Effect Of Electronic Excitation Research Articles

Ultrafast fragmentation of highly-excited doubly-ionized deoxyribose: role of the liquid water environment.

Ab initio molecular dynamics simulations are used to investigate the fragmentation dynamics following the double ionization of 2-deoxy-D-ribose (DR), a major component in the DNA chain. Different ionization scenarios are considered to provide a complete picture. First focusing on isolated DR2+, fragmentation patterns are determined for the ground electronic state, adding randomly distributed excitation energy to the nuclei. These patterns differ for the two isomers studied. To compare thermal and electronic excitation effects, Ehrenfest dynamics are also performed, allowing to remove the two electrons from selected molecular orbitals. Two intermediate-energy orbitals, localized on the carbon chain, were selected. The dissociation pattern corresponds to the most frequent pattern obtained when adding thermal excitation. On the contrary, targeting the four deepest orbitals, localized on the oxygen atoms, leads to selective ultrafast C-O and/or O-H bond dissociation. To probe the role of environment, a system consisting of a DR molecule embedded in liquid water is then studied. The two electrons are removed from either the DR or the water molecules directly linked to the sugar through hydrogen bonds. Although the dynamics onset is similar to that of isolated DR when removing the same deep orbitals localized on the sugar oxygen atoms, the subsequent fragmentation patterns differ. Sugar damage also occurs following the Coulomb explosion of neighboring H2O2+ molecules due to interaction with the emitted O or H atoms.

Modification of Cu Oxide and Cu Nitride Films by Energetic Ion Impact

We have investigated lattice disordering of cupper oxide (Cu2O) and copper nitride (Cu3N) films induced by high- and low-energy ion impact, knowing that the effects of electronic excitation and elastic collision play roles by these ions, respectively. For high-energy ion impact, degradation of X-ray diffraction (XRD) intensity per ion fluence or lattice disordering cross-section (YXD) fits to the power-law: YXD = (BXDSe)NXD, with Se and BXD being the electronic stopping power and a constant. For Cu2O and Cu3N, NXD is obtained to be 2.42 and 1.75, and BXD is 0.223 and 0.54 (kev/nm)−1. It appears that for low-energy ion impact, YXD is nearly proportional to the nuclear stopping power (Sn). The efficiency of energy deposition, YXD/Se, as well as Ysp/Se, is compared with YXD/Sn, as well as Ysp/Sn. The efficiency ratio RXD = (YXD/Se)/(YXD/Sn) is evaluated to be ~0.1 and ~0.2 at Se = 15 keV/nm for Cu2O and Cu3N, meaning that the efficiency of electronic energy deposition is smaller than that of nuclear energy deposition. Rsp = (Ysp/Se)/(Ysp/Sn) is evaluated to be 0.46 for Cu2O and 0.7 for Cu3N at Se = 15 keV/nm.

Non-thermal effect on collision cascade of tungsten

The intense electronic excitations in materials can trigger a series of non-thermal effects, seriously affecting the physical properties of materials. However, the electronic effects on collision cascade of tungsten (W) plasma-facing material in fusion reactors remains unclear. Based on the tight-binding theory, the non-thermal effects on the collision cascade of W material are extensively studied. Our results show that the non-thermal effect arising from intense electronic excitations induces substantial increase in the sputtering yield, as well as in the number of point defects during collision cascade in W slabs. Analysis shows that the observed phenomena are closely related to the deterioration of system’s stability under intense electronic excitations, since high electronic temperature results in the weakness of bonding strength among W atoms. Our work highlights the importance of electronic excitation effects in evaluating the resistance of W material to radiation damage when subjected to intense irradiation fields in fusion reactors.

