arXiv subjects

Tomás González-Lezana

Publications and source records attributed to Tomás González-Lezana.

6 recordsLinked to original sources

arXiv2026

Chemistry and ro-vibrational excitation of CH$^+$ in the planetary nebula NGC 7027

Small carbon hydride cations, such as the methylidyne ion (CH$^+$), play an important role in the chemistry of the interstellar medium (ISM). They participate in gas-phase reaction networks leading to the formation of hydrocarbon species that act as precursors to more complex organic molecules. CH$^+$ is a highly reactive ion that is rapidly destroyed by H, H$_2$, and free electrons, making its excitation challenging to model. Its level populations depend not only on radiative and inelastic processes but also on chemical formation and destruction rates, a mechanism known as chemical pumping. We investigate this effect using a new set of ab initio state-resolved ro-vibrational (reactive and inelastic) collision data to model the observed CH$^+$ emission. Multiple rotational and ro-vibrational transitions of CH$^+$ detected toward the planetary nebula NGC 7027 are analyzed. The chemical structure of CH$^+$ is modeled with the CLOUDY code using updated reaction rates, providing the temperature and density structure across the nebula. A non-local thermodynamic equilibrium (NLTE) analysis is performed using CLOUDY and the single-zone RADEX code with a comprehensive set of spectroscopic and collisional data. In addition, chemical formation and destruction processes are implemented in RADEX and explored via Markov Chain Monte Carlo sampling. The CLOUDY model reproduces the observed CH$^+$ line fluxes within a factor of 1.3 on average. It indicates that rotational and ro-vibrational lines arise from physically distinct regions, primarily differing in temperature. RADEX models show that chemical pumping significantly enhances populations above ($\upsilon = 0, J = 1$), strongly increasing ro-vibrational emission, especially in the $\upsilon =2 \to 1$ band. Single-zone models remain limited, highlighting the need for full 1D modeling including all excitation processes.

astro-ph.GA↗

arXiv2026

Coordination-driven magic numbers in protonated argon clusters

The structural properties of rare-gas clusters can be primarily described by a simple sphere packing model or by pairwise interactions. Remarkably, adding a single proton yields a large set of magic numbers that has remained unexplained. In this Letter, we unravel their origin by combining quantum Monte Carlo techniques with many-body ab initio potentials that correctly capture the proton's coordination environment. Thanks to this approach, we find that argon atoms are mainly localized around the classical minimum, resulting in a particularly rigid behavior in stark contrast to lighter rare-gas clusters. Moreover, as cluster size increases, we identify a clear structural transition from many-body coordination-driven stability to a regime dominated by two-body interactions, reflecting a reshaping of the underlying potential energy landscape.

physics.atm-clus↗

arXiv2025

Time-resolved solvation dynamics of Li$^+$, Na$^+$ and K$^+$ ions in liquid helium nanodroplets

In 2023, ultrafast pump-probe spectrocopy was used to record the solvation dynamics of a single Na$^+$ ion in a liquid helium droplet, atom-by-atom and with femtosecond time resolution [Albrechtsen \textit{et al., Nature}, 2023, \textbf{623}, 319]. Subsequently, theoretical studies showed that other alkali ions solvate in a similar manner but no experimental results were reported so far. Here, we extend the previous measurement on Na$^+$ to Li$^+$ and K$^+$ ions. A pump pulse selectively ionizes an alkali atom, initially residing at the droplet surface, and the ensuing solvation dynamics of the formed alkali cation, Ak$^+$, is followed by ionizing a Xe atom, located in the droplet interior, and recording the yields of Ak$^+$He$_n$ ions expelled from the droplet as a function of the pump-probe pulse delay. We find that Li$^+$, Na$^+$ and K$^+$ ions solvate with a binding rate of 1.8 $\pm$ 0.1, 1.8 $\pm$ 0.1 and 1.7 $\pm$ 0.1 He per ps, respectively. Furthermore, by comparing the number distribution of the Ak$^+$He$_n$ ion yields to the evaporation energies of these ion--He complexes, obtained by Path Integral Monte Carlo calculations, we identify signatures of the first solvation shells of Li$^+$, Na$^+$ and K$^+$. Lastly, we determine the time-dependent dissipation of the solvation energy from the vicinity of the three alkali ion species and find that the rate is highest (lowest) for Li$^+$ (K$^+$)

physics.chem-ph↗

arXiv2025

Femtosecond-and-atom-resolved solvation dynamics of a Na$^+$ ion in a helium nanodroplet

Recently, it was shown how the primary steps of solvation of a single Na$^+$ ion, instantly created at the surface of a nanometer-sized droplet of liquid helium, can be followed at the atomic level [Albrectsen et al. Nature $\textbf{623}$, 319 (2023)]. This involved measuring, with femtosecond time resolution, the gradual attachment of individual He atoms to the Na$^+$ ion as well as the energy dissipated from the local region of the ion. In the current work, we provide a more comprehensive and detailed description of the experimental findings of the solvation dynamics, and present an improved Poisson-statistical analysis of the time-resovled yields of the solvation complexes, Na$^+$He$_n$. For droplets containing an average of 5200 He atoms, this analysis gives a binding rate of $1.84\pm0.09$ atoms/ps for the binding of the first five He atoms to the Na$^+$ ion. Also, thanks to accurate heoretical values for the evaporation energies of the Na$^+$He$_n$ complexes, obtained by Path Integral Monte Carlo methos using a new potential energy surface presented here for the first time, we improved the determination of the time-dependent removal of the solvation energy from the region around the sodium ion. We find that it follows Newton's law of cooling for the first 6 ps. Measurements were carried out for three different average droplet sizes, $\langle N_D\rangle = $ 9000, 5200 and 3600 helium atoms, and differences between these results are discussed.

physics.atm-clus↗

arXiv2020

Experimental and Theoretical Studies of the N(2D) + H2 and D2 Reactions

This study reports the results of an experimental and theoretical investigation of the N(2D) + H2 and N(2D) + D2 reactions at room temperature and below. On the experimental side, a supersonic flow (Laval nozzle) reactor was employed to measure rate constants for these processes at temperatures as low as 127 K. N(2D) was produced indirectly by pulsed laser photolysis and these atoms were detected directly by pulsed laser induced fluorescence in the vacuum ultraviolet wavelength region. On the theoretical side, two different approaches were used to calculate rate constants for these reactions; a statistical quantum mechanical (SQM) method and a quasi-classical trajectory capture model including a semi-classical correction for tunneling (SC-Capture). This work is described in the context of previous studies, while the discrepancies between both experiment and theory, as well as between the theoretical results themselves are discussed.

physics.chem-ph↗

arXiv2019

Lithium ions solvated in helium

We report on a combined experimental and theoretical study of Li$^+$ ions solvated by up to 50 He atoms. The experiments show clear enhanced abundances associated with He$_n$Li$^+$ clusters where $n=2$, 6, 8, and 14. We find that classical methods, e.g.\ Basin-Hopping (BH), give results that qualitatively agree with quantum mechanical methods such as path integral Monte Carlo, diffusion Monte Carlo and quantum free energy, regarding both energies and the solvation structures that are formed. The theory identifies particularly stable structures for $n=4$, 6 and 8 which line up with some of the most abundant features in the experiments.

physics.atm-clus↗