Nonthermal Hot Electrons Research Articles

Hot Electrons in a Steady State: Interband vs Intraband Excitation of Plasmonic Gold.

Understanding the dynamics of "hot", highly energetic electrons resulting from nonradiative plasmon decay is crucial for optimizing applications in photocatalysis and energy conversion. This study presents an analysis of electron kinetics within plasmonic metals, focusing on the steady-state behavior during continuous-wave (CW) illumination. Using an inelastic spectroscopy technique, we quantify the temperature and lifetimes of distinct carrier populations during excitation. A significant finding is the monotonic increase in hot electron lifetime with decreases in electronic temperature. We also observe a 1.22× increase in hot electron temperature during intraband excitation compared to interband excitation and a corresponding 2.34× increase in carrier lifetime. The shorter lifetimes during interband excitation are hypothesized to result from direct recombination of nonthermal holes and hot electrons, highlighting steady-state kinetics. Our results help bridge the knowledge gap between ultrafast and steady-state spectroscopies, offering critical insights for optimizing plasmonic applications.

Vibrationally Hot Reactants in a Plasmon-Assisted Chemical Reaction.

Recent studies on plasmon-assisted chemical reactions postulate that the hot electrons of plasmon-excited nanostructures may induce a non-thermal vibrational activation of metal-bound reactants. However, the postulate has not been fully validated at the level of molecular quantum states. We directly and quantitatively prove that such activation occurs on plasmon-excited nanostructures: The anti-Stokes Raman spectra of reactants undergoing a plasmon-assisted reaction reveal that a particular vibrational mode of the reactant is selectively excited, such that the reactants possess >10 times more energy in the mode than is expected from the fully thermalized molecules at the given local temperature. Furthermore, a significant portion (∼20%) of the excited reactant is in vibrational overtone states with energies exceeding 0.5 eV. Such mode-selective multi-quantum excitation could be fully modeled by the resonant electron-molecule scattering theory. Such observations suggest that the vibrationally hot reactants are created by non-thermal hot electrons, not by thermally heated electrons or phonons of metals. The result validates the mechanism of plasmon-assisted chemical reactions and further offers a new method to explore the vibrational reaction control on metal surfaces.

Femtosecond Thermal and Nonthermal Hot Electron Tunneling Inside a Photoexcited Tunnel Junction.

Efficient operation of electronic nanodevices at ultrafast speeds requires understanding and control of the currents generated by femtosecond bursts of light. Ultrafast laser-induced currents in metallic nanojunctions can originate from photoassisted hot electron tunneling or lightwave-induced tunneling. Both processes can drive localized photocurrents inside a scanning tunneling microscope (STM) on femto- to attosecond time scales, enabling ultrafast STM with atomic spatial resolution. Femtosecond laser excitation of a metallic nanojunction, however, also leads to the formation of a transient thermalized electron distribution, but the tunneling of thermalized hot electrons on time scales faster than electron–lattice equilibration is not well understood. Here, we investigate ultrafast electronic heating and transient thermionic tunneling inside a metallic photoexcited tunnel junction and its role in the generation of ultrafast photocurrents in STM. Phase-resolved sampling of broadband terahertz (THz) pulses via the THz-field-induced modulation of ultrafast photocurrents allows us to probe the electronic temperature evolution inside the STM tip and to observe the competition between instantaneous and delayed tunneling due to nonthermal and thermal hot electron distributions in real time. Our results reveal the pronounced nonthermal character of photoinduced hot electron tunneling and provide a detailed microscopic understanding of hot electron dynamics inside a laser-excited tunnel junction.

Dust–ion acoustic solitary waves in a collisionless magnetized five components plasma

Abstract We have derived a Korteweg–de Vries–Zakharov–Kuznetsov (KdV-ZK) equation to study the nonlinear behavior of dust–ion acoustic waves in a collisionless magnetized five components dusty plasma consisting of warm adiabatic ions, nonthermal hot electrons, isothermal cold electrons, nonthermal positrons and static negatively charged dust particulates. It is found that the coefficient of the nonlinear term of the KdV-ZK equation vanishes along different family of curves in different compositional parameter planes. In this situation, to describe the nonlinear behavior of dust–ion acoustic waves, we have derived a modified KdV-ZK (MKdV-ZK) equation. When the coefficients of the nonlinear terms of both KdV-ZK and MKdV-ZK equations are simultaneously equal to zero, then we have derived a further modified KdV-ZK (FMKdV-ZK) equation which effectively describes the nonlinear behavior of dust–ion acoustic waves. Analytically and numerically, we have investigated the solitary wave solutions of different evolution equations propagating obliquely to the direction of the external static uniform magnetic field. We have seen that the amplitude of the KdV soliton strictly increases with increasing β e, whereas the amplitude of the MKdV soliton strictly decreases with increasing β e, where β e is the nonthermal parameter associated with the hot electron species. Also, there exists a critical value β r ( c ) ${\beta }_{\text{r}}^{(\text{c})}$ of β e such that the FMKdV soliton exists within the interval β r ( c ) < β e ≤ 4 7 ${\beta }_{\text{r}}^{(\text{c})}< {\beta }_{\text{e}}\le \frac{4}{7}$ , whereas the FMKdV soliton does not exist within the interval 0 < β e < β r ( c ) $0< {\beta }_{\text{e}}< {\beta }_{\text{r}}^{(\text{c})}$ . We have also discussed the effect of different parameters of the system on solitary waves obtained from the different evolution equations.

