First-excited State Research Articles

New insights into the nonadiabatic state population dynamics of model proton-coupled electron transfer reactions from the mixed quantum-classical Liouville approach.

In a previous study [F. A. Shakib and G. Hanna, J. Chem. Phys. 141, 044122 (2014)], we investigated a model proton-coupled electron transfer (PCET) reaction via the mixed quantum-classical Liouville (MQCL) approach and found that the trajectories spend the majority of their time on the mean of two coherently coupled adiabatic potential energy surfaces. This suggested a need for mean surface evolution to accurately simulate observables related to ultrafast PCET processes. In this study, we simulate the time-dependent populations of the three lowest adiabatic states in the ET-PT (i.e., electron transfer preceding proton transfer) version of the same PCET model via the MQCL approach and compare them to the exact quantum results and those obtained via the fewest switches surface hopping (FSSH) approach. We find that the MQCL population profiles are in good agreement with the exact quantum results and show a significant improvement over the FSSH results. All of the mean surfaces are shown to play a direct role in the dynamics of the state populations. Interestingly, our results indicate that the population transfer to the second-excited state can be mediated by dynamics on the mean of the ground and second-excited state surfaces, as part of a sequence of nonadiabatic transitions that bypasses the first-excited state surface altogether. This is made possible through nonadiabatic transitions between different mean surfaces, which is the manifestation of coherence transfer in MQCL dynamics. We also investigate the effect of the strength of the coupling between the proton/electron and the solvent coordinate on the state population dynamics. Drastic changes in the population dynamics are observed, which can be understood in terms of the changes in the potential energy surfaces and the nonadiabatic couplings. Finally, we investigate the state population dynamics in the PT-ET (i.e., proton transfer preceding electron transfer) and concerted versions of the model. The PT-ET results confirm the participation of all of the mean surfaces, albeit in different proportions compared to the ET-PT case, while the concerted results indicate that the mean of the ground- and first-excited state surfaces only plays a role, due to the large energy gaps between the ground- and second-excited state surfaces.

Materials for third-order nonlinear optics

Abstract

Systematic Convergence in Applying Variational Method to Double-Well Potential.

In this work, we demonstrate the application of the variational method by computing the ground- and first-excited state energies of a double-well potential. We start with the proper choice of the trial wave functions using optimized parameters, and notice that accurate expectation values in excellent agreement with the numerical results can be aquired by cautious systematic improvement of trial wave functions.

Observation and Spectroscopy of New Proton-Unbound Isotopes ³⁰Ar and ²⁹Cl: An Interplay of Prompt Two-Proton and Sequential Decay.

Previously unknown isotopes (30)Ar and (29)Cl have been identified by measurement of the trajectories of their in-flight decay products (28)S+p+p and (28)S+p, respectively. The analysis of angular correlations of the fragments provided information on decay energies and the structure of the parent states. The ground states of (30)Ar and (29)Cl were found at 2.25(-0.10)(+0.15) and 1.8±0.1 MeV above the two- and one-proton thresholds, respectively. The lowest states in (30)Ar and (29)Cl point to a violation of isobaric symmetry in the structure of these unbound nuclei. The two-proton decay has been identified in a transition region between simultaneous two-proton and sequential proton emissions from the (30)Ar ground state, which is characterized by an interplay of three-body and two-body decay mechanisms. The first hint of a fine structure of the two-proton decay of (30)Ar*(2(+)) has been obtained by detecting two decay branches into the ground and first-excited states of the (28)S fragment.

A Practical Diabatisation Scheme for Use with the Direct-Dynamics Variational Multi-Configuration Gaussian Method.

A method for diabatising multiple electronic states on-the-fly within the direct dynamics variational multi-configuration Gaussian method for calculating quantum nuclear dynamics is presented. The method is based upon the propagation of the adiabatic-diabatic transformation matrix along the paths followed by the Gaussian basis functions that constitute the nuclear wave function, by use of a well-known differential equation relating the matrix and the nonadiabatic vector coupling terms between the electronic states. The implementation of the method is described, and test calculations are presented using the ground and first-excited states of the butatriene cation as an example, allowing comparison to the earlier regularisation diabatisation scheme as well as to full nuclear dynamics on a precomputed potential energy surface. The new scheme is termed propagation diabatisation.

