arXiv subjects

Giacomo Mulas

Publications and source records attributed to Giacomo Mulas.

18 recordsLinked to original sources

arXiv2026

CosmicPAH-IRDB: A Web Application for Including Anharmonicity in Simulated PAH Spectra for Astrophysics

Advances in infrared observations with the James Webb Space Telescope (JWST) call for new tools to exploit the information associated with the Aromatic Infrared Bands (AIBs), now observed in both distant galaxies and the embryos of planetary systems. The AIBs contain information about their carriers as well as the excitation conditions, making them powerful probes of the chemical and physical conditions of their environments - if their spectral information can be decoded. While large datasets of theoretical infrared spectra are becoming available, efforts have primarily focused on the contribution of chemical diversity to the AIBs. However, the results remain limited by the impact of the excitation process on the band profiles. The AIB carriers, thought to be polycyclic aromatic hydrocarbons (PAHs), are hot molecules, which affects their spectral characteristics. We report here our approach to quantify these effects empirically by collecting IR spectra - both experimental and theoretical - and studying the evolution of band positions and widths with temperature. We present our methodology, including the development of the CosmicPAH-IRDB web application and spectral database, along with the associated cosmicPAHmfit multi-component spectral fitting tool. Our philosophy emphasizes open sharing, user-friendly access, and machine readability. The web application, tool and their update are accessible via the Cosmic PAH portal (https://cosmic-pah.irap.omp.eu/)

astro-ph.IM↗

arXiv2026

Infrared spectroscopy of gas-phase hydrogenated and methylated pyrenes: from laboratory spectra to the simulated 3.4 $\mu$m emission band

Observations of the aromatic infrared emission band at 3.3 $\mu$m often reveal satellite emission features in the 3.4 - 3.6 $\mu$m range. While the 3.3 $\mu$m band is attributed to the CH stretching vibration of polycylic aromatic hydrocarbons (PAHs), the satellite bands - particularly its prominent 3.4 $\mu$m component - is assigned to aliphatic CH stretching vibrations in hydrogenated and methylated PAH-like species. Our aim is to derive state-of-the-art infrared emission spectra for aliphatic-containing pyrene derivatives and compare them with astronomical observations. This will help refine our understanding of the contribution of these species to the 3.4 $\mu$m emission band. Mid-infrared spectra (1.4-25 $\mu$m) of gas-phase dihydropyrene, tehtrahydropyrene, hexahydropyrene methylated pyrene, and pyrene were recorded at temperatures ranging from 373 to 673 K, depending on the species. The band profiles were analyzed using a multi-component fitting tool, and empirical anharmonicity laws were derived to quantify the evolution of the band positions and widths with temperature. The obtained spectral data was combined with the results of a Monte Carlo emission model to simulate the emission spectra following UV-photon absorption, up to the dissociation limit ($\lesssim$6 eV). The resulting synthetic spectra were compared with James Webb Space Telescope observations of the Orion Bar region (PDRs4All program). Based on these state-of-the-art simulated spectra, we propose 1,2,3,6,7,8-hexahydropyrene as the carrier of the red component of the 3.4 $\mu$m band observed at 3.403 $\mu$m. While 1-methylpyrene may also contribute to the underlying emission plateau, its lack of a strong infrared band complicates detection in observed spectra, unlike hexahydropyrene. All experimental spectra and their temperature-dependent analyses are available in the new cosmicPAH-IRDB database.

astro-ph.GA↗

arXiv2022

The EDIBLES survey V: Line profile variations in the $λλ$5797, 6379, and 6614 diffuse interstellar bands as a tool to constrain carrier sizes

