arXiv subjects

O. O. Versolato

Publications and source records attributed to O. O. Versolato.

At least 19 recordsLinked to original sources

arXiv2026

Beyond-EUV spectrum of highly-charged gadolinium

We report ab initio relativistic calculations on the complex Gd$^{18+}$ and Gd$^{26+}$ highly-charged ions, which are relevant to beyond-extreme-ultraviolet (BEUV) light sources based on laser-produced gadolinium plasmas. Using the particle-hole configuration-interaction with many-body perturbation theory (CI+MBPT) method, we systematically saturate the $n=4$ configuration space and determine the influence of multiply excited states on the emissivity spectrum. For Gd$^{26+}$ we find behavior analogous to that of its isoelectronic counterpart Sn$^{12+}$, with strong contributions from multiply excited states, where even the quadruply excited states become significant at certain effective temperatures. In contrast, the industry-relevant 6.7 nm emission of Pd-like Gd$^{18+}$ is dominated by the singly excited $4d^{10} {\ }^{1}S_{0}$ -- $4d^{9}4f {\ }^{1}P_{1}$ transition, while multiply excited states primarily contribute to out-of-band emission. The calculated spectra show good agreement with experiment and demonstrate that the importance of multiply excited states depends strongly on both charge state and plasma conditions.

physics.atom-ph↗

arXiv2026

Resolving the Blueshift in Calculations of the EUV Spectrum of Multiply Charged Tin Ions

We report ab initio relativistic calculations on the complex open shell Sn$^{12+}$ highly charged ion, a prototypical plasma ion relevant for extreme ultraviolet (EUV) nanolithography. Previous calculations of EUV emissivity in tin plasmas consistently generate a spectrum in which the region of peak emissivity is blueshifted relative to experiment. By optimising our numerical methods to take full advantage of modern, high-performance CPU architectures, we are able to completely saturate the configuration interaction within the $n=4$ shell. Coupled with a thorough treatment of core-valence correlation we resolve the blueshift, finding a surprisingly large influence of highly excited states on the spectrum that is dominated by multiply excited states.

physics.atom-ph↗

arXiv2026

Rim destabilization and re-formation upon severance from its expanding sheet

Upon radial liquid sheet expansion, a bounding rim forms, with a thickness and stability governed, in part, by the liquid influx from the unsteady connected sheet. We examine how the thickness and fragmentation of such a radially expanding rim change upon its severance from its sheet, absent of liquid influx. To do so, we design an experiment enabling the study of rims pre and post severance by vaporizing the thin neck connecting the rim. We confirm that the severed rim follows a ballistic motion, with a radial velocity inherited from the sheet at severance time. We identify that the severed rim undergoes fragmentation in two types of junctions: the base of inherited, pre-severance, ligaments and the junction between nascent rim corrugations, with no significant distinction between the two associated timescales. The number of ligaments and fragments formed is captured well by the theoretical prediction of rim corrugation and ligament wavenumbers established for unsteady expanding sheets upon droplet impact on surfaces of comparable size to the droplet, and with the sheet thickness profiles in both systems having the same functional form. Our findings are robust to changes in impacting laser energy and initial droplet size. Finally, we report and analyze the re-formation of the rim on the expanding sheet and propose a prediction for its characteristic corrugation timescale. Our findings highlight the fundamental mechanisms governing interfacial destabilization of connected fluid-fed expanding rims that become severed, thereby clarifying destabilization of freely radially expanding toroidal fluid structures absent of fluid influx.

