arXiv subjects

Julianna Palotás

Publications and source records attributed to Julianna Palotás.

6 recordsLinked to original sources

arXiv2026

Development of a glow-discharge ion-trap instrument for measuring effective radiative-association rate coefficients

The ability to directly measure radiative-association rate coefficients for reactions between ions and neutral molecules has long challenged chemical physics laboratories, yet radiative association is one of the most important processes occurring in cold, diffuse regions of space. A reaction kinetics instrument has been developed for the investigation of ion--molecule radiative-association reactions, aimed at measuring slow, effective reaction rate coefficients for species relevant to astrophysical objects. The instrument consists of a glow-discharge ion source for production of bright and stable ion currents, a quadrupole mass filter for mass selection and detection, and a quadrupole ion trap capable of trapping reactants and products for the long times needed to measure slow kinetics. The performance and adaptability of the glow-discharge ion source has been evaluated using several configurations. To assess the feasibility of measuring reaction rate coefficients, the reaction of Ag$^{+}$ and O$_{2}$ was studied under pseudo-first-order conditions in the ion trap at room temperature. We present the first pressure-dependent study of this reaction and extract a lower limit of $1 \times 10^{-15}$ cm$^3$ molecule$^{-1}$ s$^{-1}$ for the Ag$^{+}$ + O$_{2}$ effective radiative-association rate coefficient. Measurements of effective radiative-association rate coefficients are possible for diverse atomic and molecular ions that react with neutral molecules over a range of rates in this versatile new instrument.

physics.chem-ph↗

arXiv2025

High Resolution Overtone Spectroscopy of HNC$^+$ and HCN$^+$

Rotationally resolved spectra of the HNC$^+$ and HCN$^+$ molecular ions have been recorded in the spectral range between 6200 and 6800 \rcm\ using a cryogenic ion trap instrument. The rovibrational transitions were probed using two different action spectroscopy schemes, namely laser-induced reaction (LIR) and leak-out spectroscopy (LOS). Various vibrational bands of HNC$^+$ and HCN$^+$ were measured with high resolution for the first time. For HNC$^+$, the $\text{X}~^2Σ^+~(20^00)-(00^00)$ overtone band was recorded using LIR, while LOS was used to probe the $\text{X}~^2Π~(000)^1-(210)^0μ$ combination band and the $\text{X}~^2Π~(000)^1-\text{A}~^2Σ^+~(10^00)$ vibronic band of HCN$^+$. Spectroscopic constants, band origins and radiative lifetimes for the observed states have been determined. The effective fit for the HCN$^+$ spectra revealed the presence of strong vibrational couplings leading to perturbations of the rovibrational levels of the excited states. The two action spectroscopy schemes are compared and their potential use to explore ion-molecule interactions is discussed.

astro-ph.SR↗

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↗