Several diffuse interstellar bands (DIBs) have profiles with resolved sub-peaks that resemble rotational bands of large molecules. Analysis of these profiles can constrain the sizes and geometries of the DIB carriers, especially if the profiles exhibit clear variations along lines of sight probing different physical conditions. Using the extensive data set from the EDIBLES survey we searched for systematic variations in the peak-to-peak separation of these sub-peaks for the $λλ$5797, 6379, and 6614 DIBs in lines of sight with a single dominant interstellar cloud. We used the spectra of twelve single-cloud sight lines to measure the peak-to-peak separation in the band profile substructures for these DIBs. We adopted the rotational contour formalism to infer the rotational constant for each DIB carrier and the rotational excitation temperature in the sight lines. We compared these to rotational constants for linear and spherical molecules to estimate the DIB carrier sizes. All three DIBs have peak separations that vary systematically between lines of sight, indicating correlated changes in the rotational excitation temperatures. We derived $B_{6614}$=$(22.2\pm8.9)\times 10^{-3}$ cm$^{-1}$, consistent with previous estimates. Assuming a similar rotational temperature for the $λ$6614 DIB carrier and assuming a linear carrier, we found B$_{5797}^{\rm linear}=(5.1\pm2.0)\times10^{-3}~{\rm cm}^{-1}$ and B$_{6379}^{\rm linear} =(2.3\pm0.9)\times10^{-3}~{\rm cm}^{-1}$. If the carriers of those DIBs however are spherical species, their rotational constants are half that value, $B_{5797}^{\rm spherical} = (2.6\pm1.0)\times10^{-3}~{\rm cm}^{-1}$ and $B_{6379}^{\rm spherical} = (1.1\pm0.4)\times10^{-3}~{\rm cm}^{-1}$. We estimate molecule sizes that range from 7--9 carbon atoms ($λ$6614 carrier, linear) to 77--114 carbon atoms ($λ$6379, spherical).

astro-ph.GA↗

arXiv2021

Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

The interaction of polycyclic aromatic hydrocarbons (PAHs) with vacuum ultraviolet (VUV) photons triggers the emission of the well-known aromatic infrared bands (AIBs) but other mechanisms such as fragmentation can be involved in this interaction. Fragmentation leads to selection effects that favor specific sizes and structures. We investigate the impact of aliphatic bonds on the VUV photo-stability of PAH cations under conditions applicable for photodissociation regions (PDRs). Cations of pyrene (C16H10) and coronene (C24H12) derivatives containing aliphatic bonds (methyl or ethyl sidegroups, superhydrogenation) are submitted to VUV (10.5 eV) photons over long timescales (~1000 s) in the cryogenic PIRENEA setup. The fragmentation cascades are analyzed with a simple kinetics model; fragmentation pathways, rates and branching ratios are derived. Aliphatic PAH derivatives are found to have a higher fragmentation rate and carbon loss compared to regular PAHs. The fragmentation of PAHs with alkylated sidegroups forms species with peripheral pentagonal cycles, which can be more stable than the bare PAH cations. This stability is quantified and the most stable species, for which there is an effective competition of the fragmentation with isomerization and radiative cooling are identified. This work supports a scenario in which the evaporation of nanograins with a mixed aliphatic and aromatic composition followed by VUV photoprocessing results in both the production of the carriers of the 3.4um AIB by methyl sidegroups and in an abundant source of small hydrocarbons at the border of PDRs. An additional side effect is the efficient formation of stable PAHs that contain some peripheral pentagonal rings. Our experiments also support the role of isomerization processes in PAH photofragmentation, including the H-migration process, which could lead to an additional contribution to the 3.4um AIB.

astro-ph.GA↗

arXiv2021

Anharmonic Infrared Spectra of Thermally Excited Pyrene(C$_{16}$H$_{10}$): the combined view of DFT-based GVPT2 with AnharmonicCaOs and approximate DFT molecular dynamics with DemonNano

