Infrared multiple-angle incidence resolution spectrometry for vapor-deposited amorphous water

Abstract

Infrared 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 out-of-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 analogs 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 cm² 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 analogs of interstellar ices including vapor-deposited amorphous water.