Infrared band strengths of dangling OH features in amorphous water at 20 K

Infrared (IR) spectra of vapor-deposited amorphous water at low temperatures show two weak peaks at around 3720 and 3696 cm-1 assigned to free-OH stretching modes of two- and three-coordinated water molecules (so-called dangling OH bonds), respectively, on the ice surface. A recent James Webb Space Telescope (JWST) observation first succeeded in detection of a potential dangling OH feature at 3664 cm-1 for ices in molecular clouds, highlighting the importance of dangling OH bonds in interstellar ice chemistry. A lack of band strengths of these features at low temperatures restricts the quantification of dangling OH bonds from IR spectra, hindering development of a molecular-level understanding of the surface structure and chemistry of ice. Using IR multiple-angle incidence resolution spectrometry, we quantified the band strengths of two- and three-coordinated dangling OH features in amorphous water at 20 K as being 4.6 plus-minus 1.6 times 10-18 and 9.1 plus-minus 1.0 times 10-18 cm molecule-1, respectively. These values are more than an order of magnitude lower than band strengths of bulk water molecules in ice and liquid water and are similar to those of H2O monomers confined in solid matrices. Adsorption of carbon monoxide with dangling OH bonds results in the appearance of a new broad dangling OH feature at 3680-3620 cm-1, with a band strength of 1.8 plus-minus 0.1 times 10-17 cm molecule-1. The band strengths of dangling OH features determined in this study advance our understanding of the surface structure of interstellar ice analogs and recent IR observations of the JWST.