Electronic Relaxation Effects Research Articles

Electron and hole relaxation effect on power conversion efficiency of selective energy contacts solar cells

Electron and hole relaxation effect on power conversion efficiency of selective energy contacts solar cells

Charge transfer and electronic relaxation effects in the photoemission of EMIM-DCA ionic liquid vapor

Ultraviolet photoelectron spectroscopy (UPS) measurements of EMIM-DCA vapors have been conducted. Density functional theory (DFT), Møller–Plesset perturbation theory (MP2) and many-body perturbation theory calculations (GW) have been used to interpret the experimentally measured spectra. The vapor of ionic liquids normally consists of isolated ion-pairs and the corresponding photoemission spectra should reflect the electronic binding energies of the molecular orbitals. The UPS spectra have revealed that many photolines are shifted when compared to ab initio calculation results. The hybrid DFT functionals, which performed very good in the description of the UPS spectra of many other ionic liquid vapors have shortcomings in the EMIM-DCA case. The MP2 calculation and the quasiparticle GW approach are shown to offer a better description of the experimental UPS spectra. The charge distribution analysis showed that there seems to be a favorable charge transfer from the cation to the anion upon ionization. The new data provides strong support to the argument that in some cases DFT still has difficulties in the modeling of IL ion-pairs.

Specific chemical bond relaxation unraveled by analysis of shake-up satellites in the oxygen single site double core hole spectrum of CO2

Important electronic relaxation and reorganization effects induced by single site double core hole formation and associated shake-up excitation are investigated in CO2 by means of an highly accurate computation method using non orthogonal basis set. Connection between the two main satellite bands and the two (non equivalent) carbon oxygen bonds are highlighted.

A correlation-relaxation-balanced direct method at the second order perturbation theory for accurate ionization potential predictions.

The development of a quasi-particle approach for an accurate yet efficient prediction of a complete ionization potential (IP) spectrum, from valence down to core electrons for the system of interest, is a long-cherished goal in quantum chemistry. Based on the physical understanding of the electron correlation and relaxation effects at the second order perturbation theory, we present here a correlation-relaxation-balanced direct method, dubbed CRB-MP2, via a parameter scaled scheme of the 2ph (two-particle, one-hole summation) and 2hp (two-hole, one-particle summation) terms. With almost no extra computational cost after a normal MP2 procedure, the CRB-MP2 method yields high quality valence and core IPs for a wide range of species. A direct approach for complete IP spectrum calculations with both computational accuracy and efficiency is therefore established.

Photoexcited Carrier Dynamics of In2S3 Thin Films.

Indium sulfide (In2S3) is a promising absorber base for substitutionally doped intermediate band photovoltaics (IBPV); however, the dynamics of charge carriers traversing the electronic density of states that determine the optical and electronic response of thin films under stimuli have yet to be explored. The kinetics of photophysical processes in In2S3 grown by oxygen-free atomic layer deposition are deduced from photoconductivity, photoluminescence (PL), and transient absorption spectroscopy. We develop a map of excited-state dynamics for polycrystalline thin films including a secondary conduction band ∼2.1 eV above the first, plus sulfur vacancy and indium interstitial defect levels resulting in long-lived (∼100 ns) transients. Band-edge recombination produces PL and stimulated emission, which both intensify and red-shift as deposition temperature and grain size increase. The effect of rapid conduction band electron relaxation (<30 ps) and deep defect levels on IBPV employing In2S3-based absorbers is finally considered.

Understanding the modulation mechanism in resonance-enhanced multiphoton probing of molecular dynamics

Time-resolved spectroscopy on isolated molecules gives fundamental insight into the conversion of light energy to other degrees of freedom. Probing of the photoinduced dynamics can be accomplished by ionization, via a single-photon or multiphoton transition. In this Rapid Communication we directly contrast transient spectra on the molecule perylene obtained with multiphoton ionization (MPI) to single-photon ionization (SPI). The photoinduced nuclear geometry relaxation modulates the MPI transient with a decay time constant of $0.9\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.28em}{0ex}}\mathrm{ps}$. In contrast, the SPI transient completely lacks any indication for relaxation. We attribute this difference to a change in resonance enhancement of the MPI probe as the molecular geometry changes. Our results underline the importance of a detailed knowledge about these resonances for a proper interpretation of transient signals of molecular dynamics subject to nuclear and electronic relaxation effects. At the same time, the direct comparison to SPI directly demonstrates the higher sensitivity of resonance-enhanced MPI as a probe in time-resolved dynamical studies.

