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

Abstract

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.

Impurity contribution to ultraviolet absorption of saturated fatty acids

“…The Beer-Lambert law gives a sample’s absorbance ( A ) at wavelength λ (in nanometers) as follows with respect to the absorption cross section σ (in square centimeters) ( 50 )A=−log100.25emIsIb=normalσnln10where I is the intensity of the transmitted UV light at wavelength λ (in nanometers); superscripts s and b indicate background and sample measurements, respectively; and n (cm −2 ) represents the column density of molecules in the sample. Considering the sample density [ D = 0.9052 g cm −3 at 20°C for nonanoic acid ( 51 )], the mean molecular mass [ M = 158.24 g mol −1 for nonanoic acid ( 52 )], and Avogadro’s constant ( N A = 6.0221 × 10 23 mol −1 ), n can be expressed as followsn=NADLMwhere L represents the optical path length (in centimeters).…”

Multiple-angle incidence resolution spectrometry: applications in nanoarchitectonics and applied physics

“…Interfacial chemistry has long been recognized to play an important role in the study of interstellar molecules, 172) and Hama and co-workers present brilliant works showing that MAIRS is useful for revealing the structure and reactions of adsorbed molecules at an interface. [173][174][175] The surface of the interstellar dust composed of a variety of solid and liquid materials works as adsorption sites of interstellar molecules. The surface of interstellar dust is covered with various solid-Fig.…”

Infrared Band Strengths of Dangling OH Features in Amorphous Water at 20 K

“…Dangling OH bonds reflect the surface structure and porosity of amorphous water, and they act as catalytic sites for adsorption and chemical reactions. For example, the 3dOH feature at 3696 cm −1 is observed for both porous and nonporous amorphous water, whereas the 2dOH feature at 3720 cm −1 appears only for porous amorphous water (Bu et al 2015;He et al 2018;Nagasawa et al 2021Nagasawa et al , 2022. Thus, the 2dOH-to-3dOH intensity ratio can be used to characterize the porosity of amorphous water.…”

“…However, 2dOH and 3dOH are located on the ice surface (including the internal pore surface in the bulk ice), so absorbance induced by them is not necessarily proportional to sample thickness (Nagasawa et al 2021). This means that application of the Beer-Lambert law (Equation (1)) is not straightforward for 2dOH and 3dOH (Nagasawa et al 2021(Nagasawa et al , 2022. Indeed, it holds only for a bulk sample whose thickness is greater than the IR wavelength (d λ), because optical interfaces (e.g., vacuum-sample-substrate interfaces) are not considered in the derivation of the law from Maxwell's equations (Hasegawa 2017).…”