Mixing Of Electronic States Research Articles

Long-lived negative molecular ions of TCNQ formed by the resonant capture of electrons with above zero energies

Two mechanisms of formation of long-lived negative molecular ions in TCNQ at resonant electron capture were considered for the case of the nonzero captured electrons. The first mechanism concerned with the ions lying in energy lower than the first molecular triplet was related to inter-systems crossing process providing formation of the long-lived ions with quartet multiplicity. The second mechanism, which yields high lifetime of the ions lying above the first triplet, was determined as a mixing of ion electronic states with the ground state contribution. The results give more knowledge of the long-lived negative ions formation.

CO Oxidation Catalyzed by a Single Ti Atom Supported on Divacancy Defective Graphene: A Dispersion‐Corrected DFT Study

AbstractWe report a detailed theoretical investigation on the possible reaction mechanisms of CO oxidation catalyzed by a single Ti atom supported on divacancy defective graphene (TiDVG). The reaction between the adsorbed O2 with CO molecules on TiDVG is compared via three different mechanisms, i. e. Eley‐Rideal (ER), Langmuir‐Hinshelwood (LH) and termolecular Eley‐Rideal (TER). According to our results, the energy barrier for the rate determining step of CO + O2 reaction in the TER mechanism is smaller than those of ER and LH ones. This can be attributed to the significant mixing of electronic states of adsorbed CO and O2 at the reaction site, as well as due to a sizable charge‐transfer from the surface to these molecules. On the other hand, the calculated adsorption energies indicate that the coadsorption of O2 and CO molecules is slightly more energetically favored with respect to the coadsorption of two CO molecules. This result together with the relatively small reaction barriers reveal that the LH mechanism is also possible to occur at low temperature. Our results reported here suggest that TiDVG can be regarded as a green, low‐cost and highly active catalyst for CO oxidation at room temperature.

Nonadiabatic electron wavepacket dynamics behind molecular autoionization.

A theoretical method for real-time dynamics of nonadiabatic reorganization of electronic configurations in molecules is developed, with dual aim that the intramolecular electron dynamics can be probed by means of direct and/or indirect photoionizations and that the physical origins behind photoionization signals attained in the time domain can be identified in terms of the language of time-dependent quantum chemistry. In doing so, we first formulate and implement a new computational scheme for nonadiabatic electron dynamics associated with molecular ionization, which well fits in the general theory of nonadiabatic electron dynamics. In this method, the total nonadiabatic electron wavepackets are propagated in time directly with complex natural orbitals without referring to Hartree-Fock molecular orbitals, and the amount of electron flux from a molecular region leading to ionization is evaluated in terms of the relevant complex natural orbitals. In the second half of this paper, we apply the method to electron dynamics in the elementary processes consisting of the Auger decay to demonstrate the methodological significance. An illustrative example is taken from an Auger decay starting from the 2a1 orbital hole-state of H2O+. The roles of nuclear momentum (kinetic) couplings in electronic-state mixing during the decay process are analyzed in terms of complex natural orbitals, which are schematically represented in the conventional language of molecular symmetry of the Hartree-Fock orbitals.

Theory of molecular nonadiabatic electron dynamics in condensed phases.

In light of the rapid progress of ultrafast chemical dynamics driven by the pulse lasers having width as short as several tens of attoseconds, we herein develop a theory of nonadiabatic electron wavepacket dynamics in condensed phases, with which to directly track the dynamics of electronic-state mixing such as electron transfer in liquid solvents. Toward this goal, we combine a theory of path-branching representation for nonadiabatic electron wavepacket dynamics in vacuum {a mixed quantum-classical representation, Yonehara and Takatsuka [J. Chem. Phys. 129, 134109 (2008)]} and a theory of entropy functional to treat chemical dynamics in condensed phases {a mixed dynamical-statistical representation, Takatsuka and Matsumoto [Phys. Chem. Chem. Phys. 18, 1771 (2016)]}. Difficulty and complexity in the present theoretical procedure arise in embedding the Schrödinger equation into classically treated statistical environment. Nevertheless, the resultant equations of motion for electronic-state mixing due to the intrinsic nonadiabatic interactions and solute-solvent interactions, along with the force matrix that drives nuclear branching paths, both turn out to be clear enough to make it possible to comprehend the physical meanings behind. We also discuss briefly the nonvalidness of naive application of the notion of nonadiabatic transition dynamics among free energy surfaces.

