A. Allouche

The adsorption of acetic acid on a proton-ordered water ice surface is modeled using periodic plane-waves density-functional theory. The structures of acetic acid adsorbed as a monomer or oligomers, hydrated or not, are calculated through gradient optimization. The resulting quantum electronic density of states are compared to metastable impact electron spectroscopy (MIES) results and lead to selection of the most plausible structures of acetic acid on water ice. Hypotheses are formulated for the structure of the acid film growing on the ice surface including mainly cyclic dimers and hydrated forms. Adsorptions of single water molecules on acetic acid crystal surfaces are also studied after optimization of the acetic acid crystal bulk and surface structure. More comparisons with spectroscopic studies are proposed in the accompanying paper.

The structure and spectroscopy (electronic and vibrational) of formic acid (HCOOH) dimers and trimers are investigated by means of the hybrid (B3LYP) density-functional theory. Adsorption of single and dimer HCOOH on amorphous water ice surface is modeled using two different water clusters. Particular attention has been given to spectroscopic consequences. Several hypotheses on formic acid film growing on ice and incorporation of a single water molecule in the formic acid film are proposed.

Boron inserted as impurity by substitution of carbon atoms in graphite is known to modify the reactivity of the surface in interaction with hydrogen. Boron induces a better H retention capability in graphite while it makes easier the recombination into molecular hydrogen under heating in thermal-desorption experimental conditions. It has already been calculated that boron modifies the electronic structure of the surface, which results in an increase of the adsorption energy for H. This result seems in good agreement with the better retention for H in doped graphite, but contradictory with the easier recombination observed. The aim of this work is to dismiss this contradiction by elucidating the modifications induced by boron in the recombination mechanism. We studied the diffusion of H on pure and boron-doped graphite in the density functional theory framework. We determined a diffusionlike mechanism leading to molecular hydrogen formation. Finally, we have shown the fundamental modifications induced by boron on the [0001] graphite surface reactivity. From these calculations it stands out that recombination is the result of desorption on pure graphite and diffusion on B-doped surfaces, while the activation energy for the rate limiting step is half reduced by boron. The results are compared to experimental observations. The connection between the cluster and periodic quantum modes for graphite is also discussed.