One-electron Excitations Research Articles

Molecular modification effects on the electrochromic and photochromic properties of diarylethene with intramolecular isomerization behavior.

Diarylethene (DAE) is one of the most widely used functional units for electrochromic or photochromic materials. To better understand the molecular modification effects on the electrochromic and photochromic properties of DAE, two modification strategies, substitution with functional groups or heteroatoms, were investigated theoretically by density functional theory calculations. It is found that red-shifted absorption spectra caused by a decreased highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap and S0 → S1 transition energy during the ring-closing reaction become more significant by adding different functional substituents. In addition, for two isomers, the energy gap and S0 → S1 transition energy decreased by heteroatom substitution of S atoms with O or NH, while they increased by replacing two S atoms with CH2. For intramolecular isomerization, one-electron excitation is the most effective way to trigger the closed-ring (O → C) reaction, while the open-ring (C → O) reaction occurs most readily in the presence of one-electron reduction. Moreover, it is confirmed that substitution with strong electron donating groups (-OCH3/-NH2) or with one O/two CH2 heteroatoms leads to a more favorable closed-ring (O → C) reaction. Functionalized with strong electron-withdrawing groups (-NO2 and -COOH) or one/two NH heteroatom substitutions, the open-ring (C → O) reaction is easier. Our results confirmed that the photochromic and electrochromic properties of DAE can be tuned effectively by molecular modifications, which provides theoretical guidance for the design of new DAE-based photochromic/electrochromic materials.

Dielectric screening versus geometry deformation in two-dimensional allotropes of silicon and germanium

The search for connections between electronic and structural features is a key factor in the synthesis of artificial materials for on-demand applications, with graphene and analogous elemental semimetals playing a distinguished role as building blocks of photonic and plasmonic systems. In particular, a diversity of arrangements and electronic-state dispersions is offered by currently synthesized two-dimensional allotropes of silicon and germanium, respectively known as silicene and germanene. These monolayers make the ideal playground to understand how their collective and single-particle electronic states, excited by electron or light beams, may be controlled by geometry rather than doping or gating. Here, we provide such a study using time-dependent density-functional theory, in the random-phase approximation, to identify the structural dependent properties of charge-density plasmon oscillations and optical absorption in flat to buckled silicene and germanene lattices. We further single out flat germanene as an unprecedented two-dimensional conductor, hosting Dirac cone fermions in parallel with metal-like charge carriers, which contribute to strong intraband plasmon modes and one-electron excitations in the far-infrared limit. Finally, we show how this atypical scenario can be tuned by external stress or strain.

Tip-induced β -hydrogen dissociation in an alkyl group bound on Si(001)

Atomic-scale chemical modification of surface-adsorbed ethyl groups on Si(001) was induced and studied by means of scanning tunneling microscopy. Tunneling at sample bias >+1.5 V leads to tip-induced C–H cleavage of a β-hydrogen of the covalently bound ethyl configuration. The reaction is characterized by the formation of an additional Si–H and a Si–C bond. The reaction probability shows a linear dependence on the tunneling current at 300 K; the reaction is largely suppressed at 50 K. The observed tip-induced surface reaction at room temperature is thus attributed to a one-electron excitation in combination with thermal activation.

Dynamics of free electron wave packets with coherent extreme ultraviolet four-wave mixing fields

We discuss the generation of cascaded extreme ultraviolet four-wave mixing fields with a single-gas-cell configuration and use them to study the dynamics of free electron wave packets of the noble gases. The temporal evolution of atomic dipoles in the vicinity of one-electron excitation Rydberg states 3s3p6np of argon and 4s4p6n′p of krypton is reflected in the intensity profiles of the four-wave mixing spectra. Different interference pathways for these dipole-allowed transitions are revealed in the time-delay-dependent phase shift of the four-wave mixing fields. The results of this study may be promising for studies of more complex systems.

Diboron- and Diaza-Doped Anthracenes and Phenanthrenes: Their Electronic Structures for Being Singlet Fission Chromophores.

