Characterization of thin film CO2 ice through the infrared ν1 + ν3 combination mode

Abstract

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 CO 2 segregates into the pure form from water-rich mixtures of ice mantles and how it then crystallizes, we used Reflection Absorption InfraRed Spectroscopy (RAIRS) to study the structural change of pure CO 2 ice as a function of both ice thickness and temperature. We found that the ν 1 + ν 3 combination mode absorption profile at 3708 cm −1 provides an excellent probe to quantify the degree of crystallinity in CO 2 ice. We also found that between 20 and 30 K, there is an ordering transition that we attribute to reorientation of CO 2 molecules, while the diffusion of CO 2 becomes significant at much higher temperatures. In the formation of pure crystalline CO 2 in ISM ices, the rate limiting process is the diffusion/segregation of CO 2 molecules in the ice instead of the phase transition from amorphous to crystalline after clusters/islands of CO 2 are formed.

Entrapment of CO in CO2 Ice

“…Both show evidence for substantial CO loss in between the CO and CO 2 desorption temperatures. This region coincides with a thickness-dependent completion of CO 2 crystallization (He & Vidali 2018). This outgassing of CO is reminiscent of the previously observed outgassing of CO from H 2 O during H 2 O crystallization (Bar-Nun et al 1985;Collings et al 2003).…”

“…Converting CO spectra into a CO quantity is complicated by a possible dependence of the CO ice band strength on ice morphology and composition, both which are expected to change during the warmup ramp due to CO and ice matrix structural changes (e.g. Schmitt et al 1989;Isokoski et al 2013;He & Vidali 2018). Figure 1 shows that indeed, while the QMS and FTIR CO measurements are broadly in agreement, there are quantitative differences.…”

Spectroscopic measurements of CH3OH in layered and mixed interstellar ice analogues

“…Also, the diffusion of various molecular species in amorphous solid water has been investigated in a series of papers (He et al 2017;He & Vidali 2018;He et al 2018a,b), where the dependence of the segregation effect with respect to temperature and concentration has been analysed in detail.…”

Formation of chiral CO polyhedral crystals on icy interstellar grains

“…Since CO 2 and CO are abundant ice components, after H 2 O, in interstellar molecular clouds (Gibb et al 2004;Boogert et al 2015), understanding the crystallinity of CO 2 and CO is essential for discussing the thermal history of low-temperature interstellar grains. The crystallinity of solid CO 2 has been examined in detail via infrared spectroscopy (Escribano et al 2013;Gerakines & Hudson 2015;Baratta & Palumbo 2017;He & Vidali 2018) and transmission electron microscopy (TEM) (Kouchi et al 2020). Additionally, the effect of UV and electron beam irradiation on solid CO 2 was investigated by Tsuge et al (2020).…”