Ab initio modelling of photocatalytic CO2 reduction reactions over Cu/TiO2 semiconductors including the electronic excitation effects

Photocatalysis is a promising method for reducing CO2 in an environmentally friendly way. Despite extensive experimental studies, theoretical studies lag behind and often resort to describing catalysts in the excited state, while the reaction mechanism is mostly studied at the ground level. We theoretically calculate a full reaction mechanism of CO2 reduction to CO in the excited state for five surfaces: Cu(111), anatase(110), rutile(101) and Cu/anatase, Cu/rutile using the ΔSCF approach. We show that excited states considerably lower activation barriers, making it necessary in describing the experimental performance. The density functional theory calculations in the excited state are used to construct a microkinetic model, which is used to predict the performance of each catalytic surface. We show that Cu(111) is photocatalytically inactive, TiO2 is only active in the UV range and catalyzes water splitting. Only Cu/rutile is active for CO2 reduction to CO, while Cu/anatase produces more H2.

Coulomb Spike Modelling of Ion Sputtering of Amorphous Water Ice

The effects of electronic excitations on the ion sputtering of water ice are not well understood even though there is a clear dependence of the sputtering yield on the electronic stopping power of high-energy ions. Ion sputtering of amorphous water ice induced by electronic excitations is modelled by using the Coulomb explosion approach. The momentum transfer to ionized target atoms in the Coulomb field that is generated by swift ion irradiation is computed. Positively charged ions produced inside tracks are emitted from the surface whenever the kinetic energy gained in the repulsive electrical field is higher than the surface binding energy. For that, the energy loss of deep-lying ions to reach the surface is taken into account in the sputtering yield and emitted ion velocity distribution. Monte Carlo simulations are carried out by taking into account the interactions of primary ions and secondary electrons (δ-rays) with the amorphous water ice medium. A jet-like anisotropic ion emission is found in the perpendicular direction in the angular distribution of the sputtering yield for normal incidence of 1-MeV protons. This directional emission decreases with an increasing incidence angle and vanishes for grazing incidence, in agreement with experimental data on several oxides upon swift ion irradiation. The role of the target material’s properties in this process is discussed.

Nonthermal effects in H-doped tungsten at high electronic temperatures

In a fusion reactor, the plasma with very high temperature intensely and continuously emits high-energy photons, which irradiate the tungsten material of the first wall. The irradiation with high-energy photons causes nonthermal events in the tungsten material. In this paper, the structural features of H-doped tungsten metal at highly electronic excitation states are studied using density functional theory. It has been found that the system can not only undergo nonthermal expansion with increasing electronic excitation energy, but also a phase transition from body-centered cubic (BCC) phase to face-centered cubic (FCC) or hexagonal close-packed cubic (HCP) phase occurs spontaneously with a threshold value of excitation energy. The relationship between the excitation energy threshold associated with the phase transition and the concentration of doped impurity in tungsten is disclosed, which can be expressed using an empirical formula. In addition, the synergistic effect of electronic excitation and impurity atoms on the phase transition is revealed. Our results give a deep insight into the irradiation effect of the high-energy photons on the first-wall tungsten in a fusion reactor.

Coherent vibrational dynamics of NbO2 film

The high thermodynamic transition temperature (\ensuremath{\approx}1080 K) of niobium dioxide (${\mathrm{NbO}}_{2}$) makes it advantageous in differentiating the effects of electronic excitation and heat accumulation in the photoinduced phase transition. Recently, a photoinduced nonthermal metallization of ${\mathrm{NbO}}_{2}$ film has been claimed [R. Rana, J. M. Klopf, J. Grenzer, H. Schneider, M. Helm, and A. Pashkin, Phys. Rev. B 99, 041102(R) (2019)], but the simultaneous lattice evolution still remains blank. In this paper, photoinduced evolution in lattice vibrations of a crystalline ${\mathrm{NbO}}_{2}$ film has been investigated using broadband coherent phonon spectroscopy. Signatures of two optical phonons at 155 and $185\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}1}$ are identified, and the corresponding lattice vibrations are found to be associated with the motions of Nb-Nb dimers. Pump fluence-dependent measurements reveal an electronic phase transition at a threshold fluence of $\ensuremath{\approx}10\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^{2}$, while the lattice structure is preserved. A transient disordering in the lattice vibration is simultaneously observed, but lattice transformation is prevented probably by the large enthalpy barrier of the strong Peierls insulation. The accompanied drastic modulation in optical properties without atomic rearrangement also suggests the potential of ${\mathrm{NbO}}_{2}$ in improving the operation speed of related devices.