We used quantum chemistry methods at the levels of mixed-reference spin-flip time-dependent density functional theory and multireference perturbation theory to study diboron- and diaza-doped anthracenes and phenanthrenes. This class of structures recently surged as potential singlet fission chromophores. We studied electronic structures of their excited states and clarified the reasons why they satisfy or fail to satisfy the energy criteria for singlet fission chromophores. Many studied structures have their S1 states not dominated by HOMO → LUMO excitation, so they cannot be described using the conventional two site model. This is attributed to frontier orbital energy shifts induced by the doping and different charge-transfer energies in different one-electron singlet excitations or, in other words, different polarizations of hole and/or particle orbitals in their S1 and T1 states. There is a mirror relation between the orbital energy shifts induced by diboron- and diaza-dopings, which together with alternant hydrocarbon pairings of occupied and unoccupied orbitals, leads to more mirror relations between the excited states of the two types of doped structures.

Charge-Transfer Excitation Energies Expressed as Orbital Energies of Kohn-Sham Density Functional Theory with Long-Range Corrected Functionals.

Previously proposed theoretical schemes for estimating one-electron excitation energies using Kohn-Sham (KS) solutions with long-range corrected (LC) functionals are applied to the charge-transfer (CT) excitations of the ethylene···tetrafluoroethylene (C2H4-C2F4) system, and the CT complex between an aromatic donor (Ar = benzene, toluene, o-xylene, naphthalene, anthracene, and various meso-substituted anthracenes) and the tetracyanoethylene (TCNE) acceptor. The CT excited state is described well as a single-electron excitation between specific orbitals of donor and acceptor. Thus, CT excitation energies are well approximated by the orbital energies because of the satisfaction of the Koopmans-type theorem and the asymptotic behavior of the LC functional. We have examined three computational schemes: scheme 1 employs the orbital energies for the neutral and cationic systems, scheme 2 utilizes orbital energies of just the cation, and in scheme 3, because the electron affinity of a molecule is the ionization energy of its anion, a scale factor is applied to enforce this identity. The present schemes reproduce the correct asymptotic behavior of CT excitation energy of C2H4···C2F4 for the long intermolecular distances and give good agreement with accurate ab initio results. Calculated CT excitation energies for Ar-TCNE are compared with those of TD-DFT and ΔSCF methods. Scheme 1 with the optimal range-separation parameter μ accurately reproduces CT excitation energies for all Ar-TCNE systems and gives good agreement with the best TD-DFT calculations and experiment. Scheme 1, scheme 3, and TD-DFT show similar tendencies with respect to the variation in μ. Scheme 2 and ΔSCF approaches are rather insensitive to changes in μ, but both considerably underestimate the CT excitation energies for these systems. KS orbital energies are physically meaningful and they are practically useful; if the range-separation parameter is tuned, then good results can be obtained.

Separation of electron-electron and electron-proton correlation in multicomponent orbital-optimized perturbation theory.

The multicomponent orbital-optimized second-order Møller-Plesset perturbation theory (OOMP2) method is the first multicomponent MP2 method that is able to calculate qualitatively accurate protonic densities, protonic affinities, and geometrical changes due to nuclear quantum effects in multicomponent systems. In this study, two approximations of the multicomponent OOMP2 method are introduced in an effort to demonstrate that, in orbital-optimized multicomponent methods, performing the orbital-optimization process with only electron-proton correlation is sufficient to obtain accurate protonic properties. Additionally, these approximations should reduce the computational expense of the multicomponent OOMP2 method. In the first approximation, the first-order wave function is written as a linear combination of one-electron one-proton excitations rather than as a linear combination of one-electron one-proton and two-electron excitations as in the original multicomponent OOMP2 method. Electron-electron correlation is included perturbatively after the orbital-optimization procedure has converged. In the second approach, the first approximation is further modified to neglect all terms in the orbital-rotation gradients that depend on the two-electron molecular-orbital integrals, which, assuming a fixed-sized protonic basis set, reduces the computational scaling for the orbital-optimization iterations to Ne 3, where Ne is a measure of the electronic system size, compared to the Ne 5 scaling of the original multicomponent OOMP2 method. The second approximation requires one Ne 5 step after orbital convergence to compute the electron-electron correlation energy. The accuracy of the calculated protonic densities, protonic affinities, and optimized geometries of these approximations is similar or improved relative to the original multicomponent OOMP2 method.