Width ofP27(1.19MeV)and theSi26(p,γ)reaction rate

I have used a simple potential model to calculate single-particle widths in $^{27}\mathrm{P}$. Assuming mirror symmetry, I have combined them with the spectroscopic factor from the reaction $^{26}\mathrm{Mg}(d,p)$ to compute proton widths for the $3/{2}^{+}$ first-excited state. I present results for the resonance strength parameter $\ensuremath{\omega}\ensuremath{\gamma}$ and for the $(p,\ensuremath{\gamma})$ reaction rate for two values of the $\ensuremath{\gamma}$ width and two values of the resonance energy.

Effect of various solvents on the absorption spectra of dithizone and DFT calculations

The electronic absorption spectra of dithizone were recorded in various solvents and the absorption maxima of the dye were found to be dependent on solvent polarity. The molecule showed two absorption peaks in the visible region, the higher wavelength band was assigned to the keto form and the lower wavelength band is an overlap of the enol and the keto forms. The absorption data fitted well to Kamlet–Abboud–Taft equation. Studies done in a binary mixture of solvents (acetone+i-PrOH and dichloromethane+i-PrOH) showed that the probe molecule tends to form a hydrogen-bonding solvated complex with the more polar solvent. The values of preferential solvation constant (KPS) were also calculated and it was found that the solute molecule preferentially solvated in i-PrOH. DFT calculations showed that the keto form absorbance occurred at 608nm and was assigned to n→π* transition. The first-excited state electron densities suggested that the excited state was favorably stabilized in less polar solvents.

Coulomb Impurity Potential RbCl Quantum Pseudodot Qubit

By employing a variational method of Pekar type, we study the eigenenergies and the corresponding eigenfunctions of the ground and the first-excited states of an electron strongly coupled to electron-LO in a RbCl quantum pseudodot (QPD) with a hydrogen-like impurity at the center. This QPD system may be used as a two-level quantum qubit. The expressions of electron’s probability density versus time and the coordinates, and the oscillating period versus the Coulombic impurity potential and the polaron radius have been derived. The investigated results indicate ① that the probability density of the electron oscillates in the QPD with a certain oscillating period of \(T_0 =7.817\mathrm{fs}\), ② that due to the presence of the asymmetrical potential in the z direction of the RbCl QPD, the electron probability density shows double-peak configuration, whereas there is only one peak if the confinement is a two-dimensional symmetric structure in the xy plane of the QPD, ③ that the oscillation period is a decreasing function of the Coulombic impurity potential, whereas it is an increasing one of the polaron radius.

Analysis of elastic, quasielastic, and inelastic scattering of lithium isotopes on aSi28target

The elastic and inelastic cross sections of $^{6}\mathrm{Li}$ on $^{28}\mathrm{Si}$ at 240 MeV, and quasielastic cross section of $^{7}\mathrm{Li}$ and $^{11}\mathrm{Li}$ on $^{28}\mathrm{Si}$ at $177.8$ and 319 MeV, respectively, are analyzed with the coupled-channels method. The collective nuclear level density is used to determine the deformation parameter regarding to the first-excited state of $^{28}\mathrm{Si}$. The results are in agreement with the experimental data and indicate the need of using a nuclear structure model such as nuclear level density to reduce the ambiguity between the optical model parameters and the deformation parameter. Additionally, the spin-orbit potential is found to have an important role in reproducing the data of the quasielastic scattering of $^{7}\mathrm{Li}$ and $^{11}\mathrm{Li}$ on a $^{28}\mathrm{Si}$ target.

