arXiv subjects

Jos Oomens

Publications and source records attributed to Jos Oomens.

8 recordsLinked to original sources

arXiv2026

Interstellar Aromatic Infrared Bands: IR absorption spectroscopy of the lowest triplet state of naphthalene

The aromatic infrared bands (AIBs), emission features observed ubiquitously across a wide variety of interstellar objects, are commonly attributed to radiative cooling of highly-excited ground-state vibrational levels of polycyclic aromatic hydrocarbons (PAHs) that are populated after UV absorption and internal conversion. However, for neutral PAHs intersystem crossing to the triplet manifold, followed by radiative cooling of vibrational levels within the lowest excited triplet state is expected to be a competing relaxation channel. This state differs in vibrational frequencies, transition moments and internal energy, and therefore gives rise to different emission spectra. Despite their significance, neutral PAHs in their triplet states have remained largely unexplored, primarily because of the lack of direct spectroscopic access. Here, we report the first measurement of the infrared absorption spectrum of the lowest triplet state of naphthalene. The spectrum is markedly different from the vibrational spectrum of the singlet electronic ground state, displaying features that can help identifying PAHs in the triplet states in astronomical spectra. We also show that experimental spectra are closely reproduced by anharmonic frequency calculations. These findings serve as a stepping stone for detecting triplet-state PAHs in the interstellar medium and provide a basis for developing more accurate infrared emission models.

astro-ph.GA↗

arXiv2026

The Computed Microwave Spectrum of the Protonated Fullerene C60H+

The largest known molecule in space, C60 , has been detected in its neutral and cationic form through its vibrational, UV-driven fluorescence emission spectrum and its electronic absorption spectrum, respectively. The detection of several polycyclic aromatic hydrocarbon molecules through their pure rotation spectrum in cold, dense, molecular cloud cores suggests that C60 might be present in these environments as well. The low flux of UV pumping photons in molecular cloud cores and the absence of suitably bright background stars, make detection of C60 and its cation through the commonly used methods impractical. As C60 has no permanent dipole moment, its pure rotational transitions are forbidden and its presence must be inferred from the rotational transitions of C60 derivatives with permanent dipole moments. Here, we present a study of the predicted rotational spectrum of protonated C60 that has a sizeable permanent dipole moment. Protonation of C60 reduces the icosahedral symmetry to Cs and results in a dipole moment of about 3.8 Debye. The resulting C60H+ is a closed shell system

astro-ph.GA↗

arXiv2025

Anharmonic infrared spectra of cationic pyrene and superhydrogenated derivatives

Studying the anharmonicity in the infrared (IR) spectra of polycyclic aromatic hydrocarbons (PAHs) at elevated temperatures is important to understand vibrational features and chemical properties of interstellar dust, especially in the James Webb Space Telescope (JWST) era. We take pyrene as an example PAH and investigate how different degrees of superhydrogenation affects the applicability of the harmonic approximation and the role of temperature in IR spectra of PAHs. This is achieved by comparing theoretical IR spectra generated by classical molecular dynamics (MD) simulations and experimental IR spectra obtained via gas-phase action spectroscopy which utilizes the Infrared Multiple Photon Dissociation (IRMPD). All simulations are accelerated by a machine learning interatomic potential, in order to reach first principle accuracies while keeping low computational costs. We have found that the harmonic approximation with empirical scaling factors is able to reproduce experimental band profile of pristine and partially superhydrogenated pyrene cations. However, a MD-based anharmonic treatment is mandatory in the case of fully superhydrogenated pyrene cation for matching theory and experiment. In addition, band shifts and broadenings as the temperature increases are investigated in detail. Those findings may aid in the interpretation of JWST observations on the variations in band positions and widths of interstellar dust.

physics.chem-ph↗

arXiv2022

Top-down formation of ethylene from fragmentation of superhydrogenated polycyclic aromatic hydrocarbons

Fragmentation is an important decay mechanism for polycyclic aromatic hydrocarbons (PAHs) under harsh interstellar conditions and represents a possible formation pathway for small molecules such as H2, C2H2, C2H4. Our aim is to investigate the dissociation mechanism of superhydrogenated PAHs that undergo energetic processing and the formation pathway of small hydrocarbons. We obtain, experimentally, the mass distribution of protonated tetrahydropyrene (C16H15 , py+5H+) and protonated hexahydropyrene (C16H17+, py+7H+) upon collision induced dissociation (CID). The IR spectra of their main fragments are recorded by infrared multiple-photon dissociation (IRMPD). Extended tight-binding (GFN2-xTB) based molecular dynamics simulations are performed in order to provide the missing structure information in experiment and identify fragmentation pathways. The pathways for fragmentation are further investigated at a hybrid-density functional theory (DFT) and dispersion corrected level. A strong signal for loss of 28 mass units of py+7H+ is observed both in the CID experiment and the MD simulation, while py+5H+ shows negligible signal for the product corresponding to a mass loss of 28. The 28 mass loss from py+7H+ is assigned to the loss of ethylene (C2H4) and a good fit between the calculated and experimental IR spectrum of the resulting fragment species is obtained. Further DFT calculations show favorable kinetic pathways for loss of C2H4 from hydrogenated PAH configurations involving three consecutive CH2 molecular entities. This joint experimental and theoretical investigation proposes a chemical pathway of ethylene formation from fragmentation of superhydrogenated PAHs. This pathway is sensitive to hydrogenated edges (e.g. the degree of hydrogenation and the hydrogenated positions). The inclusion of this pathway in astrochemical models may improve the estimated abundance of ethylene.