Discovering Hybrid Plasmonic Nanocatalysts for Solar-Chemical Energy Conversion via Femtosecond Pulsed Laser-Induced Atomic Redistribution in Bimetallic Au-Pd Nanorods

Utilizing solar energy for chemical transformations has attracted a growing interest in promoting the clean and modular chemical synthesis approach and addressing conventional thermocatalytic systems’ limitations. Photoelectrochemical (PEC) cells enabling the conversion of solar energy to storable and transportable fuels could address the existing challenge of mismatch between renewable energy supply and demand. Understanding the electronic and thermal properties of metal nanoparticles and their interactions with other materials is imperative to improve PEC cells’ efficiency. PECs typically consist of architecturally sophisticated photoelectrocatalysts assembled into a structure designed to optimize light harvesting and catalytic chemical transformations. Physical and chemical processes critical to PEC cells’ operation occur at the electrode-electrolyte interface. After plasmonic excitation of metal with incident light, the plasmon can quickly dephase through electronsurface scattering on a few femtosecond (fs) time scales, generating nonthermal electrons. Before non-thermal hot electrons lose energy to the surrounding medium, they can excite electronic or vibrational transitions in adsorbed molecules on the nanoparticle’s surface, thus enabling photocatalytic reactions.Under light irradiation, noble metal nanoparticles, particularly those characterized by localized surface plasmon resonance, commonly known as plasmonic nanoparticles, generate a strong electromagnetic field, excited hot carriers, and photothermal heating. Plasmonic nanoparticles enabling efficient absorption of light in the visible range have moderate catalytic activities. However, the catalytic performance of a plasmonic nanoparticle can be significantly enhanced by incorporating a highly catalytically active metal domain onto its surface. In this study, we demonstrate that femtosecond laser-induced atomic redistribution of metal domains in bimetallic Au-Pd nanorods (NRs) can enhance its photocurrent response by two-fold compared to parent Au-Pd NRs. We induce structure changes on Au-Pd NRs by irradiating them with femtosecond pulsed laser at 808 nm to precisely redistribute Pd atoms on AuNRs surfaces, resulting in modified electronic and optical properties thereby an enhanced catalytic activity. We also investigate the trade-off between the effect of light absorption and catalytic activity by optimizing the structure and composition of bimetallic Au-Pd nanoparticles. This work provides insight into the design of hybrid plasmonic-catalytic nanostructures with well-tailored geometry, composition, and structure for solar-fuel-based applications.This material is based upon work supported by the National Science Foundation under Grant No. 1904351. Figure 1

On the wave instability of multi component electron acoustic plasma

The stability and instability properties of electron acoustic (EA) waves which was recorded during Viking orbit 1188 in the dayside auroral zone have been theoretically investigated in this manuscript. The system under consideration is a magnetized collisionless plasma consisting of cold electron fluid, non-thermal hot electrons, electron beam and a stationary ions. The reductive perturbation theory have been used to get Zakharov–Kuznetsov (ZK) equation, obliquely propagating solitary wave solution have been derived. Nonthermality population parameter μ, densities ratios (α, β), temperatures ratios (θ, σ) and obliqueness angle ϑ affect the amplitude and width of EA solitary waves. The small k perturbation technique have been applied to ZK equation to study instability criterion in this system and finally the growth rate of the predicted instability dependence on the plasma parameters have been introduced, it was shown that plsma parameters σ, α, μ, β and the direction cosine (ι ζ , ι η ) affects the instability growth rate Γ.

