arXiv subjects

Radek Holeňák

Publications and source records attributed to Radek Holeňák.

12 recordsLinked to original sources

arXiv2026

Charge-State Dependence of Electronic Excitations in keV Ions Transmitted Through Solids Probed by Ion-Photon Coincidence Measurements

Investigating the electronic excitations caused by light keV ions in matter contributes to a better understanding of materials modification such as space weathering or plasma-wall interactions in fusion reactors as well as improved materials analysis methods such as low or medium energy ion scattering. This study investigates the dependence of specific excitations in keV He projectiles, the most common projectile employed in these analytical methods, on the kinetic energy of the ion. We report on photon emission at energies >10eV emitted by projectiles excited upon transmission through different sample systems. We separate exit charge states and report on measurements of ion-photon pairs in coincidence, i.e. we can link a photon to the specific ion emitting it. From these measurements we extracted charge state resolved photon yields and obtained a deep insight into the electronic excitation occurring. Our results show that the electronic excitation of the helium projectile is largely material independent and rising strongly with the kinetic energy of the projectile. The photon yields are in good agreement with a common excitation model initially developed for beam foil spectroscopy. We hence extend the validity of this model to higher photon energies and different sample materials. We further provide a detailed analysis of the employed coincidence approach and necessary corrections to the raw data, as the methodology has a broad applicability in fundamental research and materials analysis.

cond-mat.mtrl-sci↗

arXiv2026

Quantitative Analysis of Composition and Contamination of Atomically Thin Materials by Recoil-Projectile Coincidence in Ion Transmission

Surface contamination strongly affects the intrinsic properties of nanoscale materials, making its reliable identification and quantification crucial for both accurate experimental interpretation and nanofabrication. Although scanning transmission electron microscopy can resolve contaminants at atomic resolution within nanometer-scale regions, it cannot easily provide a quantitative, large-area contamination measure. Here, we introduce a minimally destructive recoil-projectile coincidence method for ion transmission experiments that enables element-specific identification and quantification of surface contaminants with isotopic resolution. We demonstrate this approach by comparing self-supporting graphene samples prepared using either a polymethylmethacrylate (PMMA)-based or a PMMA-free transfer process. Carbon and hydrogen are identified as the dominant surface contaminants. PMMA-free transferred graphene exhibits the lowest native contamination levels. Following in-situ thermal annealing at 400 °C for 1 h, the measured carbon areal density approaches the value expected for atomically clean single-layer graphene within the experimental uncertainty, while hydrogen coverage is strongly reduced. Unlike PMMA-transferred graphene, which rapidly recontaminates after annealing, PMMA-free transferred graphene remains nearly contamination-free for at least 140 min under ultra-high vacuum conditions ($p_{\mathrm{base}} = 2 \times 10^{-8}$ mbar). Beyond graphene, the presented method establishes a quantitative characterization platform for ultrathin materials, enabling studies of surface cleanliness, adsorption, implantation and surface interaction dynamics in such systems.

cond-mat.mtrl-sci↗

arXiv2026

Charge state dynamics of keV ions in solids

Fast dynamic processes between electrons in solids and a foreign atom represent a fundamental challenge for describing interactions in many-body systems and are a prerequisite for modelling materials modification. We experimentally determined the charge state distributions of slow He and Ne projectiles after transmission through thin single-crystalline silicon membranes. We found strong differences in velocity scaling and magnitude of the mean charge along different characteristic particle trajectories, providing direct insight on electron promotion and transfer processes inside the solid. Calculations of characteristic trajectories confirm the frequent spatial and ultrafast temporal accessibility of excitation channels commonly considered characteristic for large angle collisions. The commonly observed excess in energy deposition in amorphous targets compared to channelling trajectories and ab-initio calculations can thus be unambiguously linked to energy dissipation in frequent electron promotion as well as increased ionization density along the trajectory, driven by increased mean charge states. A quantitative comparison of energy loss and observed mean charge states further indicates complex deexcitation mechanisms at large interatomic distances masking the true equilibrium charge states along random trajectories.

cond-mat.str-el↗

arXiv2026

Rainbow Scattering from Graphene

We report the experimental observation of atomic rainbow scattering of 40 keV Xe$^+$ ions transmitted through self-supporting single-layer graphene using time-of-flight medium energy ion scattering. Supported by molecular dynamics and binary collision approximation simulations, we show that the rainbow pattern of graphene consists of a small hexagonal inner rainbow, arising from projectiles with characteristic trajectories interacting with multiple carbon atoms, and a larger circular outer rainbow, arising from close binary collisions between projectiles and individual carbon atoms.

