arXiv subjects

Sándor Demes

Publications and source records attributed to Sándor Demes.

4 recordsLinked to original sources

arXiv2026

Collisional excitation of cyclopentadiene by helium. A complete set of rate coefficients and astrophysical applications

Complex organic molecules, including large cyclic species, are prevalent in interstellar space and play a key role in various astrochemical processes. Cyclopentadiene (c-C5H6) is a five-membered cyclic hydrocarbon recently detected in TMC-1 and some other interstellar molecular clouds. While accurate spectroscopic data were available, collisional rate coefficients for its rotational transitions were missing so far, introducing a potential limitation in the interpretation of the observations. This study aims to provide a comprehensive set of state-to-state thermal rate coefficients for the rotational excitation of c-C5H6 due to collisions with helium, crucial for non-local thermodynamic equilibrium (non-LTE) radiative transfer modelling in astrophysical environments, and to examine how far the molecule is from thermalisation under the physical conditions of cold molecular clouds. The research employed accurate quantum scattering calculations using the close-coupling (CC) and coupled states (CS) methods, based on a highly-correlated three-dimensional potential energy surface for the [c-C5H6 + He] collisional complex. Calculations were performed for a wide range of rotational states (from j = 0 to j \leq 25) and kinetic temperatures (from 10 to 50 K). We calculated a complete set of thermal rate coefficients for both ortho- and para-C5H6. Radiative transfer simulations demonstrated that most rotational levels of cyclopentadiene are fully thermalized under typical cold cloud conditions and exhibit minor non-LTE effects. Nevertheless, this study is the first to utilise accurate state-to-state rate coefficients for radiative transfer simulation of a large, five-membered cyclic species detected in space. This allows to draw some general conclusions and paves the way for future studies of complex astromolecules that will enable a more precise interpretation of upcoming detections.

astro-ph.GA↗

arXiv2024

Rotational excitation cross sections for chloronium based on a new 5D interaction potential with molecular hydrogen

Chloronium (H$_2$Cl$^+$) is an important intermediate of Cl-chemistry in space. The accurate knowledge of its collisional properties allows a better interpretation of the corresponding observations in interstellar clouds and therefore a better estimation of its abundance in these environments. While the ro-vibrational spectroscopy of H$_2$Cl$^+$ is well known, the studies of its collisional excitation are rather limited and these are available for the interaction with helium atoms only. We provide a new 5-dimensional rigid-rotor potential energy surface for the interaction of H$_2$Cl$^+$ with H$_2$, calculated from explicitly correlated coupled cluster ab initio theory, which was fitted then with a set of analytical functions, allowing to perform scattering calculations using accurate quantum theories. We analyze the collision-energy-dependence of the rotational state-to-state cross sections and the temperature dependence of the corresponding thermal rate coefficients, with a particular attention on the collisional propensity rules. When comparing our results for collisions with H$_2$ with those obtained with He as a colliding partner, we found very significant differences with non-linear scaling trends, which proves again that He is not a suitable proxy for collisions between hydride molecules and molecular hydrogen, the most abundant gas particle in the interstellar medium.

physics.chem-ph↗

arXiv2024

First close-coupling study of the excitation of a large cyclic molecule: collision of c-C5H6 with He

Recent astronomical observations revealed an increasing molecular complexity in the interstellar medium through the detection of a series of large cyclic carbon species. To correctly interpret these detections, a complex analysis is necessary that takes into account the non-local thermodynamic equilibrium (non-LTE) conditions of the emitting media (e.g. when energy level populations deviate from a Boltzman distribution). This requires proper state-to-state collisional data for the excitation and de-excitation processes of the molecular levels. Cyclopentadiene (c-C5H6), which was recently detected in multiple cold interstellar clouds, is extensively studied in many aspects due to its large importance for chemistry in general. At the same time, there are no collisional data available for this species, which are necessary for a more precise interpretation of the corresponding detections. In this work, we first provide an accurate 3D rigid-rotor interaction potential for the [c-C5H6 + He] complex from high-level of ab initio theories, which has been used to study their inelastic collision by the exact close coupling quantum scattering method. To the best of our knowledge, this is the first study where this method is systematically applied to treat the dynamics of molecular collisions involving more than ten atoms. We also analyse the collisional propensity rules and the differences in contrast to calculations, where the approximate coupled states scattering methods is used.

physics.atm-clus↗

arXiv2022

An accurate set of H$_3$O$^+ -$ H$_2$ collisional rate coefficients for non-LTE modelling of warm interstellar clouds

Hydronium (H$_3$O$^+$) was first detected in 1986 in interstellar molecular clouds. It was reported in many galactic diffuse and dense regions, as well as in extragalactic sources. H$_3$O$^+$ plays a major role both in interstellar oxygen and water chemistry. However, despite the large number of H$_3$O$^+$ observations, its collisional excitation was investigated only partially. In the present work we study the state-to-state rotational de-excitation of $ortho$- and $para$-H$_3$O$^+$ in collisions both with $ortho$- and $para$-H$_2$. The cross sections are calculated within the close-coupling formalism using a highly accurate potential energy surface developed for this system. The rate coefficients are computed up to $300$ K kinetic temperature. Transitions between the lowest 21 rotation-inversion states were studied for $para$-H$_3$O$^+$, and the lowest 11 states for $ortho$-H$_3$O$^+$, i.e. all levels with rotational energies below 430 K ($\sim 300$ cm$^{-1}$) are considered (up to $j\leq5$). In order to estimate the impact of the new rate coefficients on the astrophysical models for H$_3$O$^+$, radiative transfer calculations were also carried out. We have examined how the new collisional data affect the line intensities with respect to older data previously used for the interpretation of observations. By analysing all detected transitions we find that our new, accurate rate coefficients have a significant impact (typically within a factor of 2) on radiation temperatures, allowing more accurate estimation of column densities and relative abundances of hydronium, especially in warm molecular clouds, paving the path towards better interpretation of interstellar water and oxygen chemistry.

astro-ph.GA↗