arXiv subjects

Alexey Verkhovtsev

Publications and source records attributed to Alexey Verkhovtsev.

11 recordsLinked to original sources

arXiv2026

Scaling Dependencies in Irradiation-Driven Molecular Dynamics Simulations: Case Study of W(CO)$_6$ Fragmentation

Irradiation-driven fragmentation and chemical transformations of organometallic molecules play a central role in nanofabrication techniques based on the use of focused charged-particle beams. In this paper, the electron irradiation-induced fragmentation dynamics of W(CO)$_6$, a commonly used precursor for focused electron beam-induced deposition (FEBID), is investigated using the irradiation-driven molecular dynamics (IDMD) method. Simulations are performed for gas-phase systems with different precursor densities and under different irradiation conditions. The results reveal progressive fragmentation of W(CO)$_6$ molecules into W(CO)$_n$ species, accompanied by the formation of W-rich molecular clusters. The evolution of fragment abundances shows a strong dependence on both precursor density and electron fluence. Higher densities and larger fluences result in more extensive fragmentation and promote the aggregation of tungsten atoms into small metal clusters. Under certain irradiation conditions, the studied molecular systems evolve towards a steady state characterised by slightly varying fragment abundances. The obtained scaling relations between irradiation parameters and fragment distributions provide guidance for selecting simulation parameters in IDMD simulations of the FEBID process, ensuring a quantitative description of precursor fragmentation dynamics.

physics.atm-clus↗

arXiv2020

Molecular dynamics simulations of sodium nanoparticle deposition on magnesium oxide

The interaction of mass-selected atomic clusters and nanoparticles with surfaces attracts strong interest in view of fundamental research and technological applications. Understanding dynamics of the deposition process is important for controlling structure and functioning of deposited nanoparticles on a substrate, but experimental techniques can usually observe only the final outcome of the deposition process. In this paper, the deposition of 4 nm-sized sodium nanoparticles on an experimentally relevant magnesium oxide substrate is studied by means of classical molecular dynamics simulations. An empirical force field is derived which accounts for the interaction of highly polarizable Na atoms with the surface, reproducing the results of previously reported quantum mechanics/molecular mechanics simulations. Molecular dynamics simulations permit exploring the dynamics of deposited nanoparticles on long timescales on the order of hundreds of picoseconds, thus enabling the analysis of energy relaxation mechanisms and the evolution of nanoparticle structure up to its thermalization with the substrate. Several nanoparticle characteristics, such as internal structure, contact angle, and aspect ratio are studied in a broad deposition energy range from the soft landing to multi-fragmentation regimes.

physics.atm-clus↗

arXiv2019

Generalized correction to embedded-atom potentials for modeling equilibrium and non-equilibrium properties of metals

A modification of an embedded-atom method (EAM)-type potential is proposed for a quantitative description of equilibrium and non-equilibrium properties of metal systems within the molecular-dynamics framework. The modification generalizes the previously developed linear correction to EAM-type potentials [Sushko et al., J. Phys.: Condens. Matter \textbf{28}, 145201 (2016)] and asymptotically approaches zero at large interatomic distances. A general procedure for constructing this modification is outlined and its relation to the linear correction is elaborated. To benchmark this procedure, we examine the melting phase transition and several equilibrium properties of nanosystems made of silver, gold, and titanium. The simulations performed with the modified potential predict higher bulk melting temperatures of the metals and agree better with experimental values as compared to the original EAM-type potential. Our results show that the modification works well for metals with both cubic and hexagonal crystalline lattices. The Gupta potential is chosen as an illustrative case study but the modification proposed is general and can be applied to other widely-used potentials of the EAM type.

cond-mat.mes-hall↗

arXiv2019

Reactive molecular dynamics simulations of organometallic compound W(CO)6 fragmentation

