PREFERENTIAL ADSORPTION OF ORTHO-H-2 WITH RESPECT TO PARA-H-2 ON THE AMORPHOUS ICE SURFACE

This study demonstrates preferential adsorption of ortho-H-2 with respect to para-H-2 on the surface of amorphous ice exposed to normal H-2 at 12 K. The experimental part of the study focuses on the time dependence of Fourier-transform infrared bands of ortho-H-2 and para-H-2 adsorbate and of dangling-OD bonds. In the spectrum of the newly formed adsorbate overlayer (before the ortho-to-para relaxation takes place) only the ortho-H-2 IR band is visible, identifying the initial para-H-2 content as less than 10%. In the computational part, H-2 adsorption is modeled on an amorphous cluster (H2O)450. The computational study consists of the diffusion Monte Carlo investigation of isolated H-2 states on the surface, and of path integral Monte Carlo simulation of the adsorbate overlayer. Nuclear symmetry effects are incorporated approximately in the path integral Monte Carlo simulation, which is carried out in the framework of the grand canonical ensemble. The calculated percentage of para-H-2 in the adsorbate layer is 7%, corresponding to a factor approximately 4 depletion with respect to the normal-H-2 gas phase. The para depletion on the surface is due to excess binding energy of ortho-H2 with respect to para-H-2 that originates from the anisotropy of the gas-surface potential. Para-H-2 is bound to the surface via the spherically symmetric part of the potential, while ortho-H-2 has extra binding energy from the anisotropic part.