Energy Relaxation and Electron–Phonon Coupling in Laser-Excited Metals

The rate of energy transfer between electrons and phonons is investigated by a first-principles framework for electron temperatures up to = 50,000 K while considering the lattice at ground state. Two typical but differently complex metals are investigated: aluminum and copper. In order to reasonably take the electronic excitation effect into account, we adopt finite temperature density functional theory and linear response to determine the electron temperature-dependent Eliashberg function and electron density of states. Of the three branch-dependent electron–phonon coupling strengths, the longitudinal acoustic mode plays a dominant role in the electron–phonon coupling for aluminum for all temperatures considered here, but for copper it only dominates above an electron temperature of = 40,000 K. The second moment of the Eliashberg function and the electron phonon coupling constant at room temperature K show good agreement with other results. For increasing electron temperatures, we show the limits of the approximation for the Eliashberg function. Our present work provides a rich perspective on the phonon dynamics and this will help to improve insight into the underlying mechanism of energy flow in ultra-fast laser–metal interaction.

Substrate-assisted Fermi level shifting of CVD graphene by swift heavy ions

The systematic modulation of surface electronic properties of graphene can be achieved through surface and interface engineering using swift heavy ion (SHI) irradiation. The effects of dense electronic excitation on the optical and surface electronic properties of chemical vapor deposited (CVD) graphene sheets are investigated in this work. The spectroscopic and microscopic results indicate that the sputtered substrate atoms are trapped in the graphene sheet during irradiation and alter the Fermi level of graphene. The change in the Fermi level calculated from the Raman spectra is investigated through the surface potential measurement by scanning Kelvin probe microscope (SKPM). The shifting of the Fermi level towards the valence band verifies the hole-doping of the graphene sheet by sputtered substrate atoms. The mechanism of ion interaction and its subsequent effects on the graphene and substrate material are further studied rigorously through Monte Carlo simulations using stopping and range of ions in matter (SRIM) and Surface Sputtering sub-routine of transport of ions in matter (TRIM) packages. This work offers a novel way of analyzing defect-induced modifications in the electronic properties of graphene by Raman spectroscopic features in combination with SKPM.

Thermodynamic properties of ε -Fe with thermal electronic excitation effects on vibrational spectra

The thermodynamic properties of hcp-iron ({\epsilon}-Fe) are essential for investigating planetary cores' internal structure and dynamic properties. Despite their importance to planetary sciences, experimental investigations of {\epsilon}-Fe at relevant conditions are still challenging. Therefore, ab initio calculations are crucial to elucidating the thermodynamic properties of this system. Here we use a free energy calculation scheme based on the phonon gas model compatible with temperature-dependent phonon frequencies. We investigate the effects of electronic thermal excitations, which introduces a temperature dependence on phonon frequencies, and the implication for the thermodynamic properties of {\epsilon}-Fe at extreme pressure (P) and temperature (T) conditions. We disregard phonon-phonon interactions, i.e., anharmonicity and their effect on phonon frequencies. Nevertheless, the current scheme is also applicable to T-dependent anharmonic frequencies. We conclude that the impact of thermal electronic excitations on vibrational properties is not significant up to ~ 4,000 K at 200 GPa but should not be ignored at higher temperatures or pressures. However, the static free energy, Fst, must always include the effect of thermal excitation fully in a continuum of Ts. Our results for isentropic equations of state show good agreement with data from recent ramp compression experiments up to 1,400 GPa conducted at the National Ignition Facility (NIF).

