arXiv subjects

Ramón J. Peláez

Publications and source records attributed to Ramón J. Peláez.

4 recordsLinked to original sources

arXiv2026

The Significant Role of Hydrogen in the Formation of Silicon Carbide in Evolved Stars

Cosmic dust is mainly formed in the atmospheres of evolved stars. In carbon rich stars, amorphous carbon along with silicon carbide are the main constituents of dust grains yet the mechanisms involved in the formation of these grains are still poorly understood. Several molecular precursors have been proposed to form silicon carbide grains. Here, we have simulated in the laboratory the formation of silicon carbide dust starting from atomic C, atomic Si and H$_2$ and we have clearly identified SiC$_2$ as a key molecular precursor of nanodust analogues. We show that the interaction of molecular hydrogen with atomic carbon initiates the formation of hydrocarbons, which then react with atomic silicon to produce gas-phase SiC$_2$. In our experiments, the silicon carbide nanodust analogues are partially hydrogenated. Chemical routes for the formation of SiC$_2$ and organosilicon species are discussed on the basis of thermochemical calculations and chemical kinetics modelling. Our findings reveal the central role of molecular hydrogen in the formation of SiC$_2$ and contribute to a deeper understanding of silicon carbide dust formation processes in evolved stars, from atoms to molecules, clusters, and ultimately dust grains.

astro-ph.GA↗

arXiv2025

Hydroxylamine in Astrophysical Ices: IR Spectra and Cosmic Ray-Induced Radiolytic Chemistry

Gas-phase hydroxylamine (NH$_2$OH) has recently been detected within dense clouds in the interstellar medium. However, it is also likely present within interstellar ices, as well as on the icy surfaces of outer Solar System bodies, where it may react to form more complex prebiotic molecules such as amino acids. In this work, we aimed to provide IR spectra of NH$_2$OH in astrophysical ice analogues that will help in the search for this molecule in various astrophysical environments. Furthermore, we aimed to provide quantitative information on the stability of NH$_2$OH upon exposure to ionizing radiation analogous to cosmic rays, as well as on the ensuing chemistry and potential formation of complex prebiotic molecules. Ices composed of NH$_2$OH, H$_2$O, and CO were prepared by vapor deposition and IR spectra were acquired between $4000-500$~cm$^{-1}$ ($2.5-20 μ$m) prior and during irradiation using 15 keV protons. In interstellar ice analogues, the most prominent IR absorption band of NH$_2$OH is that at about 1188~cm$^{-1}$, which may be a good candidate to use in searches of this species in icy space environments. Calculated effective destruction cross-sections and G-values for the NH$_2$OH-rich ices studied show that NH$_2$OH is rapidly destroyed upon exposure to ionizing radiation (more rapidly than a number of previously studied organic molecules), and that this destruction is slightly enhanced when it is mixed with other icy species. The irradiation of a NH$_2$OH:H$_2$O:CO ternary ice mixture leads to a rich chemistry that includes the formation of simple inorganic molecules such as NH$_3$, CO$_2$, OCN$^-$, and H$_2$O$_2$, as well as ammonium salts and, possibly, complex organic molecules relevant to life such as formamide, formic acid, urea, and glycine.

astro-ph.GA↗

arXiv2020

The chemistry of cosmic dust analogues from C, C$_2$, and C$_2$H$_2$ in C-rich circumstellar envelopes

Interstellar carbonaceous dust is mainly formed in the innermost regions of circumstellar envelopes around carbon-rich asymptotic giant branch (AGB) stars. In these highly chemically stratified regions, atomic and diatomic carbon, along with acetylene are the most abundant species after H$_2$ and CO. In a previous study, we addressed the chemistry of carbon (C and C$_2$) with H$_2$ showing that acetylene and aliphatic species form efficiently in the dust formation region of carbon-rich AGBs whereas aromatics do not. Still, acetylene is known to be a key ingredient in the formation of linear polyacetylenic chains, benzene and polycyclic aromatic hydrocarbons (PAHs), as shown by previous experiments. However, these experiments have not considered the chemistry of carbon (C and C$_2$) with C$_2$H$_2$. In this work, by employing a sufficient amount of acetylene, we investigate its gas-phase interaction with atomic and diatomic carbon. We show that the chemistry involved produces linear polyacetylenic chains, benzene and other PAHs, which are observed with high abundances in the early evolutionary phase of planetary nebulae. More importantly, we have found a non-negligible amount of pure and hydrogenated carbon clusters as well as aromatics with aliphatic substitutions, both being a direct consequence of the addition of atomic carbon. The incorporation of alkyl substituents into aromatics can be rationalized by a mechanism involving hydrogen abstraction followed by methyl addition. All the species detected in gas phase are incorporated into the nanometric sized dust analogues, which consist of a complex mixture of sp, sp$^2$ and sp$^3$ hydrocarbons with amorphous morphology.

astro-ph.GA↗

arXiv2018

Mid-to-far infrared tunable perfect absorption by a sub - λ/100 nanofilm in a fractal phasor resonant cavity

Integrating an absorbing thin film into a resonant cavity is the most practical way to achieve perfect absorption of light at a selected wavelength in the mid-to-far infrared, as required to target blackbody radiation or molecular fingerprints. The cavity is designed to resonate and enable perfect absorption in the film at the chosen wavelength λ. However, in current state-of-the-art designs, a still large absorbing film thickness (λ/50) is needed and tuning the perfect absorption wavelength over a broad range requires changing the cavity materials. Here, we introduce a new resonant cavity concept to achieve perfect absorption of infrared light in much thinner and thus really nanoscale films, with a broad wavelength tunability using a single set of cavity materials. It requires a nanofilm with giant refractive index and small extinction coefficient (found in emerging semi-metals, semi-conductors and topological insulators) backed by a transparent spacer and a metal mirror. The nanofilm acts both as absorber and multiple reflector for the internal cavity waves, which after escaping follow a fractal phasor trajectory. This enables a totally destructive optical interference for a nanofilm thickness more than 2 orders of magnitude smaller than λ. With this remarkable effect, we demonstrate angle-insensitive perfect absorption in sub - λ/100 bismuth nanofilms, at a wavelength tunable from 3 to 20 μm.

physics.optics↗