The study of the Aromatic Infrared Bands (AIBs) in astronomical environments has opened interesting spectroscopic questions on the effect of anharmonicity on the infrared (IR) spectrum of hot polycyclic aromatic hydrocarbons (PAHs) and related species in isolated conditions. The forthcoming James Webb Space Telescope will unveil unprecedented spatial and spectral details in the AIB spectrum; significant advancement is thus necessary now to model the infrared emission of PAHs, their presumed carriers, with enough detail to exploit the information content of the AIBs. This requires including anharmonicity in such models, and to do so systematically for all species included, requiring a difficult compromise between accuracy and efficiency. We performed a benchmark study to compare the performances of two methods in calculating anharmonic spectra, comparing them to available experimental data. One is a full quantum method, AnharmoniCaOs, relying on an ab initio potential, and the other relies on Molecular Dynamics simulations using a Density Functional based Tight Binding potential. The first one is found to be very accurate and detailed, but it becomes computationally very expensive for increasing temperature; the second is faster and can be used for arbitrarily high temperatures, but is less accurate. Still, its results can be used to model the evolution with temperature of isolated bands. We propose a new recipe to model anharmonic AIB emission using minimal assumptions on the general behaviour of band positions and widths with temperature, which can be defined by a small number of empirical parameters. Modelling accuracy will depend critically on these empirical parameters, allowing for an incremental improvement in model results, as better estimates become gradually available.

astro-ph.GA↗

arXiv2020

Astrochemical relevance of VUV ionization of large PAH cations

As a part of interstellar dust, polycyclic aromatic hydrocarbons (PAHs) are processed by the interaction with vacuum ultraviolet (VUV) photons that are emitted by hot young stars. This interaction leads to the emission of the well-known aromatic infrared bands but also of electrons, which can significantly contribute to the heating of the interstellar gas.Our aim is to investigate the impact of molecular size on the photoionization properties of cationic PAHs.Methods. Trapped PAH cations of sizes between 30 and 48 carbon atoms were submitted to VUV photons in the range of 9 to 20 eV from the DESIRS beamline at the synchrotron SOLEIL. All resulting photoproducts including dications and fragment cations were mass-analyzed and recorded as a function of photon energy.Photoionization is found to be predominant over dissociation at all energies, which differs from an earlier study on smaller PAHs. The photoionization branching ratio reaches 0.98 at 20 eV for the largest studied PAH. The photoionization threshold is observed to be between 9.1 and 10.2 eV, in agreement with the evolution of the ionization potential with size. Ionization cross sections were indirectly obtained and photoionization yields extracted from their ratio with theoretical photoabsorption cross sections, which were calculated using time-dependent density functional theory. An analytical function was derived to calculate this yield for a given molecular size.Large PAH cations could be efficiently ionized in H i regions and provide a contribution to the heating of the gas by photoelectric effect. Also, at the border of or in H ii regions, PAHs could be exposed to photons of energy higher than 13.6 eV. Our work provides recipes to be used in astronomical models to quantify these points.

astro-ph.GA↗

arXiv2019

Photo-processing of astro-PAHs

Polycyclic aromatic hydrocarbons (PAHs) are key species in astrophysical environments in which vacuum ultraviolet (VUV) photons are present, such as star-forming regions. The interaction with these VUV photons governs the physical and chemical evolution of PAHs. Models show that only large species can survive. However, the actual molecular properties of large PAHs are poorly characterized and the ones included in models are only an extrapolation of the properties of small and medium-sized species. We discuss here experiments performed on trapped ions including some at the SOLEIL VUV beam line DESIRS. We focus on the case of the large dicoronylene cation, C48H20+ , and compare its behavior under VUV processing with that of smaller species. We suggest that C2H2 is not a relevant channel in the fragmentation of large PAHs. Ionization is found to largely dominate fragmentation. In addition, we report evidence for a hydrogen dissociation channel through excited electronic states. Although this channel is minor, it is already effective below 13.6 eV and can significantly influence the stability of astro-PAHs. We emphasize that the competition between ionization and dissociation in large PAHs should be further evaluated for their use in astrophysical models.

astro-ph.GA↗

arXiv2019

Experimental Approach to the Study of Anharmonicity in the Infrared Spectrum of Pyrene from 14 to 723 K

Quantifying the effect of anharmonicity on the infrared spectrum of large molecules such as polycyclic aromatic hydrocarbons (PAHs) at high temperature is the focus of a number of theoretical and experimental studies, many of them motivated by astrophysical applications. We recorded the infrared spectrum of pyrene C16H10 microcrystals embedded in a KBr pellet over a wide range of temperature (14 -723 K) and studied the evolution of band positions, widths and integrated intensities with temperature. We identified jumps for some of the spectral characteristics of some bands in the [423- 473]K range. These were attributed to a change of phase from crystal to molten in condensed pyrene, which appears to affect more strongly bands involving large CH motions. Empirical anharmonic factors that describe the evolution of band positions and widths with temperature were retrieved from both phases over an unprecedented temperature range. The derived values were found to be consistent with available gas-phase data. We provide recommended values for anharmonic factors and conclude about the interest of the methodology to provide data of interest for comparison with theoretical models and as inputs of models that simulate the infrared emission of astro-PAHs.

