  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 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, ultra-high-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 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.
