Mechanisms of Auger-induced chemistry derived from wave packet dynamics

Abstract

To understand how core ionization and subsequent Auger decay lead to bond breaking in large systems, we simulate the wave packet dynamics of electrons in the hydrogenated diamond nanoparticle C 197 H 112 . We find that surface core ionizations cause emission of carbon fragments and protons through a direct Auger mechanism, whereas deeper core ionizations cause hydrides to be emitted from the surface via remote heating, consistent with results from photon-stimulated desorption experiments [Hoffman A, Laikhtman A, (2006) J Phys Condens Mater 18:S1517-S1546]. This demonstrates that it is feasible to study the chemistry of highly excited large-scale systems using simulation and analysis tools comparable in simplicity to those used for classical molecular dynamics.electron force field | fermionic molecular dynamics | floating Gaussian orbitals G reat uncertainties remain concerning how highly excited states (∼100 eV) induced by energetic photons, electrons, ions, or plasmas induce chemical processes at surfaces (1), particularly in large covalent systems (2) relevant to semiconductor fabrication (3). To determine the electron dynamics and atomic mechanisms involved in large-scale highly excited processes, we have developed the electron force field (eFF), a molecular dynamics model that includes electrons. We previously used eFF to compute the thermodynamic properties of warm dense hydrogen, and found excellent agreement with high-level theory, as well as both static compression and dynamic shock experiments (4).We report now the application of eFF to study Auger processes in a hydrogenated diamond nanoparticle C 197 H 112 (Fig. 1A). In the Auger process, ionization of a core electron leads to the collapse of a valence electron into the core hole, together with the ejection of another valence electron, all over several femtoseconds (5). During and after this time, secondary processes occur, causing protons, hydrides, and other species to desorb from hydrogen-terminated surfaces (1). We study the coupled electron-nuclear dynamics of both the primary Auger and accompanying secondary processes, ionizing core electrons both at the diamondoid surface and at different depths below the surface. In this way, we determine how the distance over which an Auger excitation relaxes and propagates affects the energies of electrons and composition of atomic species desorbed from the surface.In eFF, all electrons, valence and core, are modeled as spherical Gaussian wave packets:while nuclei are modeled as classical charged particles moving in the mean field of the electrons. The positions x i and sizes s i of the electrons are continuously variable, giving them the flexibility to participate in covalent, ionic, multicenter, and even metallic bonding; p x,i and p s,i are the corresponding conjugate momenta. This description is well-suited for representing highly excited systems, where electron density maxima and spreads may be distorted from equilibrium positions and values.Substituting the above expression into the time-dependent Schr...

High-temperature high-pressure phases of lithium from electron force field (eFF) quantum electron dynamics simulations

“…S1 and S2) (7). We also reported the application of eFF to describing the electron dynamics of the highly excited states in diamonoid nanoparticles formed by relaxation of valence electrons into core holes (Auger process) (8). These studies agree with available experimental data.…”

“…We performed the eFF dynamics for total simulation times of 1-2 ps with 1-2 ps of preequilibration and a time integration step of dt ¼ 5 attoseconds. With eFF the time-dependent Schrödinger equation uses an empirical exchange term with three universal parameters that are exactly the same as in previous publications (6)(7)(8). Individual trajectories were integrated in the microcanonical ensemble.…”

Nonadiabatic effects and excitonlike states during the insulator-to-metal transition in warm dense hydrogen

“…Here we should mention that even the proper description of the excited states of a single H 2 dimer is a complicated quantum mechanical problem that requires multi-reference methods 41 . ROKS DFT serves in this study as a computationally feasible approximation that is more realistic than the eFF model (eFF suffers from the lack of proper anti-symmetrization of the many-electron wavefunction and from the ambiguity of effective electron mass that affects the rates of nonadiabatic processes 42 ). Even this level of theory gives the possibility to justify at the semi-quantitative level the non-adiabatic nature of IMT and discuss the consequences of this thesis.…”

The dynamics of highly excited electronic systems: Applications of the electron force field

“…17. In the Auger process, a core electron is removed from an atom, which in turn causes a valence electron to collapse inward into the core, and another valence electron ͑the Auger electron͒ to be ejected.…”

“…17 These cases represent the first steps toward using eFF to model highly excited electronic processes in complex materials.…”