physics.chem-ph↗

arXiv2018

Anharmonic vibrational spectroscopy of Polycyclic Aromatic Hydrocarbons (PAHs)

While powerful techniques exist to accurately account for anharmonicity in vibrational molecular spectroscopy, they are computationally very expensive and cannot be routinely employed for large species and/or at non- zero vibrational temperatures. Motivated by the study of Polycyclic Aromatic Hydrocarbon (PAH) emission in space, we developed a new code, which takes into account all modes and can describe all IR transitions including bands becoming active due to resonances as well as overtones, combination and difference bands. In this article, we describe the methodology that was implemented and discuss how the main difficulties were overcome, so as to keep the problem tractable. Benchmarking with high-level calculations was performed on a small molecule. We carried out specific convergence tests on two prototypical PAHs, pyrene (C$_{16}$H$_{10}$) and coronene (C$_{24}$H$_{12}$), aiming at optimising tunable parameters to achieve both acceptable accuracy and computational costs for this class of molecules. We then report the results obtained at 0 K for pyrene and coronene, comparing the calculated spectra with available experimental data. The theoretical band positions were found to be significantly improved compared to harmonic Density Functional Theory (DFT) calculations. The band intensities are in reasonable agreement with experiments, the main limitation being the accuracy of the underlying calculations of the quartic force field. This is a first step towards calculating moderately high-temperature spectra of PAHs and other similarly rigid molecules using Monte Carlo sampling.

physics.chem-ph↗

arXiv2017

The ESO Diffuse Interstellar Bands Large Exploration Survey: EDIBLES I. Project description, survey sample and quality assessment

The carriers of the diffuse interstellar bands (DIBs) are largely unidentified molecules ubiquitously present in the interstellar medium (ISM). After decades of study, two strong and possibly three weak near-infrared DIBs have recently been attributed to the C60+ fullerene based on observational and laboratory measurements. There is great promise for the identification of the over 400 other known DIBs, as this result could provide chemical hints towards other possible carriers. In an effort to systematically study the properties of the DIB carriers, we have initiated a new large-scale observational survey: the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES). The main objective is to build on and extend existing DIB surveys to make a major step forward in characterising the physical and chemical conditions for a statistically significant sample of interstellar lines-of-sight, with the goal to reverse-engineer key molecular properties of the DIB carriers. EDIBLES is a filler Large Programme using the Ultraviolet and Visual Echelle Spectrograph at the Very Large Telescope at Paranal, Chile. It is designed to provide an observationally unbiased view of the presence and behaviour of the DIBs towards early-spectral-type stars whose lines-of-sight probe the diffuse-to-translucent ISM. Such a complete dataset will provide a deep census of the atomic and molecular content, physical conditions, chemical abundances and elemental depletion levels for each sightline. Achieving these goals requires a homogeneous set of high-quality data in terms of resolution (R ~ 70000 -- 100000), sensitivity (S/N up to 1000 per resolution element), and spectral coverage (305--1042 nm), as well as a large sample size (100+ sightlines). In this first paper the goals, objectives and methodology of the EDIBLES programme are described and an initial assessment of the data is provided.

astro-ph.GA↗

arXiv2017

VUV photo-processing of PAH cations: quantitative study on the ionization versus fragmentation processes

