Adsorption, desorption, and diffusion of nitrogen in a model nanoporous material. II. Diffusion limited kinetics in amorphous solid water

Gaseous "nanoprobes" for detecting gas-trapping environments in macroscopic films of vapor-deposited amorphous ice

Vapor-deposited amorphous ice, traditionally called amorphous solid water (ASW), is one of the most abundant materials in the universe and a prototypical material for studying physical vapor-deposition processes. Its complex nature arises from a strong tendency to form porous structures combined with complicated glass transition, relaxation, and desorption behavior. To gain further insights into the various gas-trapping environments that exist in ASW and hence its morphology, films in the 25-100 μm thickness range were codeposited with small amounts of gaseous "nanoprobes" including argon, methane, helium, and carbon dioxide. Upon heating in the 95-185 K temperature range, three distinct desorption processes are observed which we attribute to the gas desorption out of open cracks above 100 K, from internal voids that collapse due to the glass transition at ∼125 K and finally from fully matrix-isolated gas induced by the irreversible crystallization to stacking disordered ice (ice Isd) at ∼155 K. Nanoscale films of ASW have only displayed the latter desorption process which means that the first two desorption processes arise from the macroscopic dimensions of our ASW films. Baffling the flow of water vapor toward the deposition plate greatly reduces the first desorption feature, and hence the formation of cracks, but it significantly increases the amount of matrix-isolated gas. The complex nature in which ASW can trap gaseous species is thought to be relevant for a range of cosmological processes.

Physisorption and desorption of H2, HD and D2 on amorphous solid water ice. Effect on mixing isotopologue on statistical population of adsorption sites

We study the adsorption and desorption of three isotopologues of molecular hydrogen mixed on 10 ML of porous amorphous water ice (ASW) deposited at 10 K.

Electron-induced chemistry of methyl chloride caged within amorphous solid water

The interaction of low energy electrons (1.0-25 eV) with methyl-chloride (CD3Cl) molecules, caged within Amorphous Solid Water (ASW) films, 10-120 monolayer (ML) thick, has been studied on top of a Ru(0001) substrate under Ultra High Vacuum (UHV) conditions. While exposing the ASW film to 3 eV electrons a static electric field up to 8 × 10(8) V∕m is developed inside the ASW film due to the accumulation of trapped electrons that produce a plate capacitor voltage of exactly 3 V. At the same time while the electrons continuously strike the ASW surface, they are transmitted through the ASW film at currents of ca. 3 × 10(-7) A. These electrons transiently attach to the caged CD3Cl molecules leading to C-Cl bond scission via Dissociative Electron Attachment (DEA) process. The electron induced dissociation cross sections and product formation rate constants at 3.0 eV incident electrons at ASW film thicknesses of 10 ML and 40 ML were derived from model simulations supported by Thermal Programmed Desorption (TPD) experimental data. For 3.0 eV electrons the CD3Cl dissociation cross section is 3.5 × 10(-16) cm(2), regardless of ASW film thickness. TPD measurements reveal that the primary product is deuterated methane (D3CH) and the minor one is deuterated ethane (C2D6).

Interactions and binding energies of dimethyl methylphosphonate and dimethyl chlorophosphate with amorphous silica

The fundamental interactions of dimethyl methylphosphonate (DMMP) and dimethyl chlorophosphate (DMCP) on amorphous silica nanoparticles have been investigated with transmission infrared spectroscopy and temperature-programmed desorption (TPD). DMMP and DMCP both adsorb molecularly to silica through the formation of hydrogen bonds between isolated silanols and the phosphoryl oxygen of the adsorbate. The magnitude of the shift of the ν(OH) mode upon simulant adsorption is correlated to the adsorption strength. The activation energies for desorption for a single DMMP or DMCP molecule from amorphous silica varied with coverage. In the limit of zero coverage, after the effects of defects were excluded, the activation energies were 54.5 ± 0.3 and 48.4 ± 1.0 kJ/mol for DMMP and DMCP, respectively.

Xe interacting with porous silicon

Thin films of porous silicon (PS), structurally characterized by HR-SEM, were studied using xenon Temperature Programmed Desorption (TPD) as a probe of its inner pores. Geometric hindrance of the depth desorbing population and multiple wall collisions result in a unique double-peak structure of the TPD curve. Surface-diffusion assisted adsorption mechanism into inner pores at 48 K is proposed as the origin of these unique TPD spectra. It is experimentally verified by mild Ne(+) sputtering prior to TPD which preferentially removes Xe population from the top surfaces. A pore-diameter limited desorption kinetic model that takes into account diffusion and pore depth well explains the governing parameters that determine the experimental observations. These results suggest that TPD may be employed as a highly sensitive, non-destructive surface area determination tool.

D(2) desorption kinetics on amorphous solid water: from compact to porous ice films

The desorption kinetics of D(2) from amorphous solid water (ASW) films have been studied by the temperature-programmed desorption (TPD) technique in the 10-30 K temperature range. Compact (and nonporous) films were grown at 120 K over a copper substrate. Ultra-thin porous films were additionally grown at 10 K over the compact base. The TPD spectra from compact and from up to 20 monolayers (ML) porous films were compared. The simulation of the TPD experimental traces provides the corresponding D(2) binding-energy distributions. As compared to the compact case, the binding-energy distribution found for the 10 ML porous film clearly extends to higher energies. To study the transition from compact to porous ice, porous films of intermediate thicknesses (<10 ML), including ultra-thin films (<1 ML), were grown over the compact substrate. The thermal D(2) desorption peak was found to shift to higher temperatures as the porous ice network was progressively formed. This behavior can be explained by the formation of more energetic binding sites related to porous films. TPD spectra were also modelled by using a combination of the two energy distributions, one associated to a bare compact ice and the other associated to a 10 ML porous ice film. This analysis reveals a very fast evolution of the binding-energy distribution towards that of porous ice. Our results show that few ML of additional porous film are sufficient to produce a sample for which the D(2) adsorption can be described by the energy distribution found for the 10 ML porous film. These experiments then provide evidence that the binding energy of D(2) on ASW ice is primarily governed by the topological and morphological disorder of the surface at molecular scale.