RADIATION DAMAGE AND ASSOCIATED PHASE CHANGE EFFECT ON PHOTODESORPTION RATES FROM ICES—LYα STUDIES OF THE SURFACE BEHAVIOR OF CO₂(ICE)

Abstract

Photodesorption from a crystalline film of CO 2 (ice) at 75 K has been studied using Lyα (10.2 eV) radiation. We combine quantitative mass spectrometric studies of gases evolved and transmission IR studies of species trapped in the ice. Direct CO desorption is observed from the primary CO 2 photodissociation process, which occurs promptly for CO 2 molecules located on the outermost surface of the ice (Process I). As the fluence of Lyα radiation increases to ∼5.5 × 10 17 photons cm −2 , extensive damage to the crystalline ice occurs and photo-produced CO molecules from deeper regions (Process II) are found to desorb at a rapidly increasing rate, which becomes two orders of magnitude greater than Process I. It is postulated that deep radiation damage to produce an extensive amorphous phase of CO 2 occurs in the 50 nm ice film and that CO (and CO 2 ) diffusive transport is strongly enhanced in the amorphous phase. Photodesorption in Process II is a combination of electronic and thermally activated processes. Radiation damage in crystalline CO 2 ice has been monitored by its effects on the vibrational line shapes of CO 2 (ice).Here the crystalline-to-amorphous phase transition has been correlated with the occurrence of efficient molecular transport over long distances through the amorphous phase of CO 2 (ice). Future studies of the composition of the interstellar region, generated by photodesorption from ice layers on grains, will have to consider the significant effects of radiation damage on photodesorption rates.

On the Photodesorption of CO2 Ice Analogs: The Formation of Atomic C in the Ice and the Effect of the VUV Emission Spectrum

“…In Öberg et al (2009), the Y pd (CO2) for CO 2 deposited at 18-30 K is 2.3 × 10 −3 molecules photon −1 calculated from 1.2×10 −3 ×(1−e −x/2.9 )+1.1×10 −3 ×(1−e −x/4.6 ), where x is the ice thickness, i.e., x = 141 ML to compare with this work. The Y pd (CO 2 ) is 40% lower for CO 2 deposited at 60 K and cooled down to 18 K than that for CO 2 deposited at 18 K. This seems to verify that in the case of CO 2 deposited at 40-60 K, the UV photon energy contributed to restructure the ice from crystalline to amorphous to make an effective exit channel for molecules to desorb to the surface, in line with the conclusion from Yuan & Yates (2014). In contrast, this study demonstrates that the photodesorption yield of CO 2 is not ice structure dependent, which is incompatible with the preceding works based on IR spectroscopy for quantification ( Öberg et al 2009;Yuan & Yates 2014).…”

“…CO 2 photodesorption has already been the subject of other scientific studies, but its connection with the ice structure is still not well understood. (Yuan & Yates 2014) discussed the photodesorption rate of CO 2 ice under radiation damage, indicating that the photodesorption yield of CO 2 ice depends on its structure. In addition, the amorphous CO 2 ice provides facile pathways for CO and CO 2 molecules passing through the ice bulk and reaching the surface, which contributes to a more efficient desorption compared to crystalline CO 2 ice (Cooke et al 2018).…”

UV photoprocessing of CO2ice: a complete quantification of photochemistry and photon-induced desorption processes

“…Therefore, in these experiments UV irradiation mainly leads to photodesorption (see, e.g., Öberg et al 2007;Muñoz Caro et al 2010;Fayolle et al 2011;Chen et al 2014). In the past years, photoprocessing of other pure ices made of CO 2 , H 2 O, CH 3 OH, O 2 , or N 2 has been studied (see, e.g., Westley et al 1995;Öberg et al 2009a,b;Bahr & Baragiola 2012;Fayolle et al 2013;Yuan & Yates 2013;Fillion et al 2014;Yuan & Yates 2014;Zhen & Linnartz 2014;Cruz-Díaz et al 2015).…”

Photochemical Effects on the Ices and Dust Grains of Astronomical Importance

“…The first ice detected in the ISM was water, as it is the most abundant ice component in many molecular clouds [27]. CO and CO 2 are also common components of ices and their fraction can be as much as 30%…”

“…As shown in Figure 6 described elsewhere [27]. The grid may be heated electrically and the temperature program may be controlled electronically by a Labview program to 0.1 K in the range ~100 K -~1000 K using a type K thermocouple welded to the grid.…”