Nonadiabatic couplings and charge transfer study in H + CS+ collision using time-dependent quantum dynamics

Experiments have reported the high stability of HCS+ ion and inhibit to decompose over the range of collision energies. In this study, the various energy transfer channels of atomic H collision with CS+ molecular ion has been performed by ab initio computations at the multireference configuration interaction/aug-cc-pVQZ level of theory. The ground and several low-lying excited electronic state potential energy surfaces in three different molecular orientations, namely, two collinear configurations with, (1) H approaching the S atom (γ = 0°), (2) H approaching the C atom (γ = 180°) and one perpendicular configuration, (3) H approaching the centre of mass of CS (γ = 90°) with the diatom fixed at the equilibrium bond length, have been obtained. Nonadiabatic effects with Landau–Zener coupling leading to avoided crossings are observed between the ground- and the first-excited states in γ = 90° orientation, and also between the first- and second-excited states in γ = 180° orientation. Quantum dynamics have been performed to study the charge transfer using time-dependent wave packet method on the diabatic potential energy surfaces. The probability of charge transfer is found to be highest with 42% in γ = 180°. The high charge transfer probability result in the formation of H+ + CS channel which ascertains the high stability of HCS+ ion.

Influence of Magnetic Field and LO Phonon Effect on Properties of Magnetopolarons and Qubit in Quantum Disks

Influence of external magnetic field, longitudinal-optical (LO) phonon effect and thickness of quantum disks (QDs) on the properties of electron-LO phonon strong-coupling magnetopolarons and the qubit is studied by using Lee -Low-Pines-Huybrechts variational method. The expressions for the mean number of phonons of magnetopolarons $\bar {N}$ , electron-LO phonon interaction energy E e−ph, energies of the ground and first-excited states E 0 and E 1, oscillation period T 0 of qubits and probability distribution ρ of electrons are derived. Results of numerical calculations show that the mean number of phonons of the ground state of magnetopolarons is larger than that of the first-excited state, the mean number of phonons of magnetopolarons $\bar {N}$ increases with increasing the cyclotron frequency ω c, confinement strength ω 0 and coupling strength α, but decreases and oscillates with increasing the thickness of QDs L; E e−ph is always negative, ∣E e-ph∣ of the ground state is larger than that of the first-excited state, and ∣E e-ph∣ increases with increasing ω c, ω 0 and α but decreases with increasing L; E 0 and E 1 increase with increasing ω c, ω 0 and α, but decrease with increasing L; T 0 increases with increasing ω 0 and L, but decreases with increasing α; ρ decreases and oscillates with increasing L, and decreases with increasing ω 0; the variations of T 0 and ρ with ω c and L are influenced by ω 0 and α significantly.

Nuclear g-factor measurement with time-dependent recoil in vacuum in radioactive-beam geometry

A modified version of the time-differential recoil-in-vacuum (TDRIV) method, adapted for use with radioactive beams, was applied for the first time to perform a g-factor measurement on the first-excited state in 24Mg (Ex = 1.369 MeV, t = 1.97 ps). A high precision g-factor value was obtained using predominantly H-like ions. The results obtained demonstrate the versatility of the new approach and have the precision needed for stringent tests of shell model calculations.

Magnetism of an excited self-conjugate nucleus: precise measurement of the g factor of the 2(1)(+) state in (24)Mg.

A precise measurement of the g factor of the first-excited state in the self-conjugate (N=Z) nucleus (24)Mg is performed by a new time-differential recoil-in-vacuum method based on the hyperfine field of hydrogenlike ions. Theory predicts that the g factors of such states, in which protons and neutrons occupy the same orbits, should depart from 0.5 by a few percent due to configuration mixing and meson-exchange effects. The experimental result, g=0.538±0.013, is in excellent agreement with recent shell-model calculations and shows a departure from 0.5 by almost 3 standard deviations, thus achieving, for the first time, the precision and accuracy needed to test theory. Proof of the new method opens the way for wide applications including measurements of the magnetism of excited states of exotic nuclei produced as radioactive beams.

Decoherence of Polaron in Asymmetric Quantum Dot Qubit Under an Electromagnetic Field

In this paper, we investigate the time evolution of the quantum mechanical state of a polaron using the Pekar type variational method on the electric-LO-phonon and the magnetic-LO-phonon strong coupling in a quantum dot. We obtain the Eigen energies and the Eigen functions of the ground state and the first excited state, respectively. In a quantum dot, this system can be viewed as a two level quantum system qubit. The superposition state polaron density oscillates in the quantum dot with a period τ_0when the polaron is in the superposition of the ground and the first-excited states. The spontaneous emission of phonons causes the decoherence of the qubit. We show that the density matrix of the qubit decays with the time while the coherence term of the density matrix element 〖 p〗_01 (〖 or p〗_10) decays with the time as well for different coupling strengths, confinement lengths, and dispersion coefficients. The Shannon entropy is evaluated in order to investigate the decoherence of the system.