Reduction of the effect of electron relaxation behavior on the femtosecond laser-induced response of copper thin film by ballistic energy transfer

The thermomechanical response of a copper thin film to ultrafast laser with the pulse durations from 10 to 100 fs is investigated by the methods integrating the parabolic and hyperbolic two-temperature models with the molecular dynamics. It is found that there is an insignificant discrepancy in the results obtained by the two different two-temperature models for all the considered pulse durations, indicating the trivial effect of electron relaxation behavior on the response of the heated copper film. Further study suggests that the electron relaxation effect on the material response could be offset by the energy transport due to the ballistic electron motion.

Two-photon echo method for observing electron zitterbewegung in carbon nanotubes

The phenomenon of zitterbewegung (ZB, trembling motion) of electrons is described in zigzag carbon nanotubes (CNT) excited by laser pulses. The tight binding approach is used for the band structure of CNT and the effect of light is introduced by the vector potential. Contrary to the common theoretical practice, no a priori assumptions are made concerning electron wave packet; the latter is determined as a result of illumination. In order to overcome the problem of various electron phases in ZB, a method of two-photon echo (2PE) is considered and described using the density function formalism. The medium polarization of CNT is calculated by computing exact solutions of the time-dependent electron Hamiltonian. The signal of 2PE is extracted and it is shown that, using existing parameters of CNT and laser pulses, one should be able to observe the electron trembling motion. Effects of electron decoherence and relaxation are discussed.

On the trends of Fukui potential and hardness potential derivatives in isolated atoms vs. atoms in molecules.

In the present study, trends of electronic contribution to molecular electrostatic potential [Vel(r¯)(r=0)], Fukui potential [v(+)f|(r=0) and v(-)f|(r=0)] and hardness potential derivatives [Δ(+)h(k) and Δ(-)h(k)] for isolated atoms as well as atoms in molecules are investigated. The generated numerical values of these three reactivity descriptors in these two electronically different situations are critically analyzed through the relevant formalism. Values of Vel(r¯) (when r → 0, i.e., on the nucleus) are higher for atoms in molecules than that of isolated atoms. In contrast, higher values of v(+)|(r=0) and v(-)|(r=0) are observed for isolated atoms compared to the values for atoms in a molecule. However, no such regular trend is observed for the Δ(+)h(k) and Δ(-)h(k) values, which is attributed to the uncertainty in the Fukui function values of atoms in molecules. The sum of Fukui potential and the sum of hardness potential derivatives in molecules are also critically analyzed, which shows the efficacy of orbital relaxation effects in quantifying the values of these parameters. The chemical consequence of the observed trends of these descriptors in interpreting electron delocalization, electronic relaxation and non-negativity of atomic Fukui function indices is also touched upon. Several commonly used molecules containing carbon as well as heteroatoms are chosen to make the investigation more insightful.

Probing the Redox States at the Surface of Electroactive Nanoporous NiO Thin Films

Nanoporous NiO thin film electrodes were obtained via plasma-assisted microwave sintering and characterized by means of a combination of electrochemical techniques and X-ray photoelectron spectroscopy (XPS). The aim of this study is the elucidation of the nature of the surface changes introduced by the redox processes of this nanostructured material. NiO undergoes two distinct electrochemical processes of oxidation in aqueous electrolyte with the progress of NiO anodic polarization. These findings are consistent with the sequential formation of oxyhydroxide species at the surface, the chemical nature of which was assessed by XPS. Electronic relaxation effects in the Ni 2p spectra clearly indicated that the superficial oxyhydroxide species resulted to be β-NiOOH and γ-NiOOH. We also show for the first time spectral evidence of an electrochemically generated Ni(IV) species. This study has direct relevance for those applications in which NiO electrodes are utilized in aqueous electrolyte, namely catalytic water splitting or electrochromism, and may constitute a starting point for the comprehension of electronic phenomena at the NiO/organic electrolyte interface of cathodic dye-sensitized solar cells (p-DSCs).