Estimate of pre-thermal quench non-thermal electron density profile during Ar pellet shutdowns of low-density target plasmas in DIII-D

The radial density profile of pre-thermal quench (pre-TQ) early-time non-thermal (hot) electrons is estimated by combining electron cyclotron emission and soft x-ray data during the rapid shutdown of low-density (ne≲1019 m−3) DIII-D target plasmas with cryogenic argon pellet injection. This technique is mostly limited in these experiments to the pre-TQ phase and quickly loses validity during the TQ. Two different cases are studied: a high (10 keV) temperature target and a low (4 keV) temperature target. The results indicate that early-time, low-energy (∼10 keV) hot electrons form ahead of the argon pellet as it enters the plasma, affecting the pellet ablation rate; it is hypothesized that this may be caused by rapid cross field transport of argon ions ahead of the pellet or by rapid cross field transport of hot electrons. Fokker–Planck modeling of the two shots suggests that the hot electron current is quite significant during the pre-TQ phase (up to 50% of the total current). Comparison between modeled pre-TQ hot electron current and post-TQ hot electron current inferred from avalanche theory suggests that hot electron current increases during the high-temperature target TQ but decreases during the low-temperature target TQ. The uncertainties in this estimate are large; however, if true, this suggests that TQ radial loss of hot electron current could be larger than previously estimated in DIII-D.

Arbitrary amplitude ion acoustic solitons, double layers and supersolitons in a collisionless magnetized plasma consisting of non-thermal and isothermal electrons

Abstract Using Sagdeev pseudo-potential technique, we have studied the arbitrary amplitude ion acoustic solitons, double layers and supersolitons in a collisionless plasma consisting of adiabatic warm ions, non-thermal hot electrons and isothermal cold electrons immersed in an external uniform static magnetic field. We have used the phase portraits of the dynamical system describing the non-linear behaviour of ion acoustic waves to confirm the existence of different solitary structures. We have found that the system supports (a) positive potential solitons, (b) negative potential solitons, (c) coexistence of both positive and negative potential solitons, (d) negative potential double layers, (e) negative potential supersolitons and (f) positive potential supersolitons. Again, we have seen that the amplitude of the positive potential solitons decreases or increases with increasing n ch according to whether 0 < n c h < n c h ( c ) $0{< }{n}_{ch}{< }{n}_{ch}^{\left(c\right)}$ or n c h ( c ) < n c h ≤ 1 ${n}_{ch}^{\left(c\right)}{< }{n}_{ch}\le 1$ , where n c h ${n}_{ch}$ is the ratio of isothermal cold and non-thermal hot electron number densities, and n c h ( c ) ${n}_{ch}^{\left(c\right)}$ is a critical value of n ch . Also, we have seen that the amplitude of the positive potential solitons decreases with increasing β e , where β e is the non-thermal parameter. We have also investigated the transition of different negative potential solitary structures: negative potential soliton (before the formation of negative potential double layer) → negative potential double layer → negative potential supersoliton → negative potential soliton (after the formation of negative potential double layer) by considering the variation of θ only, where θ is angle between the direction of the external uniform static magnetic field and the direction of propagation of the ion acoustic wave.

Observation of non-thermal electron formation during the thermal quench of shattered pellet injection shutdowns in DIII-D

Formation of non-thermal (hot) electrons is studied during the thermal quench of shattered pellet injection (SPI) discharge shutdowns in the DIII-D tokamak. Both pure neon and mixed neon/deuterium shutdowns are studied. High-energy radiation (SXR and EUV) brightness levels show indications of non-thermal electron formation during the TQ. The non-thermal densities are of order 1018 m−3 and non-thermal temperatures are estimated to be of order 10 keV. The non-thermal electrons appear to be mostly lost during the subsequent current quench, as no signal of relativistic runaway electrons is typically seen at the end of the current quench. The non-thermals appear to have an effect on the deposition profile of the SPI pellets, especially at the end of the TQ, due to increased pellet ablation.

Two-color laser-plasma interactions for efficient production of non-thermal hot electrons

Efficient production of non-thermal hot electrons has been demonstrated experimentally by interactions of a plasma and two-color lasers with wavelengths of 527 and 1053 nm. The two-color-lasers and plasma interactions caused a 4.8 times enhancement of the temperature of the hot electrons and a 2.7 times enhancement of energy conversion from the lasers to the hot electrons compared with that obtained with only a 527 nm laser. These experimental results can be explained by a staging electron acceleration mechanism realized with a mixture of plasma waves excited by the two-color lasers. The primary 527 nm laser excites at least two electron plasma waves. Those have completely different phase velocities, therefore, staging acceleration cannot occur only with the 527 nm laser. The secondary 1053 nm laser excites other electron plasma waves, of which the phase velocities locate between those of the waves excited by the primary 527 nm laser. Some of the background thermal electrons can be heated contentiously to several hundred kilo-electronvolts by the electron plasma waves excited by the two-color lasers.

