Binding energies and sticking coefficients of H2 on crystalline and amorphous CO ice

Context. Molecular hydrogen (H2) is the most abundant interstellar molecule and plays an important role in the chemistry and physics of the interstellar medium. The interaction of H2 with interstellar ices is relevant for several processes (e.g., nuclear spin conversion and chemical reactions on the surface of the ice). To model surface processes, quantities such as binding energies and sticking coefficients are required. Aims. We provide sticking coefficients and binding energies for the H2/CO system. These data are absent in the literature so far and could help modelers and experimentalists to draw conclusions on the H2/CO interaction in cold molecular clouds. Methods. Ab initio molecular dynamics simulations, in combination with neural network potentials, were employed in our simulations. Atomistic neural networks were trained against density functional theory calculations on model systems. We sampled a wide range of H2 internal energies and three surface temperatures. Results. Our results show that the binding energy for the H2/CO system is low on average, − 157 K for amorphous CO and −266 K for crystalline CO. This carries several implications for the rest of the work. H2 binding to crystalline CO is stronger by 109 K than to amorphous CO, while amorphous CO shows a wider H2 binding energy distribution. Sticking coefficients are never unity and vary strongly with surface temperature, but less so with ice phase, with values between 0.95 and 0.17. With the values of this study, between 17 and 25% of a beam of H2 molecules at room temperature would stick to the surface, depending on the temperature of the surface and the ice phase. Residence times vary by several orders of magnitude between crystalline and amorphous CO, with the latter showing residence times on the order of seconds at 5 K. H2 may diffuse before desorption in amorphous ices, which might help to accommodate it in deeper binding sites. Conclusions. Based on our results, a significant fraction of H2 molecules will stick on CO ice under experimental conditions, even more so under the harsh conditions of prestellar cores. However, with the low H2–CO binding energies, residence times of H2 on CO ice before desorption are too short to consider a significant population of H2 molecules on pure CO ices. Diffusion is possible in a time window before desorption, which might help accommodate H2 on deeper binding sites, which would increase residence times on the surface.

Precise Control of Precursor Sticking Coefficient on Substrates With Large Aspect Ratios in ALD

In this paper, the Atomic Layer Deposition (ALD) process in the reactor scale was simulated using ANSYS® Fluent® 19.1 and ChemKin-PRO commercial packages in order to solve transport and chemistry equations. Trenched substrates with large aspect ratios (AR) of 33 ≤ AR ≤ 300, at a reactor scale of an ALD process, were considered a temperature of 473K and 573K. Trimethlyaluminum (TMA) and Ozone reactants were used to formulate a precise Sticking Coefficients (SC) for a satisfactory dose (precursor exposure level) in order to deposit conforming Al2O3 films. The estimated error in the empirical correlation of SCs was determined and compared to published experimental data. An empirical correlation based on simulated models was derived. This correlation can be exploited to determine precise precursor exposure levels for an ALD process irrespective of the type of reactor used.

KINETICS OF LOW PRESSURE AMMONIA OXIDATION OVER RH(111)

The kinetics of the NH3 + O2 reaction over a Rh(111) single crystal catalytic surface was explored in the 10-6 mbar pressure range at temperatures between 300-900 K. Selectivity towards N2 and NO products, and reactive sticking coefficients were monitored in situ using differentially pumped quadrupole mass spectroscopy (QMS).