Interstellar polycyclic aromatic hydrocarbons (PAHs) are strongly affected by the absorption of vacuum ultraviolet (VUV) photons in the interstellar medium (ISM), yet the branching ratio between ionization and fragmentation is poorly studied. This is crucial for the stability and charge state of PAHs in the ISM in different environments, affecting in turn the chemistry, the energy balance, and the contribution of PAHs to the extinction and emission curves. We studied the interaction of PAH cations with VUV photons in the 7-20 eV range from the synchrotron SOLEIL beamline, DESIRS. We recorded by action spectroscopy the relative intensities of photo-fragmentation and photo-ionization for a set of eight PAH cations ranging in size from 14 to 24 carbon atoms, with different structures. At photon energies below ~13.6 eV fragmentation dominates for the smaller species, while for larger species ionization is immediately competitive after the second ionization potential (IP). At higher photon energies, all species behave similarly, the ionization yield gradually increases, leveling off between 0.8 and 0.9 at ~18 eV. Among isomers, PAH structure appears to mainly affect the fragmentation cross section, but not the ionization cross section. We also measured the second IP for all species and the third IP for two of them, all are in good agreement with theoretical ones confirming that PAH cations can be further ionized in the diffuse ISM. Determining actual PAH dication abundances in the ISM will require detailed modeling. Our measured photo-ionization yields for several PAH cations provide a necessary ingredient for such models.

astro-ph.GA↗

arXiv2016

Modelling extragalactic extinction through gamma-ray burst afterglows

We analyze extragalactic extinction profiles derived through gamma-ray burst afterglows, using a dust model specifically constructed on the assumption that dust grains are not immutable but respond time-dependently to the local physics. Such a model includes core-mantle spherical particles of mixed chemical composition (silicate core, sp2 and sp3 carbonaceous layers), and an additional molecular component, in the form of free-flying polycyclic aromatic hydrocarbons. We fit most of the observed extinction profiles. Failures occur for lines of sight presenting remarkable rises blueward the bump. We find a tendency in the carbon chemical structure to become more aliphatic with the galactic activity, and to some extent with increasing redshifts. Moreover, the contribution of the moleclar component to the total extinction is more important in younger objects. The results of the fitting procedure (either successes and failures) may be naturally interpreted through an evolutionary prescription based on the carbon cycle in the interstellar medium of galaxies.

astro-ph.GA↗

arXiv2015

An optical spectrum of a large isolated gas-phase PAH cation: C78H26+

A gas-phase optical spectrum of a large polycyclic aromatic hydrocarbon (PAH) cation - C78H26 +- in the 410-610 nm range is presented. This large all-benzenoid PAH should be large enough to be stable with respect to photodissociation in the harsh conditions prevailing in the interstellar medium (ISM). The spectrum is obtained via multi-photon dissociation (MPD) spectroscopy of cationic C78H26 stored in the Fourier Transform Ion Cyclotron Resonance (FT-ICR) cell using the radiation from a mid-band optical parametric oscillator (OPO) laser. The experimental spectrum shows two main absorption peaks at 431 nm and 516 nm, in good agreement with a theoretical spectrum computed via time-dependent density functional theory (TD-DFT). DFT calculations indicate that the equilibrium geometry, with the absolute minimum energy, is of lowered, nonplanar C2 symmetry instead of the more symmetric planar D2h symmetry that is usually the minimum for similar PAHs of smaller size. This kind of slightly broken symmetry could produce some of the fine structure observed in some diffuse interstellar bands (DIBs). It can also favor the folding of C78H26 + fragments and ultimately theformation of fullerenes. This study opens up the possibility to identify the most promising candidates for DIBs amongst large cationic PAHs.

astro-ph.IM↗

arXiv2015

Modelling dust extinction in the Magellanic Clouds

We model the extinction profiles observed in the Small and Large Magellanic clouds with a synthetic population of dust grains consisting by core-mantle particles and a collection of free-flying polycyclic aromatic hydrocarbons. All different flavors of the extinction curves observed in the Magellanic Clouds can be described by the present model, that has been previously (successfully) applied to a large sample of diffuse and translucent lines of sight in the Milky Way. We find that in the Magellanic Clouds the extinction produced by classic grains is generally larger than absorption by polycyclic aromatic hydrocarbons. Within this model, the non-linear far-UV rise is accounted for by polycyclic aromatic hydrocarbons, whose presence in turn is always associated to a gap in the size distribution of classical particles. This hints either a physical connection between (e.g., a common cause for) polycyclic aromatic hydrocarbons and the absence of middle-sized dust particles, or the need for an additional component in the model, that can account for the non-linear far-UV rise without contributing to the UV bump at $\sim$217 nm, e.g., nanodiamonds.