Effect of Landau damping on ion acoustic solitary waves in a collisionless unmagnetized plasma consisting of nonthermal and isothermal electrons

We have used the Sagdeev pseudo-potential technique to investigate the arbitrary amplitude ion acoustic solitons, double layers and supersolitons in a collisionless magnetized plasma consisting of adiabatic warm ions, isothermal cold electrons and nonthermal hot electrons immersed in an external uniform static magnetic field. We have used the phase portraits of the dynamical system describing the nonlinear behaviour of ion acoustic waves to confirm the existence of different solitary structures. We have also investigated the transition of different solitary structures: soliton (before the formation of double layer) $\rightarrow$ double layer $\rightarrow$ supersoliton $\rightarrow$ soliton (soliton after the formation of double layer) by considering the variation of $\theta$ only, where $\theta$ is the angle between the direction of the external uniform static magnetic field and the direction of propagation of the wave.

Bifurcation of ion-acoustic superperiodic waves in auroral zone of Earth’s magnetosphere

Ion-acoustic (IA) superperiodic waves are explored in an unmagnetized plasma composing of mobile ions, Maxwell distributed cold electrons and non-thermal hot electrons. Implementing the phase portrait concept of the Hamiltonian systems, all distinct phase plots containing superperiodic and superhomoclinic trajectories are displayed for distinct data values of the physical parameters, such as, nonthermal parameter ($\beta $), number density ratio ($\mu $) of cold electrons and ions at equilibrium, temperature ratio ($\sigma $) of cold electrons to hot electrons and of traveling wave velocity ($v$). The IA superperiodic wave features are examined for various conditions of physical parameters by symbolic computations. The outcomes of this study may be beneficial in discerning IA superperiodic wave in auroral zone of the Earth’s magnetosphere.

Ultrafast hot-electron induced quenching of Tb 4f magnetic order

We have investigated ultrafast quenching of the Tb4f magnetic order in Co74Tb26 alloys, induced by femtosecond hot-electrons pulses. The hot-electron pulses were produced in specific non-magnetic capping layers by infrared fs laser pulses. Our experimental results show that sub-picosecond dynamics of Tb4f magnetic moments can be induced by non-thermal and thermal hot-electrons. We further demonstrate that the demagnetization efficiencies of non-thermal and thermal hot electrons are similar. However, the characteristic demagnetization times show values of 0.35 ps for non-thermal hot-electrons excitations and 1.2 ps for thermal hot-electrons excitations. We explain this temporal elongation by the propagation time of thermal hot-electrons through the 15 nm thick CoTb film.

Ion-acoustic supersolitons and double layers in plasmas with nonthermal electrons

Supersoliton (SS) can be mainly featured in two ways, namely, by focusing on subsidiary maxima on its electric field or by meeting the requirement that the appropriate Sagdeev pseudopotential (SP) has three local extrema between the equilibrium conditions and its amplitude. In this paper, by using the SP method, double layers and ion-acoustic SSs are studied in a plasma with Maxwellian cold electrons, nonthermal hot electrons, and fluid ions. The existence of the SS regime in parameter space is obtained in a methodical fashion. The existence domains for positive solitary waves are also presented. It is found that there is no SSs at the acoustic speed.

Time‐Resolved Pump–Probe Measurement of Optical Rotatory Dispersion in Chiral Metamaterial

A plasmonic chiral metamaterial is fabricated from a thin Au film and exhibits static optical rotatory power (ORP) in the visible spectral range. Transient ORP is measured to clarify the temporal development of ORP using a circularly polarized light (CPL) pump beam. Three distinct transient behaviors of ORP are identified, resulting from different energy relaxation processes of hot electrons that occur during a period of a few picoseconds after pumping. Nonthermal hot electrons experience Lorentz force from an inverse Faraday effect and electron–boundary scattering, yielding a pump beam CPL helicity‐dependent transient ORP. Once hot electrons are in thermal equilibrium with the lattice, electron energy is distributed among the occupied states, as described by Fermi–Dirac statistics. Moreover, the transient ORP is independent of pump beam CPL helicity, well explained by the selection rule of electron excitation and two‐temperature model of the electron cooling process. Theoretical analysis of the transient ORP in terms of the energy relaxation of thermal hot electrons is carried out by introducing a temperature‐dependent dielectric function and finite‐difference time‐domain simulation. It is found that the magnitude of ORP at an elevated temperature is reduced to less than that at room temperature, agreeing well with the experimental observation.

