is stabilized in the ice, hindering further conversion to CO2.
• The HCO van der Waals complex can form from the reaction of CO with H atoms produced by H2O photodissociation.

Conclusions:

• The reaction of OH with CO is a plausible, though not the most probable, route to CO2 ice formation.
• The formation of the HOCO complex is a significant competing pathway.
• The initial location of CO within interstellar ice structures is a critical factor influencing CO2 ice production.
• Alternative pathways, such as the reaction of photogenerated H atoms with CO, can lead to HCO ice formation.