nai

Project Progress

Radiation-induced chemistry of carbon dioxide (CO₂) is important in a variety of regions from the atmospheres of Venus, Earth, and Mars to the ices of comets, Mars, and planetary satellites. As carbon dioxide molecules are degraded by radiation, carbon monoxide and high energy oxygen atoms can be produced. These oxygen atoms can react with other carbon dioxide molecules to produce higher order carbon oxides, CO_n (n>2). Although higher order carbon oxides have been theoretically predicted to be stable and also important, their detection has been difficult.

We conducted 5 keV electron irradiation of solid carbon dioxide at 10 K with our ultrahigh vacuum machine. We detected the acyclic (D_3h) isomer of carbon trioxide (CO₃) via its ν¹ and ν² vibrational modes centered around 1165 cm^-1. The assignment of the IR peaks (Fig. 1) and reaction pathways (Fig. 2) were verified by the experiments of isotopic carbon dioxide (¹³CO₂ and C₁₈O₂). The structures of CO₃ isomers were determined by theoretical calculations. We also detected the C_2v isomer of carbon tetraoxide (CO₄) (Fig. 3). These isomers are important intermediates in terrestrial atmospheric chemistry to explain the quenching of O(¹D) atoms and possibly in explaining the ¹⁸O isotope enrichment in stratospheric carbon dioxide.

{{ 1 }}

{{ 2 }}

{{ 3 }}

In addition, we investigated the irradiation of N₂/CO₂ ice mixtures. In our experiments, a C_s symmetric nitroformyl radical, OCNO(X ²A^”), has been detected for the first time (Fig. 4, PCCP cover page). This experimental result is useful for understanding the icy surface of Triton, Neptune’s largest moon, where nitrogen and carbon dioxide are likely to be chemically processed by the harsh radiation environment that exists in space.

{{ 4 }}

Currently, we are conducting experiments to study the irradiation of H₂O/CO₂ ice mixtures. Those experiments probably can provide some information about the chemical processes in the Martian polar caps since the Martian polar caps are covered by both water ice and carbon dioxide ice.