Excitation energies expressed as orbital energies of Kohn-Sham density functional theory with long-range corrected functionals.

A new simple and conceptual theoretical scheme is proposed for estimating one-electron excitation energies using Kohn-Sham (KS) solutions. One-electron transitions that are dominated by the promotion from one initially occupied orbital to one unoccupied orbital of a molecular system can be expressed in a two-step process, ionization, and electron attachment. KS with long-range corrected (LC) functionals satisfies Janak's theorem and LC total energy varies almost linearly as a function of its fractional occupation number between the integer electron points. Thus, LC reproduces ionization energies (IPs) and electron affinities (EAs) with high accuracy and one-electron excitation energies are expressed as the difference between the occupied orbital energy of a neutral molecule and the corresponding unoccupied orbital energy of its cation. Two such expressions can be used, with one employing the orbital energies for the neutral and cationic systems, while the other utilizes orbital energies of just the cation. Because the EA of a molecule is the IP of its anion, if we utilize this identity, the two expressions coincide and give the same excitation energies. Reasonable results are obtained for valence and core excitations using only orbital energies.

Ensemble Green's function theory for interacting electrons with degenerate ground states

An ensemble Green's function formalism, based on the von Neumann density matrix approach, to calculate one-electron excitation spectra of a many-electron system with degenerate ground states is proposed. A set of iterative equations for the ensemble Green's function and self-energy is derived and a simplest approximation corresponding to an ensemble $GW$ approximation is naturally obtained. The derivation is based on the Schwinger functional derivative technique and does not assume any adiabatic connection between a noninteracting and an interacting ground state.

Quantum Infrastructure of Attosecond Sensors and Actuators of Nonequilibrium Physical Media in Smart Materials

The advances in new smart materials are linked with attosecond subatomic technologies which can create entangled subatomic electron pairs by attosecond hard ultraviolet and soft X rays in addition to one-electron excitations by femtosecond optical pulses. The quantum infrastructure of nonequilibrium physical media in smart materials is provided by the Fermi and Bose gas of quasi-electron excitations, and the quantum infrastructure of their space-time scales is specified by the quantum mechanisms of two-electron attophysics and one-electron femtochemistry. The primary scale is subatomic (1.0 pm to 0.1 nm), and the next scale is supra-atomic (0.1 to 10.0 nm), being the scale of nanoelectromechanical systems of sensors and actuators that provide self-organization in the space-time hierarchy of nano-, micro-, meso-, and macroscopic dissipative structures of nonequilibrium physical media in smart materials. Here we show that any quantum nanoelectromechanical system can alternately be a sensor and an actuator of dissipative structures with a two-clock cycle: an attosecond sensor of entangled two-electron excitations and a femtosecond actuator of electromechanical motion modes in a nonequilibrium physical medium. For such nanoelectromechanical sensors-actuators, the motion rhythm is three orders of magnitude faster than for femtosecond nanomolecular sensors of one-electron excitations and nanomolecular actuators of vibrationrotation modes in nonequilibrium physical media.

Synthesis, experimental and computational spectroscopic investigations of third-order nonlinear optical material (E)-N′-(benzo[d][1,3]dioxol-5-ylmethylene)benzohydrazide

An organic nonlinear optical material (E)-N′-(benzo[d][1,3]dioxol-5-ylmethylene)benzohydrazide (BDMB) has been successfully synthesized and characterized using spectroscopic techniques. Vibrational analysis has been carried out with the help of potential energy distributions to identify the functional groups. The 13C and 1H-NMR chemical shift values are in good agreement with the theoretical results. Intramolecular stabilization by donor–acceptor interactions could be witnessed through natural bond orbital analysis. The one-electron excitation from the conduction band to the valence band was expressed by π–π* transition at 324 nm. The energy gap (3.96 eV) is calculated from the optical spectrum which explains the charge transfer mechanism of BDMB. The hyperpolarizability of the BDMB molecule is 30.8538 × 10−30 esu, which is 82 times greater than that of urea (β0 = 0.3728 × 10−30 esu). The third-order nonlinear absorption coefficient (β) and nonlinear refractive index (n2) are calculated to be 6.24 × 10−5 cm W−1 and 1.19 × 10−9 cm2 W−1, respectively. Based on these points, BDMB can be projected with nonlinear properties for realizing new applications.

