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A general scheme for calculating ternary recombination rate constants of atomic species based on a hybrid quantum–classical nonadiabatic dynamics approach is presented and applied to the specific case of the ternary recombination of atomic ions of argon in cold argon plasmas. Rate constants are reported for both fine-structure states of the ion, and , T = 300 K, and for selected values of the reduced electric field. A thorough comparison with the literature data available for T = 300 K and a couple of close temperatures is performed with a favorable agreement achieved. It is shown that the excited ions may contribute to the formation of dimer ions, , as efficiently as the ground-state ions, , due to fast internal conversion of the electronic energy, which takes place in ternary collision complexes, .
We present the first theoretical line profile calculations of the ultraviolet spectral lines of carbon perturbed by helium using a semiclassical collision approach and high-quality ab initio potentials and electronic transition dipole moments. The temperature range is from 5000 to 8000 K. These results are important for astrophysical modelling of spectra in atmospheres of white dwarf stars showing atomic carbon in an helium atmosphere. Beyond the conventional symmetrical Lorentzian core at low He density, these lines exhibit a blue asymmetric behaviour. This blue asymmetry is a consequence of low maxima in the corresponding C–He potential energy difference curves at short internuclear distances. The collisional profiles are carefully examined and their perturber density dependence allow to understand the various line shapes of the observed carbon spectral lines in helium-rich white dwarf photosphere where the He perturber densities reach several 1021 cm−3.
Formation, distribution and behaviour of Complex Organic Molecules (COM's) in space is an important subject of research to the better understanding of the initial condition for the appearance of life on Earth. Furthermore, the study of high energy chemical processes in the interstellar medium (cosmic radiation's effect) and in solar system (solar wind's effect), is been of high interest. The aim of this work is to study astrophysical molecules trapped in interstellar ice systems under the effect of high energy radiation. These ices are characterised by being large systems, with large number of atoms. QM/MM hybrid method has become a very popular tool for molecular systems' simulations with a large number of atoms, appearing as a good compromise between accuracy and computational costs. We report the implementation of QM/MM hybrid method in the deMonNano software, using the Density Functional based Tight Binding (DFTB), an approximated DFT scheme, combined with Molecular Mechanic (MM) approach, namely Force Fields (FF) of class 1, such as OPLS-AA and AMBER-families of FFs. A complete implementation was performed using the QM/MM additive coupling scheme. In addition, the investigation of high energy chemical processes requires the explicit simulation of the electronic dynamics beyond the Born Oppenheimer approximation. As first step towards such dynamics, we will report the implementation of Real Time TD-DFTB in deMonNano, consisting in solving the Time-Dependent Schrödinger equation within the DFTB, where the electronic density matrix is propagated along time. We report a detailed introduction to new DFTB/MM and RT-TD-DFTB implementations as well as the complete study on glycine prebiotic molecule trapped in an interstellar ice. PAH interstellar systems will be also a matter of study.
Fullerene C 60 is one of the most iconic forms of carbon found in the interstellar medium (ISM). The interstellar chemistry of carbon-rich components, including fullerenes, is driven by a variety of energetic processes including UV and X-ray irradiation, cosmic-ray (CR) bombardment, electron impact, and shock waves. These violent events strongly alter the particle phase and lead to the release of new molecular species in the gas phase. Only a few experimental studies on the shock processing of cosmic analogs have been conducted so far. We explored in the laboratory the destruction of buckminsterfullerene C 60 using a pressure-driven shock tube coupled with optical diagnostics. Our efforts were first devoted to probing in situ the shock-induced processing of C 60 at high temperatures (≤ 4500 K) by optical emission spectroscopy. The analysis of the spectra points to the massive production of C 2 units. A broad underlying continuum was observed as well and was attributed to the collective visible emission of carbon clusters, generated similarly in large amounts. This proposed assignment was performed with the help of calculated emission spectra of various carbon clusters. The competition between dissociation and radiative relaxation, determined by statistical analysis, alludes to a predominance of clusters with less than 40 carbon atoms. Our laboratory experiments, supported by molecular dynamics simulations performed in the canonical ensemble, suggest that C 60 is very stable, and that high-energy input is required to process it under interstellar low-density conditions and to produce C 2 units and an abundance of intermediate-sized carbon clusters. These results provide some insights into the life cycle of carbon in space. Our findings hint that only J-type shocks with velocities above ~100 km s −1 or C-type shocks with velocities above 9 km s −1 can lead to the destruction of fullerenes. Observational tracers of this process remain elusive, however. Our work confirms the potential of shock tubes for laboratory astrophysics.