Impact of Electronic State Mixing on the Photoisomerization Time Scale of the Retinal Chromophore

Spectral data show that the photoisomerization of retinal protonated Schiff base (rPSB) chromophores occurs on a 100 fs time scale or less in vertebrate rhodopsins, it is several times slower in microbial rhodopsins and it is between one and 2 orders of magnitude slower in solution. These time scale variations have been attributed to specific modifications of the topography of the first excited state potential energy surface of the chromophore. However, it is presently not clear which specific environment effects (e.g., electrostatic, electronic, or steric) are responsible for changing the surface topography. Here, we use QM/MM models and excited state trajectory computations to provide evidence for an increase in electronic mixing between the first and the second excited state of the chromophore when going from vertebrate rhodopsin to the solution environments. Ultimately, we argue that a correlation between the lifetime of the first excited state and electronic mixing between such state and its higher neighbor, may have been exploited to evolve rhodopsins toward faster isomerization and, possibly, light-sensitivity.

Mixing of MnPc electronic states at the MnPc/Au(110) interface.

Manganese-phthalocyanines form assembled chains with a variety of ordered super-structures, flat lying along the Au(110) reconstructed channels. The chains first give rise to a ×5 symmetry reconstruction, while further deposition of MnPc leads to a ×7 periodicity at the completion of the first single layer. A net polarization with the formation of an interface dipole is mainly due to the molecular π-states located on the macrocycles pyrrole rings, while the central metal ion induces a reduction in the polarization, whose amount is related to the Mn-Au interaction. The adsorption-induced interface polarization is compared to other 3d-metal phthalocyanines, to unravel the role of the central metal atom configuration in the interaction process of the d-states. The MnPc adsorption on Au(110) induces the re-hybridization of the electronic states localized on the central metal atom, promoting a charge redistribution of the molecular orbitals of the MnPc molecules. The molecule-substrate interaction is controlled by a symmetry-determined mixing between the electronic states, involving also the molecular empty orbitals with d character hybridized with the nitrogen atoms of the pyrrole ring, as deduced by photoemission and X-ray absorption spectroscopy exploiting light polarization. The symmetry-determined mixing between the electronic states of the Mn metal center and of the Au substrate induces a density of states close to the Fermi level for the ×5 phase.

Ab initio calculations of exciton effects in optical spectra of an α-B12 crystal

The dielectric functions and energy loss spectrum of electrons of a rhombohedral α-B12 crystal are studied both in the single-particle and many-particle approximations using Bethe–Salpeter equations. The opposite roles of different contributions to exciton effects are discussed. The anisotropy of dielectric functions is shown, which demonstrates their high sensitivity with respect to the three-dimensional packing of icosahedrons. The influence of the coherent mixing of electronic and exciton states on the redistribution of oscillator strengths is found. The position of the plasmon of valence electrons and the high-frequency permittivity are found to be consistent with experimental data. The correlation between the distribution of oscillator strengths and features of the density of electronic states is discussed.

Energy spectrum, exchange interaction, and gate crosstalk in a system with a pair of double quantum dots: A molecular-orbital calculation

We present a theoretical study of a four-electron four-quantum-dot system based on molecular orbital methods, which hosts a pair of singlet-triplet spin qubits. We explicitly take into account of the admixture of electron wave functions in all dots, and have found that this mixing of wave functions has consequences on the energy spectrum, exchange interaction and the gate crosstalk of the system. Specifically, we have found that when the two singlet-triplet qubits are close enough, some of the states are no longer dominated by the computational basis states and the exchange interaction can not simply be understood as the energy difference between the singlet and triplet states. Using the Hund-Mulliken calculation of the Hubbard parameters, we characterize the effective exchange interaction of the system and have found good agreement with results calculated by taking energy differences where applicable. We have studied the two commonly conceived schemes coupling two qubits, the exchange and capacitive coupling, and have found that when the inter-qubit distance is at certain intermediate values, the two kinds of coupling are comparable in strength, complicating analyses of the evolution of the two qubits. We also investigate the gate crosstalk in the system due to the quantum mechanical mixing of electron states and have found that while this effect is typically very weak, it should not be ignored if the spacing between the qubits are similar to or less than the distance between the double dots that constitute the qubit.