Spectra of correlated many-electron systems: From a one- to a two-particle description

State-of-the-art spectroscopic techniques allow for a comprehensive understanding of one-electron excitations in various physically interesting and/or technologically relevant materials. While for weakly-correlated systems the corresponding one-particle spectral function A(ω, k) contains essentially all information about their physical properties the situation is much more complicated in the presence of strong electronic correlations. In fact, in the latter case different theoretical treatments often lead to very different explanations of the origin of specific features in the spectrum. A typical example is the pseudogap in the cuprates, i.e., the momentum-selective suppression of spectral weight at the Fermi level, which has been related to spin, charge or (d-wave) pairing fluctuations by different authors. This ambiguity about the underlying physical mechanism at work can be overcome by considering two-particle correlation functions as they are able to describe the collective modes of the system and can be also related to certain ground state properties. In this work, we will present different theoretical approaches for analyzing the spectrum of correlated systems by exploiting the information contained in these two-particle correlation functions. For the specific case of the pseudogap these procedures have allowed us (1) to identify antiferromagnetic spin fluctuations as microscopic origin of this spectral feature which (2) can be related to the formation of a resonating valence bond ground state in the system.

Excitation Mechanisms in Moderate-Energy Li+–He Collisions

Excitation mechanisms in Li+–He collisions were studied at laboratory collision energies of 350 ≤ Elab ≤ 2000 eV by measuring double differential cross sections (DCSs) σ(Θ)k over a wide range of center-of-mass scattering angles, 2.5 ≤ Θ ≤ 175°. At Elab ≥ 500 eV, two-electron (2e) excitations were observed as well as one-electron (1e) excitations. At the higher collision energies, excitation probabilities P(Θ)k for the 1e and 2e excitations have characteristic angular dependences, i.e., at Elab = 1500 and 2000 eV, P(Θ)1e for the 1e excitations has double maxima around Θ = 20 and 120° and P(Θ)2e for the 2e excitations has a broad maximum around Θ = 60°. As a first analysis of the experimental data, P(Θ)k, σ(Θ)k, and the integral cross sections Sk(Elab) were calculated by assuming excitations from the 11Σ state into the 11Π and 11Δ states through rotational couplings using the model potentials and couplings. As the next step, ab initio potential energies for the ground and excited states were calculated by a...

Molecular Road Map to Tuning Ground State Absorption and Excited State Dynamics of Long-Wavelength Absorbers.

Realizing chromophores that simultaneously possess substantial near-infrared (NIR) absorptivity and long-lived, high-yield triplet excited states is vital for many optoelectronic applications, such as optical power limiting and triplet-triplet annihilation photon upconversion (TTA-UC). However, the energy gap law ensures such chromophores are rare, and molecular engineering of absorbers having such properties has proven challenging. Here, we present a versatile methodology to tackle this design issue by exploiting the ethyne-bridged (polypyridyl)metal(II) (M; M = Ru, Os)-(porphinato)metal(II) (PM'; M' = Zn, Pt, Pd) molecular architecture (M-(PM')n-M), wherein high-oscillator-strength NIR absorptivity up to 850 nm, near-unity intersystem crossing (ISC) quantum yields (ΦISC), and triplet excited-state (T1) lifetimes on the microseconds time scale are simultaneously realized. By varying the extent to which the atomic coefficients of heavy metal d orbitals contribute to the one-electron excitation configurations describing the initially prepared singlet and triplet excited-state wave functions, we (i) show that the relative magnitudes of fluorescence (k0F), S1 → S0 nonradiative decay (knr), S1 → T1 ISC (kISC), and T1 → S0 relaxation (kT1→S0) rate constants can be finely tuned in M-(PM')n-M compounds and (ii) demonstrate designs in which the kISC magnitude dominates singlet manifold relaxation dynamics but does not give rise to T1 → S0 conversion dynamics that short-circuit a microseconds time scale triplet lifetime. Notably, the NIR spectral domain absorptivities of M-(PM')n-M chromophores far exceed those of classic coordination complexes and organic materials possessing similarly high yields of triplet-state formation: in contrast to these benchmark materials, this work demonstrates that these M-(PM')n-M systems realize near unit ΦISC at extraordinarily modest S1-T1 energy gaps (∼0.25 eV). This study underscores the photophysical diversity of the M-(PM')n-M platform and presents a new library of long-wavelength absorbers that efficiently populate long-lived T1 states.