Mid-infrared emission features probe the properties of ionized gas, and hot or warm molecular gas. The Orion Bar is a frequently studied photodissociation region (PDR) containing large amounts of gas under these conditions, and was observed with the MIRI IFU aboard JWST as part of the "PDRs4All" program. The resulting IR spectroscopic images of high angular resolution (0.2") reveal a rich observational inventory of mid-IR emission lines, and spatially resolve the substructure of the PDR, with a mosaic cutting perpendicularly across the ionization front and three dissociation fronts. We extracted five spectra that represent the ionized, atomic, and molecular gas layers, and measured the most prominent gas emission lines. An initial analysis summarizes the physical conditions of the gas and the potential of these data. We identified around 100 lines, report an additional 18 lines that remain unidentified, and measured the line intensities and central wavelengths. The H I recombination lines originating from the ionized gas layer bordering the PDR, have intensity ratios that are well matched by emissivity coefficients from H recombination theory, but deviate up to 10% due contamination by He I lines. We report the observed emission lines of various ionization stages of Ne, P, S, Cl, Ar, Fe, and Ni, and show how certain line ratios vary between the five regions. We observe the pure-rotational H$_2$ lines in the vibrational ground state from 0-0 S(1) to 0-0 S(8), and in the first vibrationally excited state from 1-1 S(5) to 1-1 S(9). We derive H$_2$ excitation diagrams, and approximate the excitation with one thermal (~700 K) component representative of an average gas temperature, and one non-thermal component (~2700 K) probing the effect of UV pumping. We compare these results to an existing model for the Orion Bar PDR and highlight the differences with the observations.
Sujets
CONFIGURATION-INTERACTION
Carbonaceous grains
Clusters
BOMD
Molecular dynamics
Molecular data
Dynamics
Dissociation
Alanine dipeptide
1
Approche mixte quantique/classique
Champ de forces
ISM molecules
Molecular clusters
Density functional tight binding
Agrégats protonés
Disconnectivity tree
Clustering
Dynamique moléculaire
Database
Agrégats aqueux d'ammonium/ammoniac
SCC-DFTB
Agrégats moléculaires
Quantum chemistry
Amorphous
Atomic scattering from surfaces
Anharmonic Infrared Spectroscopy
Astrochimie
ADFT
Disconnectivity Tree
Dissipation
Density functional based tight binding DFTB
Methods laboratory molecular
Dynamique Moléculaire Car-Parrinello
Optical spectra
PAH
QSAR
Car-Parrinello molecular dynamics
Probability flows
DFTB-CI
Catalysis
DUST
Chimie quantique
Barium
Auxiliary density functional theory
Line profiles
Clay mineral
Brown dwarfs
Carbon cluster
Infrared spectroscopy
Benzene dimers
Density functional theory
Chemical shift
Threshold algorithm
Atomic data
HAP
Excited states
Charged system and open shell
Ammonium/ammonia water clusters
22 pole cryogenic ion trap
Nanoparticles
Configuration interaction
Modelling
Astrochemistry
Agrégats aqueux
Molecular processes
Collision Induced Dissociation
Argon
Modélisation
CONSTANTS
Polycyclic aromatic hydrocarbon PAH
CID
Agrégats
White dwarfs
DFTB
DFT
Polycyclic Aromatic Hydrocarbons
Dusty plasma
Charge transfer state
Corannulene
Agrégats protonés uracile-eau
Agrégats d'eau
Dynamique électronique
Biodegradation
Benzene
Dftb
Catalyse
Argile
Au147
Atrazine
2
Charge resonance
Abundances -ISM
Infrared spectra
Cryogenic ion trap
Carbon clusters
CAH
Density Functional Theory
Water clusters