physics.flu-dyn↗

arXiv2026

Singular jets in free-falling droplets

We report on singular jets in a free-falling liquid tin droplet following nanosecond laser-pulse impact. Following impact, the droplet (with diameter $D_0=50$ or 70\,$μ$m) undergoes rapid radial expansion and subsequent retraction, resulting in the formation of an axisymmetric jet. Using numerical simulations in tandem with our experiments, we reveal that a delicate interplay between radial flow and the curvature of the retracting droplet governs jet formation. The resulting dynamics is characterized using the impact Weber number, $\We$ (in the experiments $2 \lesssim \We \lesssim 16$), and a pressure width, W (typically $1 \lesssim \W \lesssim 2$), which describes the angular distribution over the droplet surface of the instantaneous pressure impulse exerted by the transient laser-produced plasma. %, within the range $0-20$. For values $\We<10$, the droplet presents a pronounced forward curvature during the retraction, leading to the formation of a cavity. The collapse of such a cavity leads to a singular jet that greatly enhances the jetting velocity up to ten times the impact propulsion velocity, an effect that narrowly peaks around $\We\sim6-8$, reminiscent of singular jets in droplet-solid impact. We identify a further sensitivity of the jet velocity enhancement on the pressure width W and capture the dynamics in a phase diagram connecting the various deformation morphologies with jet velocity.

physics.flu-dyn↗

arXiv2025

Laser-driven droplet deformation at low Weber numbers

We investigate droplet deformation following laser-pulse impact at low Weber numbers (We ~ 0.1-100). Droplet dynamics can be characterized by two key parameters: the impact We number and the width, W, of the distribution of the impact force over the droplet surface. By varying laser pulse energy, our experiments traverse a phase space comprising (I) droplet oscillation, (II) breakup, or (III) sheet formation. Numerical simulations complement the experiments by determining the pressure width and by allowing We and W to be varied independently, despite their correlation in the experiments. A single phase diagram, integrating observations from both experiments and simulations, demonstrates that all phenomena can be explained by a single parameter: the deformation Weber number Wed=f(We, W) that is based on the initial radial expansion speed of the droplet, following impact. The resulting phase diagram separates (I) droplet oscillation for Wed<5, from (II) breakup for 5 60.

physics.flu-dyn↗

arXiv2022

Characterization of plasmas driven by laser wavelengths in the 1.064-10.6 $μ$m range as future extreme ultraviolet light sources

We characterize the properties of extreme ultraviolet (EUV) light source plasmas driven by laser wavelengths in the $λ_{\mathrm{laser}} = 1.064 - 10.6 $ $μ$m range. Detailed numerical simulations of laser-irradiated spherical tin microdroplet targets reveal a strong laser-wavelength dependence on laser light absorptivity and the conversion efficiency of generating EUV radiation. Radiative losses are found to dominate the power balance for all laser wavelengths, and a clear shift from kinetic to in-band radiative losses with increasing laser wavelength is identified. We find that the existence of maximum conversion efficiency, near $ λ_{\mathrm{laser}} = 4 $ $μ$m, originates from the interplay between the optical depths of the laser light and the in-band EUV photons for this specific target geometry.

physics.plasm-ph↗

arXiv2022

Review of the 1st EUV Light Sources Code Comparison Workshop

We review the results of the 1st Extreme Ultraviolet (EUV) Light Sources Code Comparison Workshop, which was held online on 3rd November 2020. The goal of this workshop was to provide a platform for specialists in EUV light source plasma modeling to benchmark and validate their numerical codes using well-defined case studies. Eight institutions spanning four countries contributed data to the workshop. Two topics were addressed, namely (i) the atomic kinetics and radiative properties of tin plasmas under EUV-generating conditions and (ii) laser absorption in a fully ionized, one-dimensional hydrogen plasma. In this paper, we summarize the key findings of the workshop and outline plans for future iterations of this code comparison activity.

physics.plasm-ph↗

arXiv2021

Characterization of angularly resolved EUV emission from 2-$μ$m-wavelength laser-driven Sn plasmas using preformed liquid disk targets

