Absolute Absorption Cross Section and Orientation of Dangling OH Bonds in Water Ice

Abstract

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 multiple‐angle incidence resolution spectrometry for vapor‐deposited amorphous water

“…For reference, the OMNIC software for IR-MAIRS analysis gives 𝐴 IP 𝜃=0° and 𝑛 4 𝐻𝐴 OP 𝜃=0° as the ordinate scales for the resultant IP and OP spectra, respectively. [63] Nagasawa et al [34] estimated the column density of three-coordinated dangling-OH bonds at 90 K using CH3OH deposition, because its hydrogen bonding with dangling OH bonds quenches the dangling-OH peak at 3696 cm −1 . [88] Figure 20A and B shows the IP and OP spectra of amorphous water at 90 K before and after CH3OH deposition in the ranges of (A) 3750-3660 cm −1 for the dangling-OH peak and (B) 1060-1010 cm −1 for the CO stretching band of CH3OH.…”

“…Nagasawa et al reported that CH3OH deposition for 6.5 ± 0.5 min is required to fully quench the peak, which corresponds to an exposure of 3.1  0.4 × 10 13 molecules cm −2 . [34] They also calculated the column density of CH3OH adsorbed on the amorphous water surface from the C-O stretching band at 1033 cm −1 (Fig. 20B).…”

“…20B). [34] Eq. 64 shows that, when the band strength 𝛽 (the integrated absorption crosssection) for the C-O stretching band of CH3OH is available, the column density N of CH3OH can be obtained from the IP and OP peak areas at 𝜈 ̃= 1050-1020 cm −1 :…”

“…66. [33,34,84] As a single peak for the CO stretching band only appeared in the OP spectra at 1033 cm −1 (Fig. 20B), all CH3OH deposited on the amorphous water surface should interact with the three-coordinated dangling-OH bonds, leading to the strong orientation of CO bonds perpendicular to the surface (Fig.…”

“…Monomer in solid O2 at 10 K [95] 3.3 × 10 −18 (3732) b a See ref [34] for details of the band strength calculations for crystalline ice and liquid water b Asymmetric OH stretching vibration…”