Effect of electronic excitation on nuclear magnetic shielding as illustrated by nuclear spin optical rotation (NSOR) in liquids

Near resonance effect of NSOR generates new resonant light shift ω well separated from the ground state optical chemical shift ω 0 , from which the shielding in the excited state can be obtained. • The near resonance effect on the optical chemical shift in NSOR is theoretically studied. • The dressed state theory is employed to explore the near resonance effect. • New optical chemical shift line in NSOR spectrum due to excited state molecule is estimated. • Possibility of detecting the new optical chemical line is discussed. Conventional NMR is generally a technique for electronic ground state molecules. In this paper, the effect of electronic excitation on nuclear magnetic shielding is theoretically illustrated by Nuclear Spin Optical Rotation (NSOR) in liquids. The near-resonance effect on the optical chemical shift in NSOR is explored by using the dressed state theory. The results indicate that in the NSOR spectrum of a molecule with nucleus I = 1/2, such effect can induce a new optical chemical shift line with the circular frequency different from that of the ground state. From it the NMR shielding of a molecule in the electronic excited state can be got directly and used to analyze excited molecule structure in liquids. For multi-nuclei molecules, the result applies to each nucleus.

Tailoring submicrometer periodic surface structures via ultrashort pulsed direct laser interference patterning

Direct laser Interference Patterning (DLIP) with ultrashort laser pulses (ULP) represents a precise and fast technique to produce tailored periodic sub-micrometer structures on various materials. In this work, an experimental and theoretical approach is presented to investigate the previously unexplored fundamental mechanisms for the formation of unprecedented laser-induced topographies on stainless steel following proper combinations of DLIP with ULP. DLIP is aimed to determine the initial conditions of the laser-matter interaction by defining an ablated region while double ULP are used to control the reorganisation of the self-assembled laser induced sub-micrometer sized structures by exploiting the interplay of different absorption and excitation levels coupled with the melt hydrodynamics induced by the first of the double pulses. A multiscale physical model is presented to correlate the interference period, polarization orientation and number of incident pulses with the induced morphologies. Special emphasis is given to electron excitation, relaxation processes and hydrodynamical effects that are crucial to the production of complex morphologies. Results are expected to derive new knowledge of laser-matter interaction in combined DLIP and ULP conditions and enable enhanced fabrication capabilities of complex hierarchical sub-micrometer sized structures for a variety of applications.

Effects of conduction electron excitation on x-ray magnetic circularly polarized emission in itinerant ferromagnets

This study provides a theoretical method to calculate x-ray magnetic circularly polarized emission spectra at the K${}_{\ensuremath{\alpha}}$ transitions in transition-metal ferromagnets. The method is applied to metallic iron. It reproduces the observed tail structures which originate from the electron excitations on the correlated $d$ bands. Cosinusoidal emission angle dependence of the spectral intensity enables one to elucidate the magnetization directions deep inside of bulk ferromagnets.

Latent Tracks in Ion-Irradiated LiTaO3 Crystals: Damage Morphology Characterization and Thermal Spike Analysis

Systematic research on the response of crystal materials to the deposition of irradiation energy to electrons and atomic nuclei has attracted considerable attention since it is fundamental to understanding the behavior of various materials in natural and manmade radiation environments. This work examines and compares track formation in LiTaO3 induced by separate and combined effects of electronic excitation and nuclear collision. Under 0.71–6.17 MeV/u ion irradiation with electronic energy loss ranging from 6.0 to 13.8 keV/nm, the track damage morphologies evolve from discontinuous to continuous cylindrical zone. Based on the irradiation energy deposited via electronic energy loss, the subsequently induced energy exchange and temperature evolution processes in electron and lattice subsystems are calculated through the inelastic thermal spike model, demonstrating the formation of track damage and relevant thresholds of lattice energy and temperature. Combined with a disorder accumulation model, the damage accumulation in LiTaO3 produced by nuclear energy loss is also experimentally determined. The damage characterizations and inelastic thermal spike calculations further demonstrate that compared to damage-free LiTaO3, nuclear-collision-damaged LiTaO3 presents a more intense thermal spike response to electronic energy loss owing to the decrease in thermal conductivity and increase in electron–phonon coupling, which further enhance track damage.

