The unbiased Diffusion Monte Carlo: a versatile tool for two-electron systems confined in different geometries

Computational codes based on the Diffusion Monte Carlo method can be used to determine the quantum state of two-electron systems confined by external potentials of various nature and geometry. In this work, we show how the application of this technique in its simplest form, that does not employ complex analytic guess functions, allows to obtain satisfactory results and, at the same time, to write programs that are readily adaptable from one type of confinement to another. This adaptability allows an easy exploration of the many possibilities in terms of both geometry and structure of the system. To illustrate these results, we present calculations in the case of two-electron hydrogen-based species (H$_2$ and H$_3^+$) and two different types of confinement, nanotube-like and octahedral crystal-field

Kinetics of Thermal Decomposition of Particulate Samples of MgCO3: Experiments and Models

In this work, we study the kinetics of thermal decomposition of MgCO3 in the form of particles of known size. In the experiments, the material is heated to a known temperature in a vacuum oven, and it is characterized, both before and after heating, by infrared spectroscopy and gravimetry. The agreement between the results of the two techniques is excellent. These results are rationalized by means of a model based on Languir's law, and the comparison with the experiments allows us to estimate the activation energy of the process. The reabsorption of atmospheric water by the oxide is shown spectroscopically, finding that is strongly influenced by the temperature of the process

Thermal decomposition of MgCO3 during the atmospheric entry of micrometeoroids

In this paper, a first study of the atmospheric entry of carbonate micrometeoroids, in an astrobiological perspective, is performed. Therefore an entry model, which includes two-dimensional dynamics, non-isothermal atmosphere, ablation and radiation losses, is build and benchmarked to literature data for silicate micrometeoroids. A thermal decomposition model of initially pure magnesium carbonate is proposed, and it includes thermal energy, mass loss and the effect of changing composition as the carbonate grain is gradually converted into oxide. Several scenarios are obtained by changing the initial speed, entry angle and grain diameter, producing a systematic comparison of silicate and carbonate grain. The results of the composite model show that the thermal behaviour of magnesium carbonate is markedly different from that of the corresponding silicate, much lower equilibration temperatures being reached in the first stages of the entry. At the same time, the model shows that the limit of a thermal protection scenario, based on magnesium carbonate, is the very high decomposition speed even at moderate temperatures, which results in the total loss of carbon already at about 100 km altitude. The present results show that, although decomposition and associated cooling are important effects in the entry process of carbonate grains, the specific scenario of pure MgCO3 micrograin does not allow complex organic matter delivery to the lower atmosphere. This suggests us to consider less volatile carbonates for further studies

Theoretical analysis of the atmospheric entry of sub-mm meteoroids of MgxCa1−xCO3 composition

Current models allow to reliably simulate mechanical and thermal phenomena associated with a mi- crometeor passage through the Earth’s atmosphere. However, these models have rarely been applied to materials other than those most common in meteorites, such as silicates and metals. A particular case that deserves attention is the one of micrograins made of minerals, in particular carbonates, which have been associated, in meteorites, with organic molecules. Carbonates are known for their decomposition in vacuum at moderate temperatures, and they might contribute to the thermal protection of organic mat- ter. In this work, a model with non isothermal atmosphere, power balance, evaporation, ablation, radia- tion losses and stoichiometry, is proposed. This paper includes the very first calculations for meteoroids with a mixed carbonate composition. Results show that the carbonate fraction of these objects always go to zero at high altitudes except for grazing entries, where the reached temperature is lower and some carbonate remains unreacted. For all entry conditions, peculiar temperature curves are obtained due to the decomposition process. Furthermore, a significant impact of decomposition cooling on the tempera- ture peak is observed for some grazing entry cases. Although specific solutions used in these calculations can be improved, this work sets a definite model and a basis for future research on sub-mm grains of relatively volatile minerals entering the Earth’s atmosphere

Atmospheric entry of sub-millimetre-sized grains into Mars atmosphere: white soft mineral micrometeoroids

In this work, we study the passage through the Martian atmosphere of micrometeorites with a white soft mineral (WSM) composition, which have been proposed as transporters of organic molecules in the solar system. The atmospheric entry model includes the dynamics of the atmospheric entry and the physico-chemical aspects of the thermal decomposition process. The results show that, due to the reduced entry speed, Mars may have been a promising collector of matter in this form. In particular, the chemical decomposition process is much more effective than in the case of the Earth's atmosphere in maintaining a moderate temperature of the micrometeorite during most of the entry process

Diffusion Monte Carlo calculations of the polarizability of a confined hydrogen atom: benchmarking and application to high symmetry wells

We present a non-perturbative direct method to calculate the polarizability of a hydrogen atom confined in a three-dimensional potential well of any geometry. The calculation is based on the diffusion Monte Carlo method. The advantage of the method is simplicity of implementation and immediate adaptability to any well shape. The method is validated for the well-studied spherically confined hydrogen atom, and demonstrated in the case of two other geometries (cube and octahedron), for which this paper provides the first set of results. Although demonstrated here for the confined hydrogen atom, the method can be immediately applied to any single-electron system placed in a three-dimensional potential well of any type and geometry. Results for a hydrogen atom confined in potential wells of cubic or spherical symmetry suggests that the polarizability in these cases is a universal function of the volume of the well. This result can simplify calculations for real situations such as in quantum dots