Ultrafast dynamics of the lowest-lying neutral states in carbon dioxide

We present a study of the ultrafast dissociation dynamics of the lowest-lying electronic excited states in ${\mathrm{CO}}_{2}$ by using ultraviolet (UV) and extreme-ultraviolet (XUV) pulses from high-order harmonic generation. We observe two primary dissociation channels: a direct dissociation channel along the $^{1}{\mathrm{\ensuremath{\Pi}}}_{g}$ electronically excited manifold, and a second channel which results from the mixing of electronic states. The direct dissociation channel is found to have a lifetime which is shorter than our experimental resolution, whereas the second channel has a significantly longer lifetime of nearly 200 fs. In this long-lived channel we observe a beating of the vibrational populations with a period of $\ensuremath{\sim}133$ fs.

Nonadiabtic electron dynamics in densely quasidegenerate states in highly excited boron cluster.

Following the previous study on nonadiabatic reaction dynamics including boron clusters [T. Yonehara and K. Takatsuka, J. Chem. Phys. 137, 22A520 (2012)], we explore deep into highly excited electronic states of the singlet boron cluster (B12) to find the characteristic features of the densely quasi-degenerate electronic state manifold, which undergo very frequent nonadiabatic transitions and thereby intensive electronic state mixing among very many of the relevant states. So much so, isolating the individual adiabatic states and tracking the expected potential energy surfaces both lose the physical sense. This domain of molecular situation is far beyond the realm of the Born-Oppenheimer approximation. To survey such a violent electronic state-mixing, we apply a method of nonadiabatic electron wavepacket dynamics, the semiclassical Ehrenfest method. We have tracked those electron wavepackets and found the electronic state mixing looks like an ultrafast diffusion in the Hilbert space, which results in huge fluctuation. Furthermore, due to such a violent mixing, the quantum phases associated with the electronic states are swiftly randomized, and consequently the coherence among the electronic states are lost quickly. Besides, these highly excited states are mostly of highly poly-radical nature, even in the spin singlet manifold and the number of radicals amounts up to 10 electrons in the sense of unpaired electrons. Thus the electronic states are summarized to be poly-radical and decoherent with huge fluctuation in shorter time scales of vibrational motions. The present numerical study sets a theoretical foundation for unknown molecular properties and chemical reactivity of such densely quasi-degenerate chemical species.

Perpendicular reading of single confined magnetic skyrmions.

Thin-film sub-5 nm magnetic skyrmions constitute an ultimate scaling alternative for future digital data storage. Skyrmions are robust noncollinear spin textures that can be moved and manipulated by small electrical currents. Here we show here a technique to detect isolated nanoskyrmions with a current perpendicular-to-plane geometry, which has immediate implications for device concepts. We explore the physics behind such a mechanism by studying the atomistic electronic structure of the magnetic quasiparticles. We investigate from first principles how the isolated skyrmion local-density-of-states which tunnels into the vacuum, when compared with the ferromagnetic background, is modified by the site-dependent spin mixing of electronic states with different relative canting angles. Local transport properties are sensitive to this effect, as we report an atomistic conductance anisotropy of up to ∼20% for magnetic skyrmions in Pd/Fe/Ir(111) thin films. In single skyrmions, engineering this spin-mixing magnetoresistance could possibly be incorporated in future magnetic storage technologies.

Emission spectra of a laser based on an In(Ga)As/GaAs quantum-dot superlattice

The spectral characteristics of a laser with an active region based on a ten-layer system of In(Ga)As/GaAs vertically correlated quantum dots with 4.5-nm GaAs spacer layers between InAs quantum dots are studied under the conditions of spontaneous and stimulated emission, depending on the current and the duration of pump pulses. Data obtained by transmission electron microscopy and electroluminescence and absorption polarization anisotropy measurements make it possible to demonstrate that the investigated system of tunnel-coupled InAs quantum dots separated by thin GaAs barriers represents a quantum-dot superlattice. With an increase in the laser pump current, the electroluminescence intensity increases linearly and the spectral position of the electroluminescence maximum shifts to higher energies, which is caused by the dependence of the miniband density-of-states distribution on the pump current. Upon exceeding the threshold current, multimode lasing via the miniband ground state is observed. One of the lasing modes can be attributed to the zero-phonon line, and the other is determined by the longitudinal-optical phonon replica of quantum-dot emission. The results obtained give evidence that, under conditions of the laser pumping of an In(Ga)As/GaAs quantum-dot superlattice, strong coupling between the discrete electron states in the miniband and optical phonons takes place. This leads to the formation of quantum-dot polarons, resulting from the resonant mixing of electronic states whose energy separation is comparable to the optical-phonon energy.

