The absolute absorption cross section of dangling OH bonds in water ice, a free OH stretch mode by three-coordinated surface water molecules, is derived experimentally as 1.0 ± 0.2 × 10−18 cm2 at 3696 cm−1 for amorphous water at 90 K using infrared multiple-angle incidence resolution spectrometry (IR–MAIRS). The integrated absorption cross section (band strength) of the dangling OH bond at 90 K (1.4 ± 0.3 × 10−17 cm molecule−1 at 3710–3680 cm−1) is found to be more than 1 order of magnitude smaller than those in bulk ice or liquid water. This indicates that a lack of hydrogen-bonding significantly decreases the band strength of dangling OH bonds. The present study also provides average molecular orientations of dangling OH bonds at 10 K and 90 K, because both the surface-parallel (in-plane) and surface-perpendicular (out-of-plane) vibration spectra of dangling OH bonds are quantitatively measured by IR–MAIRS. The intensity ratio of the dangling-OH peaks between in-plane to out-of-plane spectra shows the isotropic nature (random orientation) of the two- and three-coordinated dangling OH bonds in microporous amorphous water prepared at 10 K; however, the three-coordinated dangling OH bonds in nonporous amorphous water prepared at 90 K are dominantly located at the top ice surface and oriented perpendicular to it. These findings provide fundamental insights into the relationship between the structure and optical properties of ice surfaces, and aid quantitative understanding of the surface structure of interstellar ices and their laboratory analogs.

Infrared (IR) multiple-angle incidence resolution spectrometry (IR-MAIRS) is a recently developed spectroscopic technique that combines oblique incidence transmission measurements and chemometrics (multivariate analysis) to obtain both pure in-plane (IP) and outof-plane (OP) vibration spectra for a thin sample. IR-MAIRS is established for analyzing the molecular orientation of organic thin films at atmospheric pressure, but it should also be powerful for the structural characterization of vapor-deposited thin samples prepared in a vacuum. The application of IR-MAIRS to vapor-deposited amorphous water is particularly interesting in the fields of physical and interstellar chemistry, because it is a representative model material for interstellar icy dust grains. We recently developed an experimental setup for in situ IR-MAIRS under low-temperature, ultrahigh-vacuum conditions, which thus facilitates measurements of interstellar ice analogues such as vapor-deposited amorphous water. This review considers the theoretical framework of IR-MAIRS and our recent experimental results for vapor-deposited amorphous water. We present spectroscopic signatures for the perpendicular orientation of dangling OH bonds for three-coordinated water molecules at the surface of amorphous water at 90 K. The absolute absorption cross-section of the three-coordinated dangling OH bonds is quantitatively measured as 1.0 ± 0.2 × 10 −18 cm 2 at 3696 cm −1 . As IR-MAIRS can essentially be conducted using only a Fourier-transform IR spectrometer and an angle-controllable linear polarizer, it is a useful, low-cost, and simple spectroscopic technique for studying laboratory analogues of interstellar ices including vapor-deposited amorphous water.