Bound Magnetopolaron in an Asymmetric Cylindrical Quantum Dot Qubit in an Electric Field

We investigate a quantum mechanical system defined as an unsymmetrical bound magnetopolaron immersed in the field of the bulk longitudinal optical (LO)-phonon strong coupling. The ground and the first-excited state of the eigenenergy are derived by using variational method of Pekar type. The effect of the longitudinal and transversal confinement strengths, the effect of magnetic and electric field and the effect of the electron-phonon coupling constant on the polaron characteristics are investigated. These dependencies demonstrate that, they are more flexible tunable methods to restrain quantum decoherence and aggrandize the amplitude of the probability density.

Excited-state proton-transfer reactions of 7-azaindole with water, ammonia and mixed water–ammonia: microsolvated dynamics simulation

Dynamics simulations of excited-state multiple proton transfer (ESMPT) reactions in 7-azaindole (7AI) with ammonia, mixed water–ammonia, and water molecules were investigated by quantum dynamics simulations in the first-excited state using RI-ADC(2)/SVP-SV(P) in the gas phase. 7AI(WW), 7AI(WA), 7AI(AW) and 7AI(AA) clusters (W, water and A, ammonia) show very high probability of the excited-state triple proton transfer (ESTPT) occurrence in ranges from 20% for 7AI(WA) to 60% for 7AI(AW), respectively. Furthermore, 7AI(AW) clusters with ammonia placed near N–H of 7AI has the highest probability among other isomers. In 7AI with three molecules of bridged-planar of water, ammonia and mixed water–ammonia clusters, the excited-state quadruple proton transfer reactions occur ineffectively and rearrangement of hydrogen-bonded network on solvents also takes place prior to either ESTPT or excited-state double proton transfer. The role played by mixed-solvent is revealed with replacing H2O with NH3 in which the ESMPT is found to be more efficient corresponding to lower barrier in the excited state. The preferential number of solvent surrounding 7AI that facilitates the proton transfer process is two for methanol and water but this preferential number for ammonia is one.Highlights: (i) replacing H2O with NH3 assists ESPT corresponding to lower barrier in the excited state; (ii) the ESMPT time of 7AI with mixed water–ammonia is in the sub-picosecond timescale; (iii) the PT tends to be concerted process with at least one ammonia, but synchronous without ammonia.

Charge-carrier relaxation dynamics of poly(3-hexylthiophene)-coated gold hybrid nanoparticles

Poly(3-hexylthiophene)-coated gold (Au@P3HT) hybrid nanoparticles have been prepared via a “grafting-to” approach, and compared with pristine P3HT to investigate the dynamics of exciton relaxation using time-resolved transient-absorption and fluorescence spectroscopy. In efforts to facilitate the efficient dissociation of photo-generated excitons, we have incorporated gold nanoparticles having surface-plasmon resonances into thiol-terminated P3HT to fabricate Au@P3HT nanocomposites. The first-excited singlet state of Au@P3HT decays faster than that of pristine P3HT due to interactions with surface-plasmon resonances; excitons undergo dissociation via energy transfer from P3HT to the surface-plasmon state of a gold nanoparticle in Au@P3HT nanocomposites. The lowest triplet state of P3HT is also less populated due to the energy transfer while its lifetime is slightly reduced by the presence of gold nanoparticles. Thus, it is suggested that our incorporation of gold nanoparticles into P3HT reduces the recombination of geminate excitons and, thereby, increases the probability of exciton dissociation.

Symmetrized photoinitiated electron flow within the [myoglobin:cytochrome b₅] complex on singlet and triplet time scales: energetics vs dynamics.

