Molecular dynamics for irradiation driven chemistry:application to the FEBID process

A new molecular dynamics (MD) approach for computer simulations of irradiation driven chemical transformations of complex molecular systems is suggested. The approach is based on the fact that irradiation induced quantum transformations can often be treated as random, fast and local processes involving small molecules or molecular fragments. We advocate that the quantum transformations, such as molecular bond breaks, creation and annihilation of dangling bonds, electronic charge redistributions, changes in molecular topologies, etc., could be incorporated locally into the molecular force fields that describe the classical MD of complex molecular systems under irradiation. The proposed irradiation driven molecular dynamics (IDMD) methodology is designed for the molecular level description of the irradiation driven chemistry. The IDMD approach is implemented into the MBN Explore

Ultra-relativistic electron beams deflection by quasi-mosaic crystals

This paper provides an explanation of the key effects behind the deflection of ultra-relativistic electron beams by means of oriented quasi-mosaic Bent Crystals (qmBC). It is demonstrated that accounting for specific geometry of the qmBC and its orientation with respect to a collimated electron beam, its size and emittance is essential for an accurate quantitative description of experimental results on the beam deflection by such crystals. In an exemplary case study a detailed analysis of the recent experiment at the SLAC facility is presented. The methodology developed has enabled to understand the peculiarities in the measured distributions of the deflected electrons. This achievement constitutes an important progress in the efforts towards the practical realization of novel gamma-ray crystal-based light sources and puts new challenges for the theory and experiment in this research area.Comment: 6 pages, 4 figures plus Supplemental Materia

Simulation of Ultra-Relativistic Electrons and Positrons Channeling in Crystals with MBN Explorer

A newly developed code, implemented as a part of the \MBNExplorer package \cite{MBN_ExplorerPaper,MBN_ExplorerSite} to simulate trajectories of an ultra-relativistic projectile in a crystalline medium, is presented. The motion of a projectile is treated classically by integrating the relativistic equations of motion with account for the interaction between the projectile and crystal atoms. The probabilistic element is introduced by a random choice of transverse coordinates and velocities of the projectile at the crystal entrance as well as by accounting for the random positions of the atoms due to thermal vibrations. The simulated trajectories are used for numerical analysis of the emitted radiation. Initial approbation and verification of the code have been carried out by simulating the trajectories and calculating the radiation emitted by \E=6.7 GeV and \E=855 MeV electrons and positrons in oriented Si(110) crystal and in amorphous silicon. The calculated spectra are compared with the experimental data and with predictions of the Bethe-Heitler theory for the amorphous environment.Comment: 41 pages, 11 figures. Initially submitted on Dec 29, 2012 to Phys. Rev.

Irradiation driven molecular dynamics simulation of the FEBID process for Pt(PF3_3)4_4

This paper presents a detailed computational protocol for atomistic simulation of the formation and growth of metal-containing nanostructures during the Focused Electron Beam Induced Deposition (FEBID) process. The protocol is based upon the Irradiation-Driven Molecular Dynamics (IDMD) - a novel and general methodology for computer simulations of irradiation-driven transformations of complex molecular systems by means of the advanced software packages MBN Explorer and MBN Studio. Atomistic simulations performed following the formulated protocol provide valuable insights into the fundamental mechanisms of electron-induced precursor fragmentation and the related mechanism of nanostructure formation and growth using FEBID, which are essential for the further advancement of FEBID-based nanofabrication. The developed computational methodology is general and applicable to different precursor molecules, substrate types, irradiation regimes, etc. The methodology can also be adjusted to simulate the nanostructure formation by other nanofabrication techniques using electron beams, such as direct electron beam lithography. In the present study, the methodology is applied to the IDMD simulation of the FEBID of Pt(PF$_3$)$_4$ - a widely studied precursor molecule - on a SiO$_2$ surface. The simulations reveal the processes driving the initial phase of nanostructure formation during FEBID, including nucleation of Pt atoms, formation of small metal clusters on the surface, followed by their aggregation and the formation of dendritic platinum nanostructures. The analysis of the simulation results provides space resolved relative metal content, height and the growth rate of the deposits which represent valuable reference data for the experimental characterization of the nanostructures grown by FEBID.Comment: 19 pages, 12 figure