The emission properties of tin plasmas, produced by the irradiation of preformed liquid tin targets by several-ns-long 2-$μ$m-wavelength laser pulses, are studied in the extreme ultraviolet (EUV) regime. In a two-pulse scheme, a pre-pulse laser is first used to deform tin microdroplets into thin, extended disks before the main (2$μ$m) pulse creates the EUV-emitting plasma. Irradiating 30- to 300-$μ$m-diameter targets with 2-$μ$m laser pulses, we find that the efficiency in creating EUV light around 13.5nm follows the fraction of laser light that overlaps with the target. Next, the effects of a change in 2-$μ$m drive laser intensity (0.6-1.8$\times 10^{11}$W/cm$^2$) and pulse duration (3.7-7.4ns) are studied. It is found that the angular dependence of the emission of light within a 2\% bandwidth around 13.5nm and within the backward 2$π$ hemisphere around the incoming laser beam is almost independent of intensity and duration of the 2-$μ$m drive laser. With increasing target diameter, the emission in this 2\% bandwidth becomes increasingly anisotropic, with a greater fraction of light being emitted into the hemisphere of the incoming laser beam. For direct comparison, a similar set of experiments is performed with a 1-$μ$m-wavelength drive laser. Emission spectra, recorded in a 5.5-25.5nm wavelength range, show significant self-absorption of light around 13.5nm in the 1-$μ$m case, while in the 2-$μ$m case only an opacity-related broadening of the spectral feature at 13.5nm is observed. This work demonstrates the enhanced capabilities and performance of 2-$μ$m-driven plasmas produced from disk targets when compared to 1-$μ$m-driven plasmas, providing strong motivation for the use of 2-$μ$m lasers as drive lasers in future high-power sources of EUV light.

physics.plasm-ph↗

arXiv2020

Characterization of 1- and 2-$μ$m-wavelength laser-produced microdroplet-tin plasma for generating extreme-ultraviolet light

Experimental spectroscopic studies are presented, in a 5.5--25.5nm extreme-ultraviolet (EUV) wavelength range, of the light emitted from plasma produced by the irradiation of tin microdroplets by 5-ns-pulsed, 2-$μ$m-wavelength laser light. Emission spectra are compared to those obtained from plasma driven by 1-$μ$m-wavelength Nd:YAG laser light over a range of laser intensities spanning approximately $0.3-5 \times 10^{11}$Wcm$^{-2}$, under otherwise identical conditions. Over this range of drive laser intensities, we find that similar spectra and underlying plasma charge state distributions are obtained when keeping the ratio of 1-$μ$m to 2-$μ$m laser intensities fixed at a value of 2.1(6), which is in good agreement with RALEF-2D radiation-hydrodynamic simulations. Our experimental findings, supported by the simulations, indicate an approximately inversely proportional scaling $\sim λ^{-1}$ of the relevant plasma electron density, and of the aforementioned required drive laser intensities, with drive laser wavelength $λ$. This scaling also extends to the optical depth that is captured in the observed changes in spectra over a range of droplet diameters spanning 16-51$μ$m at a constant laser intensity that maximizes the emission in a 2\% bandwidth around 13.5nm relative to the total spectral energy, the bandwidth relevant for EUV lithography. The significant improvement of the spectral performance of the 2-$μ$m- vs 1-$μ$m driven plasma provides strong motivation for the development of high-power, high-energy near-infrared lasers to enable the development of more efficient and powerful sources of EUV light.

physics.plasm-ph↗

arXiv2018

Energy Level Structure of Sn$^{3+}$ Ions

Laser-produced Sn plasma sources are used to generate extreme ultraviolet (EUV) light in state-of-the-art nanolithography. An ultraviolet and optical spectrum is measured from a droplet-based laser-produced Sn plasma, with a spectrograph covering the range 200 - 800 nm. This spectrum contains hundreds of spectral lines from lowly charged tin ions Sn$^{1+}$ - Sn$^{4+}$ of which a major fraction was hitherto unidentified. We present and identify a selected class of lines belonging to the quasi-one-electron, Ag-like ([Kr]$4d^{10} nl$ electronic configuration), Sn$^{3+}$ ion, linking the optical lines to a specific charge state by means of a masking technique. These line identifications are made with iterative guidance from COWAN code calculations. Of the 53 lines attributed to Sn$^{3+}$, some 20 were identified from previously known energy levels, and 33 lines are used to determine previously unknown level energies of 13 electronic configurations, i.e., $ 7p $, $ (7,8)d $, $ (5,6)f $, $ (6-8)g $, $ (6-8)h $, $ (7,8)i $. The consistency of the level energy determination is verified by the quantum-defect scaling procedure. The ionization limit of Sn$^{3+}$ is confirmed and refined to 328908.4 cm$^{-1}$ with an uncertainty of 2.1 cm$^{-1}$. The relativistic Fock space coupled cluster (FSCC) calculation of the measured level energies are generally in good agreement with experiment, but fail to reproduce the anomalous behavior of the $5d$ $^2$D and $nf$ $^2$F terms. By combining the strengths of FSCC, COWAN code calculations, and configuration interaction many-body perturbation theory (CI+MBPT), this behavior is shown to arise from interactions with doubly-excited configurations.