Irradiation- and collision-induced fragmentation studies provide information about geometry, electronic properties and interactions between structural units of various molecular systems. Such knowledge brings insights into irradiation-driven chemistry of molecular systems which is exploited in different technological applications. An accurate atomistic-level simulation of irradiation-driven chemistry requires reliable models of molecular fragmentation which can be verified against mass spectrometry experiments. In this work fragmentation of a tungsten hexacarbonyl, W(CO)$_6$, molecule is studied by means of reactive molecular dynamics simulations. The quantitatively correct fragmentation picture including different fragmentation channels is reproduced. We show that distribution of the deposited energy over all degrees of freedom of the parent molecule leads to thermal evaporation of CO groups and the formation of W(CO)$_n^+$ ($n = 0-5$) fragments. Another type of fragments, WC(CO)$_n^+$ ($n = 0-4$), is produced due to cleavage of a C--O bond as a result of the localized energy deposition. Calculated fragment appearance energies are in good agreement with experimental data. These fragmentation mechanisms have a general physical nature and should take place in radiation-induced fragmentation of different molecular and biomolecular systems.

physics.chem-ph↗

arXiv2018

Phenomenon-based prediction of relative biological effectiveness of ion beams by means of the MultiScale Approach

Relative biological effectiveness (RBE) is a key quantity for the description of radiobiological effects induced by charged-particle irradiation in the context of ion-beam cancer therapy. Since RBE is a complex function that depends on different physical, chemical and biological parameters, a fundamental understanding of these effects becomes increasingly important for clinical applications. The phenomenon-based MultiScale Approach to the physics of radiation damage with ions (MSA) provides a tool for a molecular-level understanding of physical and chemical mechanisms of radiation biodamage and allows to quantify macroscopic biological effects caused by ion irradiation. This study reports the first application of the MSA for the analysis of RBE of ion beams. As a case study, we quantify the response of human normal tissue cell lines to carbon-ion irradiation at different values of linear energy transfer. RBE corresponding to different biological endpoints as well as other radiobiological parameters, such as clonogenic cell survival as a function of dose and inactivation cross section, are analyzed and compared with experimental data. Good agreement with an extensive data set emphasizes the predictive power of the MSA. The method is also used to make predictions of RBE at high values of LET where RBE decreases due to the "overkill" effect. In this regime, the dose needed to achieve a given biological effect is deposited by a few ions. The fact that a given number of ions may produce more damage than needed for a given effect leads to a significant (up to 20%) variation in RBE. This effect should be taken into consideration in the analysis of experimental data on irradiation with high-LET ions.

physics.med-ph↗

arXiv2018

Transport of secondary electrons through coatings of ion-irradiated metallic nanoparticles

The transport of low-energy electrons through the coating of a radiosensitizing metallic nanoparticle under fast ion irradiation is analyzed theoretically and numerically. As a case study, we consider a poly(ethylene glycol)-coated gold nanoparticle of diameter 1.6~nm excited by a carbon ion in the Bragg peak region in water as well as by more energetic carbon ions. The diffusion equation for low-energy electrons emitted from a finite-size spherical source representing the surface of the metal core is solved to obtain the electron number density as a function of radial distance and time. Information on the atomistic structure and composition of the coating is obtained from molecular dynamics simulations performed with the MBN Explorer software package. Two mechanisms of low-energy electron production by the metallic core are considered: the relaxation of plasmon excitations and collective excitations of valence $d$ electrons in individual atoms of gold. Diffusion coefficients and characteristic lifetimes of electrons propagating in gold, water, and poly(ethylene glycol) are obtained from relativistic partial wave analysis and the dielectric formalism, respectively. On this basis, the number of electrons released through the organic coating into the surrounding aqueous medium and the number of hydroxyl radicals produced are evaluated. The largest increase of the radical yield due to low-energy electrons is observed when the nanoparticle is excited by an ion with energy significantly exceeding that in the Bragg peak region. It is also shown that the water content of the coating, especially near the surface of the metal core, is crucial for the production of hydroxyl radicals.

physics.atm-clus↗

arXiv2017

Classical molecular dynamics simulations of fusion and fragmentation in fullerene-fullerene collisions

We present the results of classical molecular dynamics simulations of collision-induced fusion and fragmentation of C$_{60}$ fullerenes, performed by means of the MBN Explorer software package. The simulations provide information on structural differences of the fused compound depending on kinematics of the collision process. The analysis of fragmentation dynamics at different initial conditions shows that the size distributions of produced molecular fragments are peaked for dimers, which is in agreement with a well-established mechanism of C$_{60}$ fragmentation via preferential C$_2$ emission. Atomic trajectories of the colliding particles are analyzed and different fragmentation patterns are observed and discussed. On the basis of the performed simulations, characteristic time of C$_2$ emission is estimated as a function of collision energy. The results are compared with experimental time-of-flight distributions of molecular fragments and with earlier theoretical studies. Considering the widely explored case study of C$_{60}$--C$_{60}$ collisions, we demonstrate broad capabilities of the MBN Explorer software, which can be utilized for studying collisions of a broad variety of nanoscale and biomolecular systems by means of classical molecular dynamics.