Electron relaxation effect on the sub-100-fs laser interaction with gold thin film

The heating of a gold thin film by a single 10 fs laser pulse is modeled by a combined continuum-atomistic method considering the electron relaxation effect. Numerical results show that the temperature evolution and stress propagation proceed in the same manners as those for the subpicosecond laser irradiation. It is also found that the electron relaxation effect is insignificant and could be considerably overestimated by neglecting the ballistic energy transfer in the film.

Valence one-electron and shake-up ionization bands of fluorene, carbazole and dibenzofuran

A comprehensive study of the He (I) ultra-violet photoelectron spectra of fluorene, carbazole and dibenzofuran is presented with the aid of one-particle Green’s Function calculations employing the outer-valence Green’s Function (OVGF) approach and the third-order algebraic diagrammatic construction [ADC(3)] scheme, along with Dunning’s correlation consistent basis sets of double and triple zeta quality (cc-pVDZ, cc-pVTZ). Extrapolations of the ADC(3) results for the outermost one-electron π-ionization energies to the cc-pVTZ basis set enable theoretical insights into He (I) measurements within ∼0.15eV accuracy, up to the σ-ionization onset. The lower ionization energy of carbazole is the combined result of mesomeric and electronic relaxation effects. OVGF/cc-pVDZ or OVGF/cc-pVTZ pole strengths smaller than 0.85 systematically corroborate a breakdown of the orbital picture of ionization at the ADC(3) level. Comparison is made with calculations of the lowest doublet–doublet excitation energies of the radical cation of fluorene, by means of time-dependent density functional theory (TDDFT).

Atomistic modeling of electron relaxation effect on femtosecond laser-induced thermoelastic response of gold films

The thermoelastic response of thin gold films induced by femtosecond laser irradiation is numerically simulated using a modified combined two-temperature model (TTM) and molecular dynamics (MD) method, with focus placed upon the influence of the electron relaxation effect. The validity of the numerical approach is checked against existing experimental results. While the electron relaxation effect is found negligible when the laser duration is much longer than the electron thermal relaxation time, it becomes significant if the laser duration matches the electron relaxation time, especially when the former is much shorter than the latter. The characteristics of thermo-mechanical interaction in the thin film are analyzed, and the influence of temperature-dependent material properties upon the thermoelastic response of the film quantified.

Unified analysis for calculating the amplification gain of Cerenkov laser in the single-particle and collective regimes

In this paper, we introduce a unified analytical model to describe both the single-electron and the collective modes of interaction in the Cerenkov laser. Although the basic formulations of the model are based on the single-electron interaction with the electromagnetic wave, the characteristics of the wave-wave interaction associated with the collective model are also determined. The transition regime between the single-electron and the collective models is explored. The electron relaxation effect caused by the asymmetric Coulomb forces among the electrons is taken into account. The essential results of this paper agree with those predicted by previous theories at the limits of the single-electron and collective.

Energies, Auger and radiative probabilities of the doubly-excited 1s23l3l′ states for Be-like neon

Energy levels, Auger decay and radiative transition rates of the doubly-excited 1s23l3l′ states for Be-like neon are calculated using relativistic Multiconfiguration Dirac-Fock (MCDF) method. Electron correlation and relaxation effects are taken into account. The magnitude of Auger rates for the 35 doubly-excited states and their decay channels are discussed. Present calculated values are in agreement with the latest experimental electron spectra as well as other theoretical results.