cond-mat.mtrl-sci↗

arXiv2025

Probing formation and epitaxy of ultrathin Titanium Silicide using low and medium energy ion scattering

Titanium silicide is a key contact material in advanced three-dimensional semiconductor device architectures. Here, we examine the formation of ultrathin Ti-silicide on Si(100) using a combination of non-destructive in-situ and ex-situ ion scattering techniques capable of resolving composition and structure at the nanoscale. In-situ Time-of-Flight Low-Energy Ion Scattering (ToF-LEIS) indicates intermixing after annealing at 350 °C, with further compositional changes after annealing at 500 °C, including the emergence of a Si terminating layer at the surface. Consecutive ex-situ Time-of-Flight Medium-Energy Ion Scattering (ToF-MEIS) reveals a Ti-rich polycrystalline surface layer and a Si-rich interface layer exhibiting strong ordering along the Si [100] axis. High-Resolution Transmission Electron Microscopy (HR-TEM) images confirm these findings, revealing a $\approx$1.5 nm thick epitaxial silicide layer at the interface. The presence of an epitaxial interface is particularly promising for minimizing contact resistivity in ultrathin contact layers, where interfacial order can dominate electronic performance. In addition, both ToF-MEIS and HR-TEM unveil significant variations in the thickness of the silicide layer, with a substantial interface roughness but no translation of this roughness to the surface.

cond-mat.mtrl-sci↗

arXiv2025

The Role of 11B4C Interlayers in Enhancing Fe/Si Multilayer Performance for Polarized Neutron Mirrors

This study investigates the effects of incorporating 11B4C interlayers into Fe/Si multilayers, with a focus on interface quality, reflectivity, polarization, and magnetic properties for polarized neutron optics. It is found that the introduction of 1 Å and 2 Å 11B4C interlayers significantly improves the interface sharpness, reducing interface width and preventing excessive Si diffusion into the Fe layers. X-ray reflectivity and polarized neutron reflectivity measurements show enhanced reflectivity and polarization, with a notable increase in polarization for 30 Å period multilayers. The inclusion of interlayers also helps prevent the formation of iron-silicides, improving both the magnetic properties and neutron optical performance. However, the impact of interlayers is less pronounced in thicker-period multilayers (100 Å), primarily due to the ratio between layer and interface widths. These results suggest that 11B4C interlayers offer a promising route for optimizing Fe/Si multilayer performance in polarized neutron mirrors.

cond-mat.mtrl-sci↗

arXiv2024

The influence of geometry and specific electronic and nuclear energy deposition on ion-stimulated desorption from thin self-supporting membranes

We investigate the dependence of the yield of positive secondary ions created upon impact of primary He, B and Ne ions on geometry and electronic and nuclear energy deposition by the projectiles. We employ pulsed beams in the medium energy regime and a large position-sensitive, time-of-flight detection system to ensure accurate quantification. As a target, we employ a single crystalline Si(100) self-supporting 50 nm thick membrane thus featuring two identical surfaces enabling simultaneous measurements in backscattering and transmission geometry. Electronic sputtering is identified as the governing mechanism for the desorption of hydrogen and molecular species found on the surfaces. Nevertheless, larger energy deposition to the nuclear subsystem by heavier projectiles as well as due to the directionality of the collision cascade appears to act in synergy with the electronic energy deposition leading to an overall increase in secondary ion yields. A higher yield of ions sputtered from the matrix is observed in transmission geometry only for B and Ne ions, consistent with the observed role of nuclear stopping.

physics.chem-ph↗

arXiv2024

Local electronic excitations by slow light ions in tungsten

Accurately predicting the electronic energy deposition of ions in materials is an important challenge in both fundamental and applied research. While employing ab initio simulations to investigate electronic stopping of ions in matter holds promise, its combined use with experimental measurements paves the way for obtaining reliable data. In this paper, we present a collaborative study using real-time time-dependent density functional theory and experimental methods to determine the electronic stopping power of hydrogen and helium ions in tungsten, a primary candidate material for future nuclear fusion devices. While calculated stopping powers in hyperchanneling trajectories are significantly lower than the experimental data, off-center and random geometries demonstrate a better agreement. We show that the deviation from velocity proportionality for both projectiles traversing the hyperchanneling directions can be explained through the existence of a threshold velocity leading to the activation of semicore states. Additionally, we analyse the pseudopotential and the trajectory dependence of computed electronic energy losses. It is demonstrated that the role of including inner-shell electrons varies depending on the velocity range. While these states play a crucial role at high projectile velocities by introducing additional dissipation channels, their impact diminishes in the low-velocity range. Finally, we introduce a simple expression that links electronic energy losses in different trajectories to local electron density, and we show that utilizing this formula allows for quite accurate predictions of stopping powers around the Bragg peak.