We report here that photoinitiated electron flow involving a metal-substituted (M = Mg, Zn) myoglobin (Mb) and its physiological partner protein, cytochrome b5 (cyt b5) can be “symmetrized”: the [Mb:cyt b5] complex stabilized by three D/E → K mutations on Mb (D44K/D60K/E85K, denoted MMb) exhibits both oxidative and reductive ET quenching of both the singlet and triplet photoexcited MMb states, the direction of flow being determined by the oxidation state of the cyt b5 partner. The first-excited singlet state of MMb (1MMb) undergoes ns-time scale reductive ET quenching by Fe2+cyt b5 as well as ns-time scale oxidative ET quenching by Fe3+cyt b5, both processes involving an ensemble of structures that do not interconvert on this time scale. Despite a large disparity in driving force favoring photooxidation of 1MMb relative to photoreduction (δ(−ΔG0) ≈ 0.4 eV, M = Mg; ≈ 0.2 eV, M = Zn), for each M the average rate constants for the two reactions are the same within error, 1kf > 108 s–1. This surprising observation is explained by considering the driving-force dependence of the Franck–Condon factor in the Marcus equation. The triplet state of the myoglobin (3MMb) created by intersystem crossing from 1MMb likewise undergoes reductive ET quenching by Fe2+cyt b5 as well as oxidative ET quenching by Fe3+cyt b5. As with singlet ET, the rate constants for oxidative ET quenching and reductive ET quenching on the triplet time scale are the same within error, 3kf ≈ 105 s–1, but here the equivalence is attributable to gating by intracomplex conversion among a conformational ensemble.

Delocalization of electrons by cavity photons in transport through a quantum dot molecule

We present results on cavity-photon-assisted electron transport through two lateral quantum dots embedded in a finite quantum wire. The double quantum dot system is weakly connected to two leads and strongly coupled to a single quantized photon cavity mode with initially two linearly polarized photons in the cavity. Including the full electron–photon interaction, the transient current controlled by a plunger-gate in the central system is studied by using quantum master equation. Without a photon cavity, two resonant current peaks are observed in the range selected for the plunger gate voltage: The ground state peak, and the peak corresponding to the first-excited state. The current in the ground state is higher than in the first-excited state due to their different symmetry. In a photon cavity with the photon field polarized along or perpendicular to the transport direction, two extra side peaks are found, namely, photon-replica of the ground state and photon-replica of the first-excited state. The side-peaks are caused by photon-assisted electron transport, with multiphoton absorption processes for up to three photons during an electron tunneling process. The inter-dot tunneling in the ground state can be controlled by the photon cavity in the case of the photon field polarized along the transport direction. The electron charge is delocalized from the dots by the photon cavity. Furthermore, the current in the photon-induced side-peaks can be strongly enhanced by increasing the electron–photon coupling strength for the case of photons polarized along the transport direction.

Enhancing optical characteristics of InAs/InGaAsSb quantum dot structures with long-excited state emission at 1.31 μm.

In this study, the optical properties of InAs quantum dots (QDs) with various strain-reducing layers (SRLs) of GaAsSb and InGaAsSb are characterized using photoluminescence (PL) and time-resolved PL (TRPL) measurements. The room-temperature PL results for the InAs/InGaAsSb QDs revealed stronger emission intensities than InAs QDs capped with an GaAs(1-x)Sb(x) (x = 20%)SRL, although both samples were grown under the same Sb flux during the molecular beam epitaxy process. The InAs/InGaAsSb QDs showed a significant elongation of emission wavelengths to 1450 and 1310 nm for the ground and first-excited state at room temperature. The energy band alignment of the InAs QD heterostructures was found tailoring from type II to type I as the GaAsSb SRL was replaced by InGaAsSb layer, which improved the radiative efficiency and was verified by power-dependent PL and TRPL measurements. Post-growth rapid thermal annealing was applied on the InAs/InGaAsSb QDs to further enhance the QD quality and PL emission efficiency. The greatly improved PL intensity, reduced linewidth, shortened radiative lifetime, with increasing annealing temperature were demonstrated, and InAs/InGaAsSb QDs exhibited enhanced optical characteristics for long-wavelength emission applications.