Loading...
Derniers dépôts, tout type de documents
Recently, some of us reviewed and studied the photoionization dynamics of C<sub>60</sub> that are of great interest to the astrochemical community as four of the diffuse interstellar bands (DIBs) have been assigned to electronic transitions in the C<sub>60</sub>$^+$ cation. Our previous analysis of the threshold photoelectron spectrum (TPES) of C<sub>60</sub> [Hrodmarsson et al., Phys. Chem. Chem. Phys. 22, 13880–13892 (2020)] appeared to give indication of D<sub>3d</sub>$^+$ ground state symmetry, in contrast to theoretical predictions of D<sub>5d</sub>$^+$ symmetry. Here, we revisit our original measurements taking account of a previous theoretical spectrum presented in the work of Manini et al., Phys. Rev. Lett. 91(19), 196402 (2003), obtained within a vibronic model parametrized on density functional theory/local-density approximation electronic structure involving all h<sub>g</sub> Jahn–Teller active modes, which couple to the $^2$H<sub>u</sub> components of the ground state of the C<sub>60</sub>$^+$ cation. By reanalyzing our measured TPES of the ground state of the C<sub>60</sub> Buckminsterfullerene, we find a striking resemblance to the theoretical spectrum calculated in the work of Manini et al., Phys. Rev. Lett. 91(19), 196402 (2003), and we provide assignments for many of the hg modes. In order to obtain deeper insights into the temperature effects and possible anharmonicity effects, we provide complementary modeling of the photoelectron spectrum via classical molecular dynamics (MD) involving density functional based tight binding (DFTB) computations of the electronic structure for both C<sub>60</sub> and C<sub>60</sub>$^+$. The validity of the DFTB modeling is first checked vs the IR spectra of both species which are well established from IR spectroscopic studies. To aid the interpretation of our measured TPES and the comparisons to the ab initio spectrum we showcase the complementarity of utilizing MD calculations to predict the PES evolution at high temperatures expected in our experiment. The comparison with the theoretical spectrum presented in the work of Manini et al., Phys. Rev. Lett. 91(19), 196402 (2003), furthermore, provides further evidence for a DC<sub>5d</sub> symmetric ground state of the C<sub>60</sub>$^+$ cation in the gas phase, in complement to IR spectroscopy in frozen noble gas matrices. This not only allows us to assign the first adiabatic ionization transition and thus determine the ionization energy of C<sub>60</sub> with greater accuracy than has been achieved at 7.598 ± 0.005 eV, but we also assign the two lowest excited states ($^2E$<sub>1u</sub> and $^2E$<sub>2u</sub>) which are visible in our TPES. Finally, we discuss the energetics of additional DIBs that could be assigned to C<sub>60</sub>$^+$ in the future.
This article corresponds to part II of a series about singly and multiply charged gold clusters. From their total energies in the size range n = 3 -20 and charge q = 0-4 determined in part I one of the series, it aims to present a Density Functional based Tight Binding approach of their stability/metastability versus atomization and fragmentation, as well as their ionization properties to different charge states. The present DFTB results are discussed with respect to previous theoretical or experimental investigations.
This series of two papers reports the investigation of the properties of multiply charged gold clusters cations Aunq+ (n = 3 -20) up to charge q = 4. In the present part I of the study, a global exploration of their potential energy surface has been performed using a combination of parallel tempering molecular dynamics and quenches performed at the DFTB level. When increasing the charge of the clusters, their structure was found to evolve from a compact form to an elongated one, shifting the 2D/3D transition toward larger sizes. Such structure elongation is explained by the minimization of coulombic destabilization. In the further part II of this study, the stability of these low-energy isomers will be discussed, with a focus on the cluster's ionization and fragmentation.
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.
This work examines the reliability of the Self Consistent Charge Density Functional based Tight Binding (SCC-DFTB) scheme to derive geometrical and thermochemistry observables for complexes and clusters made of Ag, C and H atoms. In addition to the currently available DFTB parameterization DFTBhyb, it proposes a new SCC-DFTB parameterization based on DFT Slater Koster integrals and recalibrated on atomic pairs MRCI calculations for clusters made of Ag, C and H atoms. Two sets of parameters were determined, one for restricted open shell SCC-DFTB, the other for spin-polarized SCC-DFTB. These two new sets of parameters, namely DFTB$^γ$ and DFTB$^{γpol}$ respectively, along with DFTB$^{hyb}$ , are first tested on Ag$_n$, Ag$_n$C and Ag$_n$H clusters. A key issue being the transferability of such parameters on different types of Ag-X bonds, the three sets of parameters are then tested on Ag$_m$C$_n$H$_p$ (m=1-3, n=2, p=0-2) complexes involving covalent and π metal-ligand bonds. The particular case of naphthalene C$_{10}$H$_8$ as a πligand is also investigated. In general, with respect to DFTB$^{hyb}$ results, using DFTB$^γ$ parameters leads to an improvement of geometries and energetics. In the case of Ag$_n$C$_{10}$H$_8$ clusters, the role of dispersion is evidenced. However, in a few cases, the geometries may distort due to a questionable description of charge transfer with DFTB$^γ$ and DFTB$^{γpol}$. The spin-polarized version of SCC-DFTB is suited to correctly describe open-shell species with more than one unpaired electron in their ground electronic state but is shown not to improve the results otherwise.
Sujets
Alanine dipeptide
CONSTANTS
Agrégats moléculaires
Barium
Anharmonic Infrared Spectroscopy
Excited states
Catalyse
SCC-DFTB
DFTB
Auxiliary density functional theory
Molecular processes
Carbonaceous grains
DFTB-CI
Electronic structure
Dehydrogenation mechanisms
Dynamique moléculaire
Quantum chemistry
Astrochemistry
White dwarfs
Atomic data
Modelling
Clusters
Charge
Charge transfer state
Optical spectra
Biodegradation
Catalysis
ISM molecules
Chimie quantique
Molecular data
Dynamique électronique
Au147
Collision Induced Dissociation
Carbon clusters
ADFT
Probability flows
Water clusters
Infrared spectroscopy
Astrochimie
HAP
Brown dwarfs
Approche mixte quantique/classique
Agrégats protonés
CONFIGURATION-INTERACTION
Polycyclic Aromatic Hydrocarbons
Argon
1
Density functional theory
Cryogenic ion trap
PAH
Abundances -ISM
PTMD
CID
DFT
Disconnectivity tree
CAH
Car-Parrinello molecular dynamics
Atomic scattering from surfaces
Gold clusters
Amorphous
Corannulene
Density Functional Theory
Polycyclic aromatic hydrocarbon PAH
Ammonium/ammonia water clusters
22 pole cryogenic ion trap
Carbon cluster
Gold
2
Benzene
DUST
Agrégats d'eau
BOMD
Threshold algorithm
Clay mineral
Benzene dimers
Clustering
Chemical shift
Infrared ISM
QSAR
Photon-dominated region PDR
Molecular clusters
Atrazine
Nanoparticles
Infrared spectra
Champ de forces
Polycyclic aromatic hydrocarbons
Agrégats aqueux d'ammonium/ammoniac
Charged system and open shell
Molecular dynamics
Modélisation
Agrégats aqueux
Agrégats protonés uracile-eau
Argile
Agrégats
Methods laboratory molecular
Configuration interaction
Line profiles
Database
Charge resonance