physics.atm-clus↗

arXiv2017

Ion-impact-induced multifragmentation of liquid droplets

An instability of a liquid droplet traversed by an energetic ion is explored. This instability is brought about by the predicted shock wave induced by the ion. An observation of multifragmentation of small droplets traversed by ions with high linear energy transfer is suggested to demonstrate the existence of shock waves. A number of effects are analysed in effort to find the conditions for such an experiment to be signifying. The presence of shock waves crucially affects the scenario of radiation damage with ions since the shock waves significantly contribute to the thermomechanical damage of biomolecules as well as the transport of reactive species. While the scenario has been upheld by analyses of biological experiments, the shock waves have not yet been observed directly, regardless of a number of ideas of experiments to detect them were exchanged at conferences.

physics.med-ph↗

arXiv2016

Photoionization of multishell fullerenes studied by ab initio and model approaches

Photoionization of two buckyonions, C$_{60}$@C$_{240}$ and C$_{20}$@C$_{60}$, is investigated by means of time-dependent density-functional theory (TDDFT). The TDDFT-based photoabsorption spectrum of C$_{60}$@C$_{240}$, calculated in a broad photon energy range, resembles the sum of spectra of the two isolated fullerenes, thus illustrating the absence of strong plasmonic coupling between the fullerenes which was proposed earlier. The calculated spectrum of the smaller buckyonion, C$_{20}$@C$_{60}$, differs significantly from the sum of the cross sections of the individual fullerenes because of strong geometrical distortion of the system. The contribution of collective electron excitations arising in individual fullerenes is evaluated by means of plasmon resonance approximation (PRA). An extension of the PRA formalism is presented, which allows for the study of collective electron excitations in multishell fullerenes under photon impact. An advanced analysis of photoionization of buckyonions, performed using modern computational and analytical approaches, provides valuable information on the response of complex molecular systems to the external electromagnetic field.

physics.atm-clus↗

arXiv2016

Reconciling simulated melting and ground-state properties of metals with a modified embedded-atom method potential

We propose a modification of the embedded-atom method-type potential aiming at reconciling simulated melting and ground-state properties of metals by means of classical molecular dynamics. Considering titanium, magnesium, gold, and platinum as case studies, we demonstrate that simulations performed with the modified force field yield quantitatively correctly both the melting temperature of the metals and their ground-state properties. It is shown that the accounting for the long-range interatomic interactions noticeably affect the melting point assessment. The introduced modification weakens the interaction at interatomic distances exceeding the equilibrium one by a characteristic vibration amplitude defined by the Lindemann criterion, thus allowing for the correct simulation of melting, while keeping its behavior in the vicinity of the ground state minimum. The modification of the many-body potential has a general nature and can be applicable to metals with different characteristics of the electron structure as well as for many different molecular and solid state systems experiencing phase transitions.

cond-mat.mes-hall↗

arXiv2015

Comparative analysis of the secondary electron yield from carbon nanoparticles and pure water medium

The production of secondary electrons generated by carbon nanoparticles and pure water medium irradiated by fast protons is studied by means of model approaches and Monte Carlo simulations. It is demonstrated that due to a prominent collective response to an external field, the nanoparticles embedded in the medium enhance the yield of low-energy electrons. The maximal enhancement is observed for electrons in the energy range where plasmons, which are excited in the nanoparticles, play the dominant role. Electron yield from a solid carbon nanoparticle composed of fullerite, a crystalline form of C60 fullerene, is demonstrated to be several times higher than that from liquid water. Decay of plasmon excitations in carbon-based nanosystems thus represents a mechanism of increase of the low-energy electron yield, similar to the case of sensitizing metal nanoparticles. This observation gives a hint for investigation of novel types of sensitizers to be composed of metallic and organic parts.

physics.atm-clus↗