Optical spectroscopy of cryogenic metalated flavins: The O2(+) isomers of M+lumiflavin (M=Li–Cs)

Flavin complexes are used by nature in many photobiological processes, because their photochemical response in the visible range can be strongly modulated by their environment. Herein, we report optical spectra of mass-selected metal complexes of lumiflavin (LF) with alkali metal ions (M=Li–Cs) recorded in the visible range by photodissociation (VISPD) at cryogenic temperatures (T<20 K). VISPD spectra are measured in a tandem mass spectrometer coupled to a cryogenic ion trap and an electrospray ionization source. The vibrationally-resolved VISPD spectra obtained in the 420–450 nm range are assigned to the S0←S1 (ππ*) transition of the O2(+) isomers of M+LF by comparison to time-dependent density functional theory calculations (PBE0/cc-pVDZ) coupled to multidimensional Franck–Condon (FC) simulations. The preferred binding motif and interaction strength strongly depend on the size of M+. The spectra of M+LF with the smaller M+ ions (M=Li and Na) are attributed to the most stable O2+ isomers characterized by a N1–M–O2 chelate binding motif in which M+ can interact with the lone pairs of both N1 and O2 of LF. Because of steric interaction with the CH3 group at N10, the tricyclic aromatic ring is slightly bent and the VISPD spectra feature low-frequency out-of-plane bending modes of LF. Such an O2+ structure is sterically repulsive for the larger M+ ions (M=Rb and Cs), which instead form planar O2 minima with nearly linear C2–O2–M bonds. Their VISPD spectra are characterized by low-frequency in-plane bending modes (β) describing the M+…LF interaction. The VISPD spectrum of K+LF with the intermediate-size K+ ion is more complex and features very low-frequency modes resulting from a very shallow potential along the O2↔O2+ isomerization coordinate, which cannot be described well by harmonic FC simulations. Electronic S1 excitation slightly weakens the M+…LF interaction (6–9%), as deduced from the small ΔS1 blueshifts upon metalation at the O2(+) binding site, consistent with the charge reorganization deduced from the molecular orbitals involved in ππ* excitation. Overall, the effects of O2(+) complexation of LF are drastically different from those of the previously studied O4+ isomers, which are characterized by large ΔS1 redshifts based on the strong increase in the M+…LF interaction (up to 20%) upon the same ππ* excitation of the LF chromophor. The drastic site-specific variation in the photophysical response of the O2(+) and O4+ isomers arises mainly from the different effect of electronic excitation on the M+…LF bond strength rather than from the metal-induced change of the electronic structure and molecular orbitals of LF responsible for ππ* excitation.

Investigation on elastic and thermodynamic properties of Fe25Cr20NiMnNb austenitic stainless steel at high temperatures from first principles

Temperature-dependent properties are very useful in modeling the behavior of materials servicing at high temperature such as austenitic stainless steels. Besides, for predicting the mechanical properties of such alloys, experimental data at high temperature are very important, but scarce. In this work, we use first-principles calculations combined with a quasi-harmonic Debye model to evaluate the Helmholtz free energy of paramagnetic Fe25Cr20NiMnNb austenitic stainless steel as a function of temperature and volume. The contributions due to electronic excitations, magnetic fluctuations, and lattice vibrations are taken into account. Elastic and other thermodynamic properties of the material, such as the equilibrium lattice parameter, elastic moduli, entropy, are derived from the free energy in the temperature range from 800 to 1100 K and compared with available experimental data. The influence of Nb concentration, as well as the effects of electronic excitations and lattice vibrations, on the elastic and thermodynamic properties are discussed in detail. The elastic moduli are found to decrease with increasing temperature. This elastic softening phenomenon is mainly due to thermal expansion; the contribution of thermal electronic excitations is found to be relatively small.

