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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.

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The radiative cooling of naphthalene dimer cations, (C$_{10}$H$_8$)$_2^+$ was studied experimentally through action spectroscopy using two different electrostatic ion-beam storage rings, DESIREE in Stockholm and Mini-Ring in Lyon. The spectral characteristics of the charge resonance (CR) band were observed to vary significantly with storage time of up to 30 seconds in DESIREE. In particular, the position of the CR band shifts to the blue, with specific times (inverse of rates) of 0.64 s and 8.0 s in the 0-5 s and 5-30 s storage time ranges, respectively. Such long-time scales evoke that the internal energy distribution of the stored ions evolves by vibrational radiative cooling, which is consistent with the absence of fast radiative cooling via recurrent fluorescence for (C$_{10}$H$_8$)$_2^+$. Density Functional based Tight Binding calculations with local excitations and Configuration Interactions (DFTB-EXCI) were used to simulate the absorption spectrum for ion temperatures between 10 and 500 K. The evolution of the band width and position with temperature is in qualitative agreement with the experimental findings. Furthermore, these calculations yielded linear temperature dependencies for both the shift and the broadening. Combining the relation between the CR band position and the ion temperature with the results of the statistical model, we demonstrate that the observed blue shift can be used to determine the radiative cooling rate of (C$_{10}$H$_8$)$_2^+$.

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Context. Mid-infrared observations of photodissociation regions (PDRs) are dominated by strong emission features called aromatic infrared bands (AIBs). The most prominent AIBs are found at 3.3, 6.2, 7.7, 8.6, and 11.2 µm. The most sensitive, highest-resolution infrared spectral imaging data ever taken of the prototypical PDR, the Orion Bar, have been captured by JWST. These high-quality data allow for an unprecedentedly detailed view of AIBs.Aims. We provide an inventory of the AIBs found in the Orion Bar, along with mid-IR template spectra from five distinct regions in the Bar: the molecular PDR (i.e. the three H2 dissociation fronts), the atomic PDR, and the H II region.Methods. We used JWST NIRSpec IFU and MIRI MRS observations of the Orion Bar from the JWST Early Release Science Program, PDRs4All (ID: 1288). We extracted five template spectra to represent the morphology and environment of the Orion Bar PDR. We investigated and characterised the AIBs in these template spectra. We describe the variations among them here.Results. The superb sensitivity and the spectral and spatial resolution of these JWST observations reveal many details of the AIB emission and enable an improved characterization of their detailed profile shapes and sub-components. The Orion Bar spectra are dominated by the well-known AIBs at 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7 µm with well-defined profiles. In addition, the spectra display a wealth of weaker features and sub-components. The widths of many AIBs show clear and systematic variations, being narrowest in the atomic PDR template, but showing a clear broadening in the H II region template while the broadest bands are found in the three dissociation front templates. In addition, the relative strengths of AIB (sub-)components vary among the template spectra as well. All AIB profiles are characteristic of class A sources as designated by Peeters (2022, A&A, 390, 1089), except for the 11.2 µm AIB profile deep in the molecular zone, which belongs to class B11.2. Furthermore, the observations show that the sub-components that contribute to the 5.75, 7.7, and 11.2 µm AIBs become much weaker in the PDR surface layers. We attribute this to the presence of small, more labile carriers in the deeper PDR layers that are photolysed away in the harsh radiation field near the surface. The 3.3/11.2 AIB intensity ratio decreases by about 40% between the dissociation fronts and the H II region, indicating a shift in the polycyclic aromatic hydrocarbon (PAH) size distribution to larger PAHs in the PDR surface layers, also likely due to the effects of photochemistry. The observed broadening of the bands in the molecular PDR is consistent with an enhanced importance of smaller PAHs since smaller PAHs attain a higher internal excitation energy at a fixed photon energy.Conclusions. Spectral-imaging observations of the Orion Bar using JWST yield key insights into the photochemical evolution of PAHs, such as the evolution responsible for the shift of 11.2 µm AIB emission from class B11.2 in the molecular PDR to class A11.2 in the PDR surface layers. This photochemical evolution is driven by the increased importance of FUV processing in the PDR surface layers, resulting in a “weeding out” of the weakest links of the PAH family in these layers. For now, these JWST observations are consistent with a model in which the underlying PAH family is composed of a few species: the so-called ‘grandPAHs’.Key words: astrochemistry / infrared: ISM / ISM: molecules / ISM: individual objects: Orion Bar / photon-dominated region (PDR) / techniques: spectroscopic★ The 5 template spectra are available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/685/A75★★ Tim Lee sadly passed away on Nov. 3, 2022.