Quantum characteristics of stimulated Cherenkov radiation in dielectric-lined waveguide operating at optical wavelengths

Abstract In this paper, a quantum mechanical model is proposed to describe the basic features of stimulated Cherenkov radiation in the small-signal low-gain regime. In this model, the electron is described by a wavepacket with finite spreading length and the electron wave function is a solution of the Schrodinger equation. We show that the quantum effects are manifested when the spreading length of the electron wave is much longer than the electromagnetic (EM) wavelength such as in the optical wavelength range. The effect of electron relaxation due to Coulomb's collisions with neighboring electrons is introduced to characterize the damping of the vibration of the electron wave with time. When the relaxation effect is neglected, we prove that our essential results match with other classical and quantum approaches based on different theoretical concepts.

Current Excitation Model for Cerenkov Lasers with a Planar Waveguide

A unified theoretical analysis of the spontaneous and stimulated emissions in Cerenkov free-electron lasers (CFELs) with a planar dielectric waveguide is shown. The excitation of the electromagnetic (EM) wave is formulated with current sources of the electron beam. A relativistic classical analysis is applied to this model, where the electron is represented as a spatially localized point particle. The spontaneous emission is evaluated through spatially varying field components associated with the motion of an electron. The stimulated emission is derived on the basis of the modulations of electron velocity and density by the EM field. Interaction regimes induced by the stimulated emission are classified into transition and steady states with the time variation. The effect of electron relaxation is introduced to explain the operation in the steady state. The compatibility of this analysis with already known analyses is also confirmed.

ChemInform Abstract: Variable Photon Energy Photoelectron Spectroscopy of OsO4 and Pseudopotential Calculations of the Valence Ionization Energies of OsO4 and RuO4.

Relative partial photoionization cross sections and photoelectron branching ratios have been obtained for the valence bands of osmium tetraoxide in the ionization energy range 12-18 eV. The photon energies used ranged between 24 and 115 eV. The ionization cross sections of the 2t2, 1e, and 2a1 orbitals show evidence of substantial metal character. By use of an ab initio many-body Green's function formalism that takes into account the effect of electron correlation and relaxation, ionization energies of OsO4 and RuO4 have been calculated. Considerations of the spectral features and the predictions of the calculation lead to an assignment of ion state ordering of 2T1 < 2T2 < 2A1 < 2E < 2T2. The presence of a substantial p-d resonance feature in the cross section of the upper 2T2 and 2E ion states shows that the 2t2 and le orbitals have significant Os 5d character. The lower 2T2 ion state (arising from ionization from the 3t2 orbital) undergoes a spin-orbit splitting of 0.4 eV resulting from an Os 6p contribution to the 3t2 molecular orbital.

Transparency of a thin absorber in Moessbauer optics: Effect of electron relaxation

A model of Moessbauer absorption taking into account the effects of the electron relaxation and the nuclear levels anticrossing simultaneously has been proposed. The dependence of the absorption deficit (transparency) on the relaxation and mixing parameters has been obtained using the stochastic theory of Moessbauer relaxation spectra. The role of quantum interference in occurrence of a partial transparency on Moessbauer transitions has been explored under these conditions.

Full-electron calculation of effective electronic couplings and excitation energies of charge transfer states: Application to hole transfer in DNA pi-stacks.

In this work I develop and apply a theoretical method for calculating effective electronic couplings (or transfer integrals) between redox sites involved in hole or electron transfer reactions. The resulting methodology is a refinement and a generalization of a recently developed approach for transfer integral evaluation. In fact, it holds for any overlap between the charge-localized states used to represent charge transfer (CT) processes in the two-state model. The presented theoretical and computational analyses show that the prototype approach is recovered for sufficiently small overlaps. The method does not involve any empirical parameter. It allows a complete multielectron description, therefore including electronic relaxation effects. Furthermore, its theoretical formulation holds at any value of the given reaction coordinate and yields a formula for the evaluation of the vertical excitation energy (i.e., the energy difference between the adiabatic ground and first-excited electronic states) that rests on the same physical quantities used in transfer integral calculation. In this paper the theoretical approach is applied to CT in B-DNA base dimers within the framework of Density Functional Theory (DFT), although it can be implemented in other computational schemes. The results of this work, as compared with previous Hartree-Fock (HF) and post-HF evaluations, support the applicability of the current implementation of the method to larger pi-stacked arrays, where post-HF approaches are computationally unfeasible.