Characterization of thin film CO₂ ice through the infrared v₁ + v₃ combination mode

Carbon dioxide is abundant in ice mantles of dust grains; some is found in the pure crystalline form as inferred from the double peak splitting of the bending profile at about 650 cm^-1. To study how CO₂ segregates into the pure form from water-rich mixtures of ice mantles and how it then crystallizes, we used Reflection Absorption InfraRed Spectroscopy to study the structural change of pure CO₂ ice as a function of both ice thickness and temperature. We found that the v₁ +v₃ combination mode absorption profile at 3708 cm^-1 provides an excellent probe to quantify the degree of crystallinity in CO₂ ice. We also found that between 20 and 30 K, there is an ordering transition that we attribute to reorientation of CO₂ molecules, while the diffusion of CO₂ becomes significant at much higher temperatures. In the formation of pure crystallineCO₂ in interstellarmedium ices, the rate limiting process is the diffusion/segregation of CO₂ molecules in the ice instead of the phase transition from amorphous to crystalline after clusters/islands of CO₂ are formed.