Systematic First-Principles Calculations of Charge Transfer Transitions of Rare Earth Ions in Oxides Using Simple Cubic Model Cluster

[Introduction] It is important for trap levels to capture carriers for persistent phosphors, and the properties depend on the positions of the trap levels within the band gap [1]. It is necessary to estimate the positions of impurity levels which work as trap levels for material design of the persistent phosphors. As one of the ways, the position of the impurity level within the band gap can be estimated by calculating the charge transfer transition energy from the valence band (Ligand to Metal Charge Transfer: LMCT). However, calculation of the LMCT based on quantum theory is exceedingly difficult in d electron system like transition metal and f electron system like rare earth ion because the calculation requires to consider many-electron effect, symmetry and relativistic effect such as spin-orbit interaction. Recently, we successfully reproduced the LMCT energies and their experimental trend of transition metal ions in α-Al2O3 by performing configuration interaction (CI) calculation using the DVME method [2]. From these results, we expect that the relativistic DVME method can be also applied to rare earth ions. In this study, as a first approach, in order to obtain the LMCT energy for various trivalent RE ions in REO8 13- cubic model cluster, we performed first-principles calculations using the DVME method and constructed the theoretical energy diagram of the rare earth ions, and compared it with the reported LMCT energies in YPO4 [3] and YAG [4]. [Computational Method] We constructed a 9-atom cluster consisting of the central RE ion and the 8 oxygen ions with Oh symmetry (cubic) and 2.4 Å bond length. We considered the ground state configuration 4f n 2p48 and one-electron excitation configuration 4f n +12p47, where n is the number of 4f electrons. In order to improve the computational accuracy, the configuration dependent correction (CDC) was considered. The LMCT energy was calculated as the transition energy from the top of the valence band corresponding to the highest one of the MOs mainly consisting of oxygen 2p orbital to the MO mainly consisting of RE 4f orbital based on the MO calculation using the DV-Xα method and the CI calculation using the DVME method. [Result] The theoretical LMCT energies calculated with CDC are shown as the diagram in Fig. 1 together with the diagram for the reported values in YPO4 and YAG. The figure shows that the reported trend, especially around Eu to Gd, was successfully reproduced by first-principles calculations. The overestimation for Ho to Tm is probably due to the insufficient consideration of electron correlation. [1] J. Ueda, et al,. Appl. Phys. Lett. 104, 101904, (2014). [2] K. Ogasawara, et al., Phys. Rev. B, 64, 115413, (2001). [3] E. Nakazawa, J. Lumin. 100, 89, (2002). [4] P. Dorenbos, J. Lumin. 134, 310, (2013). Figure 1

Electronic fine structure calculation of metal complexes with three-open-shell s, d, and p configurations.

The ligand field density functional theory (LFDFT) algorithm is extended to treat the electronic structure and properties of systems with three-open-shell electron configurations, exemplified in this work by the calculation of the core and semi-core 1s, 2s, and 3s one-electron excitations in compounds containing transition metal ions. The work presents a model to non-empirically resolve the multiplet energy levels arising from the three-open-shell systems of non-equivalent ns, 3d, and 4p electrons and to calculate the oscillator strengths corresponding to the electric-dipole 3d m →ns 13d m 4p 1 transitions, with n=1, 2, 3 and m=0, 1, 2, …, 10 involved in the s electron excitation process. Using the concept of ligand field, the Slater-Condon integrals, the spin-orbit coupling constants, and the parameters of the ligand field potential are determined from density functional theory (DFT). Therefore, a theoretical procedure using LFDFT is established illustrating the spectroscopic details at the atomic scale that can be valuable in the analysis and characterization of the electronic spectra obtained from X-ray absorption fine structure or electron energy loss spectroscopies.