physics.atom-ph↗

arXiv2018

Short-wavelength out-of-band EUV emission from Sn laser-produced plasma

We present the results of spectroscopic measurements in the extreme ultraviolet (EUV) regime (7-17 nm) of molten tin microdroplets illuminated by a high-intensity 3-J, 60-ns Nd:YAG laser pulse. The strong 13.5 nm emission from this laser-produced plasma is of relevance for next-generation nanolithography machines. Here, we focus on the shorter wavelength features between 7 and 12 nm which have so far remained poorly investigated despite their diagnostic relevance. Using flexible atomic code calculations and local thermodynamic equilibrium arguments, we show that the line features in this region of the spectrum can be explained by transitions from high-lying configurations within the Sn$^{8+}$-Sn$^{15+}$ ions. The dominant transitions for all ions but Sn$^{8+}$ are found to be electric-dipole transitions towards the $n$=4 ground state from the core-excited configuration in which a 4$p$ electron is promoted to the 5$s$ sub-shell. Our results resolve some long-standing spectroscopic issues and provide reliable charge state identification for Sn laser-produced plasma, which could be employed as a useful tool for diagnostic purposes.

physics.atom-ph↗

arXiv2016

Optical spectroscopy of complex open 4$d$-shell ions Sn$^{7+}$-Sn$^{10+}$

We analyze the complex level structure of ions with many-valence-electron open [Kr] 4$d^\textrm{m}$ sub-shells ($\textrm{m}$=7-4) with ab initio calculations based on configuration-interaction many-body perturbation theory (CI+MBPT). Charge-state-resolved optical and extreme ultraviolet (EUV) spectra of Sn$^{7+}$-Sn$^{10+}$ ions were obtained using an electron beam ion trap. Semi-empirical spectral fits carried out with the orthogonal parameters technique and Cowan code calculations lead to 90 identifications of magnetic-dipole transitions and the determination of 79 energy ground-configuration levels, questioning some earlier EUV-line assignments. Our results, the most complete data set available to date for these ground configurations, confirm the ab initio predictive power of CI+MBPT calculations for the these complex electronic systems.

physics.atom-ph↗

arXiv2016

Analysis of the fine structure of Sn$^{11+...14+}$ ions by optical spectroscopy in an electron beam ion trap

We experimentally re-evaluate the fine structure of Sn$^{11+...14+}$ ions. These ions are essential in bright extreme-ultraviolet (EUV) plasma-light sources for next-generation nanolithography, but their complex electronic structure is an open challenge for both theory and experiment. We combine optical spectroscopy of magnetic dipole $M1$ transitions, in a wavelength range covering 260\,nm to 780\,nm, with charge-state selective ionization in an electron beam ion trap. Our measurements confirm the predictive power of \emph{ab initio} calculations based on Fock space coupled cluster theory. We validate our line identification using semi-empirical Cowan calculations with adjustable wavefunction parameters. Available Ritz combinations further strengthen our analysis. Comparison with previous work suggests that line identifications in the EUV need to be revisited.

physics.atom-ph↗

arXiv2014

Measurement of the half-life of the T=$\frac{1}{2}$ mirror decay of $^{19}$Ne and its implication on physics beyond the standard model

