Infrared Spectroscopy and Optical Constants of Porous Amorphous Solid Water

Reflection-absorption infrared spectra (RAIRS) of amorphous solid water (ASW) films grown at 20 K on a Pt(111) substrate at various angles (theta(Beam) = 0-85 degrees) using a molecular beam are reported. They display complex features arising from the interplay between refraction, absorption within the sample, and interference effects between the multiple reflections at the film-substrate and film-vacuum interfaces. Using a simple classical optics model based on Fresnel equations, we obtain optical constants [i.e., n(omega) and k(omega)] for porous ASW in the 1000-4000 cm(-1) (10-2.5 mu m) range. The behavior of the optical properties of ASW in the intramolecular OH stretching region with increasing theta(Beam) is shown to be strongly correlated with its decreasing density and increasing surface area. A direct comparison between the RAIRS and calculated vibrational spectra shows a large difference (similar to 200 cm(-1)) in the position of the coupled H-bonded intramolecular OH stretching vibrations spectral feature. Moreover, this band shifts in opposite directions with increasing theta(Beam) in RAIRS and vibrational spectra demonstrating RAIRS spectra cannot be interpreted straightforwardly as vibrational spectra due to severe optical distortions from refraction and interference effects.

报道了使用分子束在不同角度（θ（束）=0 - 85度）下于20 K在Pt(111)基底上生长的无定形固态水（ASW）薄膜的反射吸收红外光谱（RAIRS）。它们呈现出复杂的特征，这些特征源于折射、样品内的吸收以及薄膜 - 基底和薄膜 - 真空界面处多次反射之间的干涉效应之间的相互作用。使用基于菲涅耳方程的简单经典光学模型，我们得到了1000 - 4000 cm⁻¹（10 - 2.5 μm）范围内多孔ASW的光学常数[即n(ω)和k(ω)]。随着θ（束）的增加，ASW在分子内OH伸缩区域的光学性质的行为显示出与其密度降低和表面积增加密切相关。RAIRS与计算的振动光谱之间的直接比较表明，耦合的氢键分子内OH伸缩振动光谱特征的位置存在较大差异（约200 cm⁻¹）。此外，在RAIRS和振动光谱中，随着θ（束）的增加，该谱带向相反方向移动，这表明由于折射和干涉效应造成的严重光学畸变，RAIRS光谱不能直接解释为振动光谱。