Enhancement of the critical temperature of d-wave superconductors by odd-parity electronic mixing

We study the dependence of the critical temperature of a two-band d-wave superconductor on the intensity of an odd parity hybridization between these bands. We consider a two-dimensional (2D) system in the presence of an attractive inter-band interaction. The odd-parity k-dependent hybridization is appropriate to describe mixing of electronic states arising from orbitals with angular momenta that differ by an odd number, such as, sp, pd or df bands. We show that increasing the strength of the odd-parity hybridization enhances superconductivity and also changes the nature of superconductivity from a Bardeen–Cooper–Schrieffer (BCS) type of pairing to a Bose–Einstein condensation (BEC) characteristic of a strong coupling regime, even for weak-couplings. This crossover can also be driven by increasing the binding energy, that appears in the two-body interaction of these 2D systems. Finally, we investigate the effect of the variation in the carrier density on the superconducting phase diagram.

Spin-dependent recombination at arsenic donors in ion-implanted silicon

Spin-dependent transport processes in thin near-surface doping regions created by low energy ion implantation of arsenic in silicon are detected by two methods, spin-dependent recombination using microwave photoconductivity and electrically detected magnetic resonance monitoring the direct current through the sample. The high sensitivity of these techniques allows the observation of the magnetic resonance, in particular, of As in weak magnetic fields and at low resonance frequencies (40–1200 MHz), where high-field-forbidden transitions between the magnetic sublevels can be observed due to the mixing of electron and nuclear spin states. Several implantation-induced defects are present in the samples studied and act as spin readout partner. We explicitly demonstrate this by electrically detected electron double resonance experiments and identify a pair recombination of close pairs formed by As donors and oxygen-vacancy centers in an excited triplet state (SL1) as the dominant spin-dependent process in As-implanted Czochralski-grown Si.

On the use of reduced active space in CASSCF calculations

The complete active space self-consistent field (CASSCF) method is one of the most used approaches to explore theoretically non-adiabatic photochemical processes in medium-size molecular systems. It is also frequently utilized in direct dynamics studies because this method offers a reliable description of the electronic state mixing in the regions of surface crossings (conical intersections). In this context, however, the large computational demand required to perform such on-the-fly molecular dynamics simulations using CASSCF as the electronic structure method means that only relatively small active space can be handled (typically not more than 10 electrons distributed in 10 orbitals). In this article, we present some guidelines to reduce the active space size in CASSCF calculations in order to decrease the computational cost necessary to perform, for example, molecular dynamics simulations. Three important criteria are considered: (i) the degradation of the accuracy in the description of the potential energy surface following the reduction of the active space, (ii) the stability of the reduced active space across the global potential energy surface, and (iii) the cost of the CASSCF calculation using the reduced active space. Two illustrations are given. The first one is the benzene to benzvalene S1 photochemical pathway, the second one is the photoisomerization of thiomethyl para-coumaric acid, the chromophore in photoactive yellow protein.

Theoretical Analysis of the Relaxation Dynamics in Perylene Bisimide Dimers Excited by Femtosecond Laser Pulses

We present a model for the relaxation dynamics in perylene bisimide dimers, which is based on ab initio electronic structure and quantum dynamics calculations including effects of dissipation. The excited-state dynamics proceeds via a mixing of electronic states of local Frenkel and charge-transfer characters, which becomes effective upon a small distortion of the dimer geometry. In this way, it is possible to explain the fast depopulation of the photoexcited state, which we characterize by femtosecond transient absorption measurements. The combined theoretical and experimental analysis hints at a trapping mechanism, which involves nonadiabatic and dissipative dynamics in an excited-state vibronic manifold and provides an atomistic picture that might prove valuable for future design of photovoltaic materials.

