Experimental and simulated vibrational spectra of H2 absorbed in amorphous ice: Surface structures, energetics, and relaxations

Abstract

Infrared spectra are reported for thin films of deuterated microporous amorphous ice formed at 12 K and saturated with absorbed molecular hydrogen. This paper focuses on both the influence of the surface-bound H2 on the absorption bands of the OD groups that dangle from the micropore surfaces and the behavior of the induced infrared bands of the stretching mode of H2 itself. Both structural changes and the relaxation of ortho-H2 to para-H2 are apparent from variations in the observed spectra with time and temperature. A reasonably detailed interpretation of the complex spectral behavior has been possible through simulation of spectra for H2 interacting with the surface of amorphous ice clusters generated previously in a classical trajectory study of the cluster growth through the accumulation and relaxation of individual water molecules. Potential minima were calculated with respect to H2 coordinates on the cluster surface and a qualitative interpretation of adsorbate–surface bonding provided via the partitioning of the H2⋅⋅D2O interactions into relatively weak van der Waals interactions, and stronger electrostatic interactions between the H2 quadrupole and the H2O dipole and quadrupole. The net binding of H2 to the surface is dominated by several van der Waals interactions with neighboring D2O molecules, but in most of the calculated minima, H2 also forms a single electrostatic bond to a surface molecule. While such electrostatic bonds provide only a small fraction of the binding energy, they appear to influence strongly the observed spectra. Observed H2-induced shifts in the dangling OD bands appear to be caused by electrostatic bonding of H2 to a D atom of a dangling OD and a significant fraction of the H2 intensity is proposed to originate from H2 molecules which are electrostatically bonded to dangling oxygen atoms on the surface. Calculations (which do not contain adjustable parameters) reproduce quite well several features of the measured spectra, including the splitting of the dangling OD band, the shift of this band due to binding of H2, and the frequency and the width of the H2 band.