An EOM-CCSD-PCM Benchmark for Electronic Excitation Energies of Solvated Molecules.

In this work, we benchmark the equation of motion coupled cluster with single and double excitations (EOM-CCSD) method combined with the polarizable continuum model (PCM) for the calculation of electronic excitation energies of solvated molecules. EOM-CCSD is one of the most accurate methods for computing one-electron excitation energies, and accounting for the solvent effect on this property is a key challenge. PCM is one of the most widely employed solvation models due to its adaptability to virtually any solute and its efficient implementation with density functional theory methods (DFT). Our goal in this work is to evaluate the reliability of EOM-CCSD-PCM, especially compared to time-dependent DFT-PCM (TDDFT-PCM). Comparisons between calculated and experimental excitation energies show that EOM-CCSD-PCM consistently overestimates experimental results by 0.4-0.5 eV, which is larger than the expected EOM-CCSD error in vacuo. We attribute this decrease in accuracy to the approximated solvation model. Thus, we investigate a particularly important source of error: the lack of H-bonding interactions in PCM. We show that this issue can be addressed by computing an energy shift, ΔHB, from bare-PCM to microsolvation + PCM at DFT level. Our results show that such a shift is independent of the functional used, contrary to the absolute value of the excitation energy. Hence, we suggest an efficient protocol where the EOM-CCSD-PCM transition energy is corrected by ΔHB(DFT), which consistently improves the agreement with the experimental measurements.

Antiproton stopping in atomic targets

Stopping powers of antiprotons in H, He, Ne, Ar, Kr, and Xe targets are calculated using a semiclassical time-dependent convergent close-coupling method. The helium target is treated using both frozen-core and multiconfiguration approximations. The electron-electron correlation of the target is fully accounted for in both cases. Double ionization and ionization with excitation channels are taken into account using an independent-event model. The Ne, Ar, Kr, and Xe atom wave functions are described in a model of six $p$-shell electrons above a frozen Hartree-Fock core with only one-electron excitations from the outer $p$ shell allowed. Results obtained for helium in the multiconfiguration treatment are in better agreement with experimental measurements than other theories.

Construction of the energy matrix for complex atoms Part IV: Excitation of one electron from a closed shell into an open shell

This work is a continuation of the previous series related to the construction of the energy matrix for complex atoms. The effects of the second-order configuration interaction perturbations on the energy-level structure of \(nl^{N}\), \( nl^{N}n_{1}l_{1}^{N_{1}}\) and \( nl^{N}n_{1}l_{1}^{N_{1}}n_{2}l_{2}\) configurations were studied. In this paper we consider one-electron core excitations for and between the configurations under study. This work combined with Part II and III is a complete description of an electrostatic interaction. In Part I we presented a method which allows to analyse complex electronic systems. They constitute the basis for the design of an efficient computer program package allowing large scale calculations which provide accurate wave functions.

Antiproton-impact ionization of Ne, Ar, Kr, Xe, andH2O

We calculate antiproton-impact total single ionization of Ne, Ar, Kr, Xe, and ${\mathrm{H}}_{2}\mathrm{O}$ using a time-dependent convergent close-coupling approach. The Ne, Ar, Kr, and Xe atom wave functions are described in a model of six $p$-shell electrons above a frozen Hartree-Fock core with only one-electron excitations from the outer $p$ shell allowed. For treating the water molecule we use a neonization method recently proposed by Montanari and Miraglia [J. Phys. B: At. Mol. Opt. Phys. 47, 015201 (2014)], which describes the ten-electron water molecule as a dressed Ne-like atom in a pseudospherical potential. In the present work the target states of noble gas atoms and water are obtained using a Laguerre basis expansion. For the noble gas atoms there is reasonably good agreement with the calculated single-ionization cross sections.