The $\frac{1}{2}^+ \rightarrow \frac{1}{2}^+$ superallowed mixed mirror decay of $^{19}$Ne to $^{19}$F is excellently suited for high precision studies of the weak interaction. However, there is some disagreement on the value of the half-life. In a new measurement we have determined this quantity to be $T_{1/2}$ = $17.2832 \pm 0.0051_{(stat)}$ $\pm 0.0066_{(sys)}$ s, which differs from the previous world average by 3 standard deviations. The impact of this measurement on limits for physics beyond the standard model such as the presence of tensor currents is discussed.

nucl-ex↗

arXiv2013

First Test of Lorentz Invariance in the Weak Decay of Polarized Nuclei

A new test of Lorentz invariance in the weak interactions has been made by searching for variations in the decay rate of spin-polarized 20Na nuclei. This test is unique to Gamow-Teller transitions, as was shown in the framework of a recently developed theory that assumes a Lorentz symmetry breaking background field of tensor nature. The nuclear spins were polarized in the up and down direction, putting a limit on the amplitude of sidereal variations of the form |(Γ_{up} - Γ_{down})| / (Γ_{up} + Γ_{down}) < 3 * 10^{-3}. This measurement shows a possible route toward a more detailed testing of Lorentz symmetry in weak interactions.

nucl-ex↗

arXiv2013

Decay rate measurement of the first vibrationally excited state of MgH$^+$ in a cryogenic Paul trap

We present a method to measure the decay rate of the first excited vibrational state of simple polar molecular ions being part of a Coulomb crystal in a cryogenic linear Paul trap. Specifically, we have monitored the decay of the $|ν$=$1,J$=$1 \rangle_X$ towards the $|ν$=$0,J$=$0 \rangle_X$ level in MgH$^+$ by saturated laser excitation of the $|ν$=$0,J$=$2 \rangle_X$-$|ν$=$1,J$=$1 \rangle_X$ transition followed by state selective resonance enhanced two-photon dissociation out of the $|ν$=$0,J$=$2 \rangle_X$ level. The technique enables the determination of decay rates, and thus absorption strengths, with an accuracy at the few percent level.

physics.atom-ph↗

arXiv2011

Isotope Shifts of the $6d\,^2$D$_{3/2}\,$ - $7p\,^2$P$_{1/2}\,$ Transition in Trapped Short-Lived $^{209-214}$Ra$^+$

Laser spectroscopy of short-lived radium isotopes in a linear Paul trap has been performed. The isotope shifts of the $6d\,^2$D$_{3/2}\,$ - $7p\,^2$P$_{1/2}\,$ transition in $^{209-214}$Ra$^+$ were measured, which are sensitive to the short range part of the atomic wavefunctions. The results are essential experimental input for improving the precision of atomic structure calculation. This is indispensable for parity violation in Ra$^+$ aiming at the determination of the weak mixing angle.

physics.atom-ph↗

arXiv2011

Radium single-ion optical clock

We explore the potential of the electric quadrupole transitions $7s\,^2S_{1/2}$ - $6d\,^2D_{3/2}$, $6d\,^2D_{5/2}$ in radium isotopes as single-ion optical frequency standards. The frequency shifts of the clock transitions due to external fields and the corresponding uncertainties are calculated. Several competitive $^A$Ra$^+$ candidates with $A=$ 223 - 229 are identified. In particular, we show that the transition $7s\,^2S_{1/2}\,(F=2,m_F=0)$ - $6d\,^2D_{3/2}\,(F=0,m_F=0)$ at 828 nm in $^{223}$Ra$^+$, with no linear Zeeman and electric quadrupole shifts, stands out as a relatively simple case, which could be exploited as a compact, robust, and low-cost atomic clock operating at a fractional frequency uncertainty of $10^{-17}$. With more experimental effort, the $^{223,225,226}$Ra$^+$ clocks could be pushed to a projected performance reaching the $10^{-18}$ level.

physics.atom-ph↗