arXiv subjects

Milan Ončák

Publications and source records attributed to Milan Ončák.

6 recordsLinked to original sources

arXiv2026

Wavelength-Resolved Timing of Near-Infrared Scintillation in Gaseous Xenon

The near-infrared (NIR) scintillation signal of xenon has the potential to enhance the performance of xenon-based detectors, but a robust understanding of its properties is a prerequisite for its application. In this work, we measure the NIR time response for $α$-particle interactions in gaseous xenon in 12 wavelength bands spanning approximately $(950-1620)\,$nm. The time response is decomposed into a fast (ns-scale) component and a dominant slow ($μ$s-scale) component. The fast-component spectrum coincides with strong atomic emission lines, predominantly 6p$\,\to\,$6s, whereas the slow-component spectrum follows the broad continuum centred around $1.3\,μ$m. The pressure dependence of the continuum can be described by a two-step kinetic model in which the emitting state is formed by a three-body process and depopulated by a two-body collisional process, with density-independent terms in both rates. Interpreting the slower of these as radiative decay yields an intrinsic lifetime of $\sim3\,μ$s. Theoretical calculations identify a neutral-excimer transition $0_u^- \rightarrow 0_g^-$ and an ionic-dimer transition $1/2_{u} \rightarrow 1/2_g$ as plausible candidates for the continuum emission, with the ionic-dimer transition showing better overall agreement with the measurements.

physics.ins-det↗

arXiv2025

Infrared Spectroscopy of Pentagon-Containing PAHs: Indenyl and Fluorenyl Anions and Indenyl Cation

Polycyclic aromatic hydrocarbon (PAH) ions are crucial intermediates in interstellar chemistry and may play a key role in the infrared emission features observed in space. Here, we investigate the infrared spectra of the indenyl (C$_9$H$_7^-$) and fluorenyl (C$_{13}$H$_9^-$) anions and the indenyl cation (C$_9$H$_7^+$) using infrared pre-dissociation (IRPD) spectroscopy. The experiments were performed in a cryogenic 22 pole ion trap at the FELion beamline of the tunable free-electron laser FELIX. Spectral analysis of the two anionic PAHs, in combination with density functional theory (DFT) computations, revealed key vibrational modes near 1300 cm$^{-1}$, making these ions potential carriers of the 7.7 μm PAH emission band seen in many astronomical objects. The feature-rich spectrum of cationic indenyl could not be entirely explained by modeling through time-independent anharmonic DFT calculations. Although a better match has been achieved through molecular dynamics simulations, we cannot completely rule out the presence of multiple cationic isomers of the H-loss fragments of indene in the experiments.

astro-ph.GA↗

arXiv2025

Cyclic C$_4^+$ as the carrier of the diffuse interstellar band at 503.9 nm?

The diffuse interstellar bands (DIBs) have remained a mystery in astronomy since their discovery over a century ago. The only currently known carrier is C$_{60}^+$ responsible for five DIBs, while more than 550 are yet to be interpreted. The spectra of short carbon chain cations C$_n^+$, which are considered one of the most promising classes of species for the role of carriers of DIBs, are successfully recorded using He-tagging spectroscopy. The comparison of laboratory spectra with the observations demonstrates a close match of two absorption bands of C$_4^+$ with the broad DIB at 503.9 nm. This defines a high abundance of these ions in the interstellar medium (ISM), which should exceed those of other similar-sized carbon chain cations. It is anticipated that all other short carbon chain cations will exhibit linear geometry and, as a consequence, will have a long vibrational progression. However, the distinctive cyclic geometry of C$_4^+$ is postulated to underpin the elevated abundance of these ions in the ISM, as well as the distinctive spectrum of this ion, which displays a single strong, relatively narrow absorption band that exceeds in intensity all other absorption bands in the visible range.

astro-ph.GA↗

arXiv2024

Unimolecular processes in diatomic carbon anions at high rotational excitation

On the millisecond to second time scale, stored beams of diatomic carbon anions C$_2{}^-$ from a sputter ion source feature unimolecular decay of yet unexplained origin by electron emission and fragmentation. To account for the magnitude and time dependence of the experimental rates, levels with high rotational and vibrational excitation are modeled for the lowest electronic states of C$_2{}^-$, also including the lowest quartet potential. Energies, spontaneous radiative decay rates (including spin-forbidden quartet-level decay), and tunneling dissociation rates are determined for a large number of highly excited C$_2{}^-$ levels and their population in sputter-type ion sources is considered. For the quartet levels, the stability against autodetachment is addressed and recently calculated rates of rotationally assisted autodetachment are applied. Non-adiabatic vibrational autodetachment rates of high vibrational levels in the doublet C$_2{}^-$ ground potential are also calculated. The results are combined to model the experimental unimolecular decay signals. Comparison of the modeled to the experimental rates measured at the Croygenic Storage Ring (CSR) gives strong evidence that C$_2{}^-$ ions in quasi-stable levels of the quartet electronic states are the so far unidentified source of unimolecular decay.

physics.chem-ph↗

arXiv2024

Autodetachment of diatomic carbon anions from long-lived high-rotation quartet states

Highly excited C$_2{}^{-}$ ions prominently feature electron detachment at a mean decay time near 3 milliseconds with hitherto unexplained origin. Considering various sources of unimolecular decay, we attribute the signal to the electronic C$^4Σ^+_u$ state. Quartet C$_2{}^{-}$ levels are found to be stabilized against autodetachment by high rotation. Time constants of their rotationally assisted autodetachment into levels opening energetically at lower rotation are calculated by a theory based on the non-local resonance model. For some final levels of significantly less rotation the results conclusively explain the puzzling observations.

physics.chem-ph↗

arXiv2023

Charging up the cold: Formation of doubly- and triply-charged fullerene dimers in superfluid helium nanodroplets

Sequential ionization of fullerene cluster ions (C$_{60}$)$_{n}^{+}$ within multiply-charged helium nanodroplets leads to the intriguing phenomenon of forming and stabilizing doubly- and triply-charged fullerene oligomers. Surprisingly, we have detected (C$_{60}$)$_{2}^{2+}$ and (C$_{60}$)$_{2}^{3+}$, indicating that dimers, rather than the previously established pentamers and dodecamers, are the smallest fullerene cluster sizes capable of stabilizing two and even three charges. This remarkable resilience against Coulomb explosion is achieved through efficient cooling within the superfluid environment of helium nanodroplets, and a sequential ionization scheme that populates covalently bound or physisorbed fullerene dimers. Calculations support the stability of four differently bonded (C$_{60}$)$_{2}^{2+}$ and (C$_{60}$)$_{2}^{3+}$ isomers and predict a low Coulomb barrier (<0.4 eV) preventing even dissociation of cold van der Waals complexes.

physics.atm-clus↗