Can Plasmon Change Reaction Path? Decomposition of Unsymmetrical Iodonium Salts as an Organic Probe.

Plasmon-assisted transformations of organic compounds represent a novel opportunity for conversion of light to chemical energy at room temperature. However, the mechanistic insights of interaction between plasmon energy and organic molecules is still under debate. Herein, we proposed a comprehensive study of the plasmon-assisted reaction mechanism using unsymmetric iodonium salts (ISs) as an organic probe. The experimental and theoretical analysis allow us to exclude the possible thermal effect or hot electron transfer. We found that plasmon interaction with unsymmetrical ISs led to the intramolecular excitation of electron followed by the regioselective cleavage of C-I bond with the formation of electron-rich radical species, which cannot be explained by the hot electron excitation or thermal effects. The high regioselectivity is explained by the direct excitation of electron to LUMO with the formation of a dissociative excited state according to quantum-chemical modeling, which provides novel opportunities for the fine control of reactivity using plasmon energy.

Atomic scattering of H and N on W(100): Effect of lattice vibration and electronic excitations on the dynamics

Scattering of H and N atoms from W(100) surface has been theoretically studied by means of classical trajectories based on an electronically adiabatic potential energy surface (PES). The PES has been constructed by interpolation of density functional theory (DFT) calculations following a corrugation reducing procedure (CRP). Van der Waals interactions are taken into account by using the vdW-DF2 exchange- correlation functional. In the dynamics a special focus is made on the influence of energy release through surface atoms motion and electrons of the metal. Most part of the H-atoms sticking is mediated through penetration into the sub-surface area where electron-hole pairs excitation plays the major role. For N-atoms the sticking is mainly due to the coupling with lattice vibrations, as expected when considering heavy atoms. Adsorption and absorption processes have been characterized for both H/W(100) and N/W(100) exhibiting differences between those benchmark light and heavy atoms in interaction with metals.

Phase stability and mechanical response of tantalum nitrides to electronic excitation effect

Based on an analysis ofelectronic structures and phonon spectra calculationsas a function of electronic temperature Te using the density functional theory, we investigated the phase stability and mechanical responseto electronic excitation effect on variousphases of tantalum nitrides TaNx with fully or partially occupied nitrogen sublattices. The results suggest that the cubic phase exhibits lattice instability at room temperature, while the hexagonal phases are dynamically stable. The phase stability increases in the sequence TaN (δ-type), TaN0.43, TaN0.5, TaN (AsNi type), TaN0.83, TaN (θ-type), and TaN (ε-type). The nonmetal vacancies exhibit enhanced latticestability with hexagonal phasescompared to the cubicphase. The electronic excitation effect providedextra stability for cubic δ-TaNwith the increase inelectronic temperature, whilethe excitation results in a lower degree of stability for hexagonal phases.The physical origin of electronic excitation effects on both the cubic and hexagonal phase stability of TaN can be attributed to the peculiarities of their electronicdensities of states near the Fermi level and a combination of three different types of bonding characteristics.The counterintuitivebehavior oflattice stabilities appears or disappears as a response to mechanical properties under electronic excitation.

Channel-closing effects of electronic excitation in solids.

We investigate the electronic excitation of solids in strong fields by solving the time-dependent Schrödinger equation. The excitation probability exhibits a strong modulation as a function of laser intensity when the initial states fill in the whole valence band. To have a clear insight into the modulation, we further study the electronic excitation from a single eigenstate in solids. A series of resonance-like enhancements of excitation probability are produced by changing the laser intensity and wavelength. We attribute the resonance-like enhancements to the channel-closing effects in solids. It is shown that the excitation probability exhibits enhancements when the value of channel is odd for intracycle interference and an integer for intercycle interference. This is different from the atom that the enhancement occur in the integer channels. We also reveal that the channel-closing effects can be observed by solid high-order harmonic generation.