Dopant concentration effects on Si 1 - x Ge x crystals for emerging light-source technologies: a molecular dynamics study

In this study, we conduct atomistic-level molecular dynamics simulations on fixed-sized silicon-germanium (Si1-xGex) crystals to elucidate the effects of dopant concentration on the crystalline inter-planar distances. Our calculations consider a range of Ge dopant concentrations between pure Si (0%) and 15%, and for both the optimised system state and a temperature of 300K. We observe a linear relationship between Ge concentration and inter-planar distance and lattice constant, in line with the approximation of Vegard’s Law, and other experimental and computational results. These findings will be employed in conjunction with future studies to establish precise tolerances for use in crystal growth, crucial for the manufacture of crystals intended for emerging gamma-ray crystal-based light source technologies

A small-amplitude crystalline undulator based on 20 GeV electrons and positrons: Simulations

This paper presents the results of numerical simulations of a crystalline undulator based on the channeling of 20 GeV electrons and positrons. The device considered is characterized by a small amplitude and a short period of periodic bending. Calculations have been performed accounting for all-atom interactions using the MBN Explorer software package. The effect of low crystal thickness (less than a channeling oscillations period) on radiation spectrum was studied. A new scheme to product a high-energy radiation was proposed. It is based on short-period small-amplitude crystalline undulator and allows decreasing the intensity of the non-undulator part of the spectrum

Electron and positron propagation in straight and periodically bent axial and planar silicon channels

In this paper the results of simulations of axial and planar channeling of electrons and positrons in straight and periodically bent Si crystals are presented. Simulations with direct calculation of trajectories of projectiles accounting for all-atom interactions were carried out using the MBN Explorer software package. The full atomistic approach for particle trajectories simulation allows to quantitatively compare axial and planar channeling processes. The results of the simulations show significantly lower dechanneling length and number of channeling projectiles in the axial channeling case. For this case the dependence of characteristics of the channeling process on the choice of an axis direction and on a direction of the crystal bending has been investigated

All-atom relativistic molecular dynamics simulations of channeling and radiation processes in oriented crystals

We review achievements made in recent years in the field of numerical modeling of ultra-relativistic projectiles propagation in oriented crystals, radiation emission and related phenomena. This topic is highly relevant to the problem of designing novel gamma-ray light sources based on the exposure of oriented crystals to the beams of ultra-relativistic charged particles. The paper focuses on the approaches that allow for advanced computation exploration beyond the continuous potential and the binary collisions frameworks. A comprehensive description of the multiscale all-atom relativistic molecular dynamics approach implemented in the MBN Explorer package is given. Several case studies related to modeling of ultra-relativistic projectiles (electrons, positron and pions) channeling and photon emission in oriented straight, bent and periodically bent crystals are presented. In most cases, the input data used in the simulations, such as crystal orientation and thickness, the bending radii, periods and amplitudes, as well as the energies of the projectiles, have been chosen to match the parameters used in the accomplished and the ongoing experiments. Wherever available the results of calculations are compared with the experimental data and/or the data obtained by other numerical means.Comment: 60 pages, 34 figure

Extremely brilliant crystal-based light sources

The feasibility of novel gamma-ray light sources based on the channeling phenomenon of ultrarelativistic charged particles in oriented crystals is demonstrated by means of rigorous numerical modeling that accounts for the interaction of a projectile with all atoms of the crystalline environment. Accurate predictions are provided for the brilliance of radiation emitted in a diamond-based crystalline undulator by a 10 GeV positron beam available at present at the SLAC facility. It is shown that in the photon energy range $$\hbar \omega \gtrsim 1$$ MeV the brilliance is higher than that predicted in the Gamma Factory proposal for CERN. Novel crystal-based light sources may have a broad range of applications