PNNL

Abstract

Reflection-absorption infrared spectra (RAIRS) of amorphous solid water (ASW) films grown at 20K on a Pt(111) substrate at various incidence angle (?Beam = 0-85o) 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(?) and k(?)] for porous ASW in the 1000-4000cm-1 (10-2.5 µm) range. The behaviour of the optical properties of ASW in the intramolecular OH stretching region with increasing ?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 (~200cm-1) in the position of the coupled H-bonded intramolecular OH stretching vibrations spectral feature. Moreover, this band shifts in opposite directions with increasing ?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.