cond-mat.mtrl-sci↗

arXiv2023

Trajectory dependence of electronic energy-loss straggling at keV ion energies

We have measured the electronic energy-loss straggling of protons, helium, boron and silicon ions in silicon using a transmission time-of-flight approach. Ions with velocities between 0.25 and 1.6 times the Bohr velocity were transmitted through single-crystalline Si(100) nanomembranes in either channelling or random geometry to study the impact parameter dependence of energy-loss straggling. Nuclear and path length contributions to the straggling were determined with the help of Monte Carlo simulations. Our results exhibit an increase in straggling with increasing ion velocity for channelled trajectories for all projectiles as well as for protons and helium in random geometry. In contrast for heavier ions, electronic straggling at low velocities does not decrease further but plateaus and even seems to increase again. We compare our experimental results with transport cross section calculations. The satisfying agreement for helium shows that electronic stopping for light ions is dominated by electron-hole pair excitations, and that the previously observed trajectory dependence can indeed be attributed to a higher mean charge state for random trajectories. No agreement is found for boron and silicon indicating the breakdown of models based solely on electron-hole pair excitations, and that local electron-promotion and charge-exchange events significantly contribute to energy loss at low velocities.

cond-mat.mtrl-sci↗

arXiv2021

On the correlation of angular distributions of keV ions and trajectory-dependent electronic excitations in transmission channelling geometry

We use energy discrimination of keV ions transmitted through a thin, single-crystalline silicon membrane to correlate specific angular distribution patterns formed in channelling geometry with trajectory-dependent electronic energy loss. The integral energy and intensity distribution of transmitted ions can thus be dissected into on one side axially channelled projectiles travelling along rather straight trajectories and on the other side dechannelled projectiles predominantly experiencing blocking. Angular distributions of transmitted ions are further simulated with two different Monte-Carlo codes.

physics.app-ph↗

arXiv2020

Trajectory-dependent electronic excitations by light and heavy ions around and below the Bohr velocity

We present experiments demonstrating trajectory-dependent electronic excitations at low ion velocities, where ions are expected to primarily interact with delocalized valence electrons. The energy loss of H$^+$, H$_2 ^+$, He$^+$, B$^+$, N$^+$, Ne$^+$, $^{28/29}$Si$^+$ and Ar$^+$ in self-supporting silicon membranes was analysed along channelled and random trajectories in a transmission approach. For all ions, we observe a difference in electronic stopping dependent on crystal orientation. For heavier ions, the energy-loss difference between channelling and random geometry is generally found more pronounced, and, in contrast to protons, increases for decreasing ion energy. Due to the inefficiency of core-electron excitations at employed ion velocities, we explain these results by reionization events occurring in close collisions of ions with target atoms, which are heavily suppressed for channelled trajectories. These processes result in trajectory-dependent mean charge states, which strongly affects the energy loss. The strength of the effect seems to exhibit a Z$_1$ oscillation with an observed minimum for Ne. We, furthermore, demonstrate that the simplicity of our experimental geometry leads to results that can serve as excellent benchmark systems for dynamic calculations of the electronic systems of solids using time-dependent density functional theory.

cond-mat.mtrl-sci↗

arXiv2020

Assessing electron emission induced by pulsed ion beams: a time-of-flight approach

We present a method to measure the kinetic energy of electrons emitted upon ion impact via their time-of-flight. Pulsed beams of H$^{+}$, D$_2^{+}$, He$^{+}$ and Ne$^{+}$ ions with velocities between 0.4 and 3.5 a.u. are transmitted through thin, self-supporting carbon and gold foils. Transmitted ions and secondary electrons are detected with a position-sensitive detector behind the sample and their respective energies are determined via their flight times. A coincidence criterium can be applied in the acquisition software. Measured electron energies range between 10 and 400 eV. Above ion velocities of 1 a.u. the most probable electron energy scales with ion velocity pointing towards a kinetic emission mechanism. At lower ion velocities, the electron energy stays constant and lies above the maximum energy transfer possible in a classical binary collision between ion and electron. Potential applications and technical challenges of measuring electron energies and yields with a time-of-flight approach are discussed.

physics.ins-det↗