astro-ph.GA↗

arXiv2013

Detection of the buckminsterfullerene cation (C60+) in space

In the early 90s, C60+ was proposed as the carrier of two diffuse interstellar bands (DIBs) at 957.7 and 963.2 nm, but a firm identification still awaits gas-phase spectroscopic data. Neutral C60, on the other hand, was recently detected through its infrared emission bands in the interstellar medium and evolved stars. In this contribution, we present the detection of C60+ through its infrared vibrational bands in the NGC 7023 nebula, based on spectroscopic observations with the Spitzer space telescope, quantum chemistry calculation, and laboratory data from the literature. This detection supports the idea that C60+ could be a DIB carrier, and provides robust evidence that fullerenes exist in the gas-phase in the interstellar medium. Modeling efforts to design specific observations, combined with new gas-phase data, will be essential to confirm this proposal. A definitive attribution of the 957.7 and 963.2 nm DIBs to C60+ would represent a significant step forward in the field.

astro-ph.GA↗

arXiv2012

Interstellar C60+

Buckminsterfullerene (C60) was recently detected through its infrared emission bands in the interstellar medium (ISM), including in the proximity of massive stars, where physical conditions could favor the formation of the cationic form, C60+. In addition, C60+ was proposed as the carrier of two diffuse interstellar bands in the near-IR, although a firm identification still awaits for gas-phase spectroscopic data. We examined in details the Spitzer IRS spectra of the NGC 7023 reflection nebula, at a position close (7.5") to the illuminating B star HD 200775, and found four previously unreported bands at 6.4, 7.1, 8.2 and 10.5 μm in addition to the classical bands attributed to Polycylic Aromatic Hydrocarbons (PAHs) and neutral C60. These 4 bands are observed only in this region of the nebula, while C60 emission is still present slightly further away from the star, and PAH emission even further away. Based on this observation, on theoretical calculations we perform, and on laboratory studies, we attribute these bands to C60+. The detection of C60+ confirms the idea that large carbon molecules exist in the gas-phase in these environments. In addition, the relative variation of the C60, and C60+, band intensities constitutes a potentially powerful probe of the physical conditions in highly UV-irradiated regions.

astro-ph.GA↗

arXiv2009

Large prebiotic molecules in space: photo-physics of acetic acid and its isomers

An increasing number of large molecules have been positively identified in space. Many of these molecules are of biological interest and thus provide insight into prebiotic organic chemistry in the protoplanetary nebula. Among these molecules, acetic acid is of particular importance due to its structural proximity to glycine, the simplest amino acid. We compute electronic and vibrational properties of acetic acid and its isomers, methyl formate and glycolaldehyde, using density functional theory. From computed photo-absorption cross-sections, we obtain the corresponding photo-absorption rates for solar radiation at 1 AU and find them in good agreement with previous estimates. We also discuss glycolaldehyde diffuse emission in Sgr B2(N), as opposite to emissions from methyl formate and acetic acid that appear to be concentrate in the compact region Sgr B2(N-LMH).

astro-ph.SR↗

arXiv2007

Efficient calculation of van der Waals dispersion coefficients with time-dependent density functional theory in real time: application to polycyclic aromatic hydrocarbons

The van der Waals dispersion coefficients of a set of polycyclic aromatic hydrocarbons, ranging in size from the single-cycle benzene to circumovalene (C66H20), are calculated with a real-time propagation approach to time-dependent density functional theory (TDDFT). In the non-retarded regime, the Casimir-Polder integral is employed to obtain C6, once the dynamic polarizabilities have been computed at imaginary frequencies with TDDFT. On the other hand, the numerical coefficient that characterizes the fully retarded regime is obtained from the static polarizabilities. This ab initio strategy has favorable scaling with the size of the system - as demonstrated by the size of the reported molecules - and can be easily extended to obtain higher order van der Waals coefficients.

cond-mat.mtrl-sci↗