[P-CG21] Origin and evolution of materials in space

[PCG21-04] Experimental analysis of the surface sites on porous amorphous H₂O ice at 20 K

*Takeshi Hasegawa¹, Ian Fitch Mochida¹, Tetsuya Hama¹ (1.The University of Tokyo Graduate School of Arts and Science)

Keywords:Icy dust grain, dangling OH bonds, infrared spectrometry, temperature programmed desorption

In interstellar space, various kinds of molecules are detected in spite of low-temperature and high-vacuum environments. Chemical reactions on the interstellar dust surface are considered to be important for the formation of these molecules. Observations suggest that interstellar dust is covered with amorphous ice mainly composed of water (H₂O). Amorphous H₂O ice has thus been used as a model of interstellar dust in experimental and theoretical studies [1]. Recently, the James Webb Space Telescope (JWST) succeeded in detecting “dangling OH (dOH)” features in a dense cloud, which are assigned to OH groups on the ice surface not forming hydrogen bond with other water molecules [2]. dOH bonds are used as an indicator of ice morphology and serve as adsorption and reaction sites on the ice surface. The coverage of dOH bonds is proposed to be pivotal to a reaction on the ice surface mimicking interstellar environment [3]. For crystalline H₂O ice, the coverage of dOH bonds was determined to be around 10 % using atomic force microscopy (AFM) [4]. However, quantification of dOH bonds on the amorphous water surface is difficult by AFM measurements because of its rough surface nature.
In this study, we combined infrared multiple-angle incidence-resolved spectroscopy (IR-MAIRS) and temperature-programmed desorption (TPD) to determine the number density of surface water molecules (including those in pores) in porous amorphous water ice vapor-deposited at 20 K. Compared with the number density of dOH bonds previously obtained [5], we found that the ratio of dOH bonds to surface water molecules in amorphous ice is almost equal to that in crystalline H₂O ice [4]. This study will contribute to improving modelling studies of interstellar dust surface chemistry, which eventually leads to a deeper understanding of chemical evolution in the interstellar medium.

[1] Hama, T., & Watanabe, N. (2013). Surface processes on interstellar amorphous solid water: Adsorption, diffusion, tunneling reactions, and nuclear-spin conversion. Chemical reviews, 113(12), 8783-8839.
[2] Noble, J. A., Fraser, H. J., Smith, Z. L., Dartois, E., Boogert, A. C. A., Cuppen, H. M., ... & Urso, R. G. (2024). Detection of the elusive dangling OH ice features at~ 2.7 μm in Chamaeleon I with JWST NIRCam. Nature Astronomy, 8(9), 1169-1180.
[3] Bovolenta, G. M., Silva-Vera, G., Bovino, S., Molpeceres, G., Kästner, J., & Vogt-Geisse, S. (2024). In-depth exploration of catalytic sites on amorphous solid water: I. The astrosynthesis of aminomethanol. Physical Chemistry Chemical Physics, 26(27), 18692-18706.
[4] Kawakami, N., Iwata, K., Shiotari, A., & Sugimoto, Y. (2020). Intrinsic reconstruction of ice-I surfaces. Science Advances, 6(37), eabb7986.
[5] Hasegawa, T., Yanagisawa, H., Nagasawa, T., Sato, R., Numadate, N., & Hama, T. (2024). Infrared band strengths of dangling OH features in amorphous water at 20 K. The Astrophysical Journal, 969(2), 134.