astro-ph.GA↗

arXiv2021

Infrared action spectroscopy of doubly charged PAHs and their contribution to the aromatic infrared bands

The so-called aromatic infrared bands are attributed to emission of polycyclic aromatic hydrocarbons. The observed variations toward different regions in space are believed to be caused by contributions of different classes of PAH molecules, i.e. with respect to their size, structure, and charge state. Laboratory spectra of members of these classes are needed to compare them to observations and to benchmark quantum-chemically computed spectra of these species. In this paper we present the experimental infrared spectra of three different PAH dications, naphthalene$^{2+}$, anthracene$^{2+}$, and phenanthrene$^{2+}$, in the vibrational fingerprint region 500-1700~cm$^{-1}$. The dications were produced by electron impact ionization of the vapors with 70 eV electrons, and they remained stable against dissociation and Coulomb explosion. The vibrational spectra were obtained by IR predissociation of the PAH$^{2+}$ complexed with neon in a 22-pole cryogenic ion trap setup coupled to a free-electron infrared laser at the Free-Electron Lasers for Infrared eXperiments (FELIX) Laboratory. We performed anharmonic density-functional theory calculations for both singly and doubly charged states of the three molecules. The experimental band positions showed excellent agreement with the calculated band positions of the singlet electronic ground state for all three doubly charged species, indicating its higher stability over the triplet state. The presence of several strong combination bands and additional weaker features in the recorded spectra, especially in the 10-15~$μ$m region of the mid-IR spectrum, required anharmonic calculations to understand their effects on the total integrated intensity for the different charge states. These measurements, in tandem with theoretical calculations, will help in the identification of this specific class of doubly-charged PAHs as carriers of AIBs.

astro-ph.GA↗

arXiv2019

The infrared spectrum of protonated buckminsterfullerene, C60H+

Although fullerenes have long been hypothesized to occur in interstellar environments, their actual unambiguous spectroscopic identification is of more recent date. C60, C70 and C60+ now constitute the largest molecular species individually identified in the interstellar medium (ISM). Fullerenes have significant proton affinities and it was suggested that C60H+ is likely the most abundant interstellar analogue of C60. We present here the first laboratory infrared (IR) spectrum of gaseous C60H+. Symmetry breaking relative to C60 produces an IR spectrum that is much richer than that of C60. The experimental spectrum is used to benchmark theoretical spectra indicating that the B3LYP density functional with the 6-311+G(d,p) basis set accurately reproduces the spectrum. Comparison with IR emission spectra from two planetary nebulae, SMP LMC56 and SMC16, that have been associated with high C60 abundances, indicate that C60H+ is a plausible contributor to their IR emission.

physics.chem-ph↗

arXiv2015

High-resolution IR absorption spectroscopy of polycyclic aromatic hydrocarbons: the realm of anharmonicity

We report on an experimental and theoretical investigation of the importance of anharmonicity in the 3 micron CH stretching region of Polycyclic Aromatic Hydrocarbon (PAH) molecules. We present mass-resolved, high-resolution spectra of the gas-phase cold (~4K) linear PAH molecules naphthalene, anthracene, and tetracene. The measured IR spectra show a surprisingly high number of strong vibrational bands. For naphthalene, the observed bands are well separated and limited by the rotational contour, revealing the band symmetries. Comparisons are made to the harmonic and anharmonic approaches of the widely used Gaussian software. We also present calculated spectra of these acenes using the computational program SPECTRO, providing anharmonic predictions enhanced with a Fermi-resonance treatment that utilises intensity redistribution. We demonstrate that the anharmonicity of the investigated acenes is strong, dominated by Fermi resonances between the fundamental and double combination modes, with triple combination bands as possible candidates to resolve remaining discrepancies. The anharmonic spectra as calculated with SPECTRO lead to predictions of the main modes that fall within 0.5% of the experimental frequencies. The implications for the Aromatic Infrared Bands, specifically the 3 micron band are discussed.

astro-ph.GA↗

arXiv2009

Action spectroscopy of gas-phase carboxylate anions by multiple photon IR electron detachment/attachment

We report on a form of gas-phase anion action spectroscopy based on infrared multiple photon electron detachment and subsequent capture of the free electrons by a neutral electron scavenger in a Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometer. This method allows one to obtain background-free spectra of strongly bound anions, for which no dissociation channels are observed. The first gas-phase spectra of acetate and propionate are presented using SF6 as electron scavenger and a free electron laser as source of intense and tunable infrared radiation. To validate the method, we compare infrared spectra obtained through multiple photon electron detachment/attachment and multiple photon dissociation for the benzoate anion. In addition, different electron acceptors are used, comparing both associative and dissociative electron capture. The relative energies of dissociation (by CO2 loss) and electron detachment are investigated for all three anions by DFT and CCSD(T) methods. DFT calculations are also employed to predict vibrational frequencies, which provide a good fit to the infrared spectra observed. The frequencies of the symmetric and antisymmetric carboxylate stretching modes for the aliphatic carboxylates are compared to those previously observed in condensed-phase IR spectra and to those reported for gas-phase benzoate, showing a strong influence of the solution environment and a slight substituent effect on the antisymmetric stretch.

physics.chem-ph↗