The Fe substitution in [formula omitted] (M = Si, Ge and Sn): A first-principles study

The preferential site substitution of Fe by Si, Ge and Sn and its effect on magnetic moments have been studied in Nd2Fe14B by a first-principles density functional calculation. The substitution energies of Si and Ge are positive at the sites of 4e, 8j2 and 16k1 while the values are negative at the sites of 4c, 8j1 and 16k2. However, the substitution energies of Sn are only negative at the sites of 4c and 8j1 in Nd2(Fe,Sn)14B. Based on Maxwell–Boltzmann distribution, Si, Ge and Sn prefer strongly to enter the site of 4c and exhibit a moderate preference at the site of 8j1, in good agreement with the powder neutron diffraction results. Upon doping Si, Ge and Sn, the average Fe magnetic moments remain almost unchanged in addition to the dilute effect of the non-magnetic elements of Si, Ge and Sn. The slight enhancement of Nd magnetic moment results from the reduction of the net Nd 5d spin moment, which is antiparallel to the total Nd 4f moment. The reduction of Nd 5d spin moment is ascribed to the hybrid mixing of the Nd 5d electron states and the p electron states of the Si, Ge, and Sn nearest neighbors.

Ion imaging studies of the photodissociation dynamics of CH2I2 at 248 nm

The photodissociation of CH2I2 at 248nm is investigated using velocity map ion imaging with photoionization detection of the I∗ (2P1/2) products. The velocity distribution of the I∗ products reveals that only a small fraction of the available energy is released as translational energy, consistent with a simple impulsive model. Most of the excess energy is channeled into internal excitation with 〈Eint〉 = 36.3kcalmol−1 for CH2I radical fragments produced with I∗ atoms. The anisotropy angular distribution of the I∗ fragments and corresponding positive anisotropy parameter are indicative of prompt dissociation and electronic state mixing.

Excited Electronic State Mixing in 7-Azaindole. Quantitative Measurements Using the Stark Effect

Stark effect measurements of the +280 cm(-1) vibronic band at ∼286 nm in the high resolution S1-S0 fluorescence excitation spectrum of 7-azaindole (7AI) in a molecular beam show that the permanent (electric) dipole moment (PDM) of the upper state vibrational level reached in this transition is 4.6 D, twice as large as the PDM of the zero-point level of the S1 state. This large difference is attributed to state mixing with a more polar state. EOM-CSSD calculations suggest that this more polar state is σπ* in nature and that it crosses the ππ* state in energy along the coordinate connecting the two potential energy minima. Such state mixing apparently provides more facile access to conical intersections with the ground state, and subsequent hydrogen atom detachment reactions, since independent studies by Sakota and Sekiya have shown that the N-H stretching frequency of 7AI is significantly reduced when it is excited to the +280 cm(-1) vibrational level of the S1 state.

Comparison of vertical and adiabatic harmonic approaches for the calculation of the vibrational structure of electronic spectra

The calculation of the vibrational structure associated to electronic spectra in large molecules requires a Taylor expansion of the initial and final state potential energy surface (PES) around some reference nuclear structure. Vertical (V) and adiabatic (A) approaches expand the final state PES around the initial-state (V) or final-state (A) equilibrium structure. Simplest models only take into account displacements of initial- and final-state minima, intermediate ones also allow for difference in frequencies and more accurate models introduce the Dushinsky effect through the computation of the Hessians of both the initial and final state. In this contribution we summarize and compare the mathematical expressions of the complete hierarchy of V and A harmonic models and we implement them in a numerical code, presenting a detailed comparison of their performance on a number of prototypical systems. We also address non-Condon effects through linear expansions of the transition dipole as a function of nuclear coordinates (Herzberg-Teller effect) and compare the results of expansions around initial and final state equilibrium geometries. By a throughout analysis of our results we highlight a number of general trends in the relative performance of the models that can provide hints for their proper choice. Moreover we show that A and V models including final state PES Hessian outperform the simpler ones and that discrepancies in their predictions are diagnostic for failure of harmonic approximation and/or of Born-Oppenheimer approximation (existence of remarkable geometry-dependent mixing of electronic states).