Infrared Spectroscopy and Optical Constants of Porous Amorphous Solid Water

Abstract

Reflection−absorption infrared spectra (RAIRS) of amorphous solid water (ASW) films grown at 20 K on a Pt(111) substrate at various angles (θ_Beam <i>=</i> 0−85°) 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., <i>n</i>(ω) and <i>k</i>(ω)] for porous ASW in the 1000−4000 cm⁻¹ (10−2.5 μm) range. The behavior of the optical properties of ASW in the intramolecular OH stretching region with increasing θ_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 (∼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 θ_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.