Modulated 3D electron-acoustic rogue waves in magnetized plasma with nonthermal electrons

Modulational instability analysis of Electron-acoustic (EA) rogue waves is carried out in a three-dimensional magnetized plasma holding two electron populations (nonthermal hot electrons following Cairns distribution and cold electron fluid) and stationary ions. A \((3+1)\)-dimensional nonlinear Schrodinger (NLS) equation that governs the evolution of the EA rogue waves in the current plasma system is derived through derivative expansion method. The existence domains for the first and second order rogue waves are investigated. Effects of the plasma parameters on the propagation of the first and second order rogue waves are examined analytically and numerically. Furthermore, the role of the different plasma parameters as well as the critical threshold of the obliqueness \(\theta \) on the first and second order rogue waves for parameters regimes corresponding to dayside auroral zone plasma are emphasized.

Reducing the photocatalysis induced by hot electrons of plasmonic nanoparticles due to tradeoff of photothermal heating.

Silver nanotetrahedron (AgNT) two-dimensional arrays on a glass substrate generate nonthermal hot electrons, which induced photocatalytic oxidation of 4,4-dimercaptoazobenzene (DMAB) adsorbed onto its surface. The rate of the photocatalytic oxidation of DMAB is found to decrease upon increasing the intensity of the excitation light, unlike traditional photocatalysis reactions. The amount of 4-nitrothiophenol (4-NTP) resulting from the photocatalytic oxidation of DMAB is lower than expected, suggesting the partial desorption of DMAB or 4-NTP molecules from the surface of the AgNTs. However, the hot electrons remaining from the photocatalysis reaction thermalize and transfer their energy to the nanoparticle lattices, generating heat. The photothermal heat is sufficient to dissociate the Ag-S bond causing desorption of both DMAB and 4-NTP.

Three dimensional clyindrical Kadomtsev Petviashvili equation in two temperature charged dusty plasma

The properties of solitary waves in an unmagnetized, collisionless dusty plasma consisting of nonthermal ions, cold and hot dust grains and Maxwellian electrons have been investigated. Under a suitable coordinate transformation, the three-dimensional cylindrical Kadomtsev-Petviashvili (3D-CKP) equation is obtained. The effect of the nonthermal parameter, the negative charge number of hot and cold dust on the solitary properties are investigated. Furthermore, the solitary profile in the radial, axial, and polar angle coordinates with the time is examined. The present investigation may be applicable in space plasma such as F-ring of Saturn.

Electron acoustic solitary waves in a magnetized plasma with nonthermal electrons and an electron beam

A theoretical investigation is carried out to study the obliquely propagating electron acoustic solitary waves having nonthermal hot electrons, cold and beam electrons, and ions in a magnetized plasma. We have employed reductive perturbation theory to derive the Korteweg-de-Vries-Zakharov-Kuznetsov (KdV-ZK) equation describing the nonlinear evolution of these waves. The two-dimensional plane wave solution of KdV-ZK equation is analyzed to study the effects of nonthermal and beam electrons on the characteristics of the solitons. Theoretical results predict negative potential solitary structures. We emphasize that the inclusion of finite temperature effects reduces the soliton amplitudes and the width of the solitons increases by an increase in the obliquity of the wave propagation. The numerical analysis is presented for the parameters corresponding to the observations of “burst a” event by Viking satellite on the auroral field lines.

Efficiency of intramolecular electron transfer from the second excited state of the donor in molecular triads D–A1–A2

It is found that intramolecular and intermolecular electron transfer from the second singlet excited state of the donor in all molecular dyads studied up to now is accompanied by ultrafast recombination into the first excited state, resulting in a low quantum yield of the thermalized state with separated charges. The ultrafast photoinduced intramolecular charge transfer in donor‒acceptor 1‒acceptor 2 molecular triads is studied to ascertain the possibilities of increasing the quantum yield of ionic state. It is demonstrated that nonthermal (hot) electron transfer from the primary acceptor to the secondary acceptor can, in parallel with relaxation of a polar solvent, efficiently suppress the ultrafast recombination of charges into the first excited state of the donor and increase the yield of the ionic state. It is established that the angle between the directions of reaction coordinates corresponding to the electron transfer from the donor to the primary acceptor and from the primary acceptor to the secondary acceptor play the most important role in describing these processes. It is concluded that the value of this angle is governed by the ratio between the reorganization energies of the three possible electron transfers in the triad and can vary within wide limits. The parametric regions with maximum quantum yield of the thermalized ionic state are revealed. The strong effect the geometry of a studied triad has on charge separation efficiency is observed.