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Recently, some of us reviewed and studied the photoionization dynamics of C60 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 C60+ cation. Our previous analysis of the threshold photoelectron spectrum (TPES) of C60 [Hrodmarsson et al., Phys. Chem. Chem. Phys. 22, 13880–13892 (2020)] appeared to give indication of D3d ground state symmetry, in contrast to theoretical predictions of D5d 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 hg Jahn–Teller active modes, which couple to the 2Hu components of the ground state of the C60+ cation. By reanalyzing our measured TPES of the ground state of the C60 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 C60 and C60+. 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 D5d symmetric ground state of the C60+ 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 C60 with greater accuracy than has been achieved at 7.598 ± 0.005 eV, but we also assign the two lowest excited states (2E1u and 2E2u) which are visible in our TPES. Finally, we discuss the energetics of additional DIBs that could be assigned to C60+ in the future.

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(Abridged) We investigate the impact of radiative feedback from massive stars on their natal cloud and focus on the transition from the HII region to the atomic PDR (crossing the ionisation front (IF)), and the subsequent transition to the molecular PDR (crossing the dissociation front (DF)). We use high-resolution near-IR integral field spectroscopic data from NIRSpec on JWST to observe the Orion Bar PDR as part of the PDRs4All JWST Early Release Science Program. The NIRSpec data reveal a forest of lines including, but not limited to, HeI, HI, and CI recombination lines, ionic lines, OI and NI fluorescence lines, Aromatic Infrared Bands (AIBs including aromatic CH, aliphatic CH, and their CD counterparts), CO2 ice, pure rotational and ro-vibrational lines from H2, and ro-vibrational lines HD, CO, and CH+, most of them detected for the first time towards a PDR. Their spatial distribution resolves the H and He ionisation structure in the Huygens region, gives insight into the geometry of the Bar, and confirms the large-scale stratification of PDRs. We observe numerous smaller scale structures whose typical size decreases with distance from Ori C and IR lines from CI, if solely arising from radiative recombination and cascade, reveal very high gas temperatures consistent with the hot irradiated surface of small-scale dense clumps deep inside the PDR. The H2 lines reveal multiple, prominent filaments which exhibit different characteristics. This leaves the impression of a "terraced" transition from the predominantly atomic surface region to the CO-rich molecular zone deeper in. This study showcases the discovery space created by JWST to further our understanding of the impact radiation from young stars has on their natal molecular cloud and proto-planetary disk, which touches on star- and planet formation as well as galaxy evolution.

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Sujets

Chimie quantique Atomic data Database Charge transfer state Water clusters Dissociation 22 pole cryogenic ion trap Infrared spectroscopy Clusters Ammonium/ammonia water clusters Molecular processes Anharmonic Infrared Spectroscopy Disconnectivity Tree CAH CID Alanine dipeptide Astrochemistry Molecular dynamics Cryogenic ion trap Infrared ISM ISM molecules Argon Density functional theory Charged system and open shell Car-Parrinello molecular dynamics Carbon cluster Quantum chemistry Champ de forces Charge resonance Nanoparticles Polycyclic Aromatic Hydrocarbons DFT Approche mixte quantique/classique Modelling Infrared spectra Astrochimie Biodegradation Au147 Dissipation Agrégats aqueux d'ammonium/ammoniac Atrazine Collision Induced Dissociation DFTB-CI Dftb Density functional based tight binding DFTB Atomic scattering from surfaces Excited states Dynamique électronique Chemical shift HAP 2 Benzene dimers Barium Methods laboratory molecular Agrégats aqueux Line profiles Disconnectivity tree Agrégats protonés uracile-eau Dynamique moléculaire ADFT SCC-DFTB Carbonaceous grains White dwarfs Molecular data DUST BOMD Auxiliary density functional theory Amorphous Corannulene Catalysis Benzene Polycyclic aromatic hydrocarbon PAH CONFIGURATION-INTERACTION Argile Agrégats moléculaires Agrégats Configuration interaction 1 Clustering Threshold algorithm DFTB Optical spectra CONSTANTS Molecular clusters Agrégats d'eau Photon-dominated region PDR Modélisation Density functional tight binding Agrégats protonés Carbon clusters Catalyse Density Functional Theory Probability flows PAH Clay mineral Abundances -ISM Dusty plasma QSAR Brown dwarfs

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