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. © 2016 The Authors.