Excited Resonance States Research Articles

Magnetic Resonance of Excited States in Semiconductors and Related Nanostructures

Magnetic Resonance of Excited States in Semiconductors and Related Nanostructures

Tunnelling of electrons via the neighboring atom

As compared to the intuitive process that the electron emits straight to the continuum from its parent ion, there is an alternative route that the electron may transfer to and be trapped by a neighboring ionic core before the eventual release. Here, we demonstrate that electron tunnelling via the neighboring atomic core is a pronounced process in light-induced tunnelling ionization of molecules by absorbing multiple near-infrared photons. We devised a site-resolved tunnelling experiment using an Ar-Kr+ ion as a prototype system to track the electron tunnelling dynamics from the Ar atom towards the neighboring Kr+ by monitoring its transverse momentum distribution, which is temporally captured into the resonant excited states of the Ar-Kr+ before its eventual releasing. The influence of the Coulomb potential of neighboring ionic cores promises new insights into the understanding and controlling of tunnelling dynamics in complex molecules or environment.

Soft Dipole Resonances in 8He and 8C

Soft dipole resonances in 8He and 8C are investigated by applying the complex scaling method to the α + N + N + N + N five-body model for the isospin T = 2 system. The 1− excited resonant states are calculated at (Ex, Γ) = (14.0, 21.1) MeV and (12.7, 24.0) MeV for 8He and 8C, respectively. From the analyses of the configuration mixing and the spatial properties, the soft dipole resonances of two nuclei are understood to be the vibrational states of valence nucleons and the α core. They are also seen to have a good isospin symmetry. In contrast, the ground states of two nuclei show different spatial properties indicating the symmetry breaking due to the Coulomb repulsion of valence protons in 86C2.

Dispersion interaction between thin conducting cylinders.

The ground state and excited state resonance dipole-dipole interaction energy between two elongated conducting molecules is explored in this study. We review the current status for ground state interactions. This interaction is found to be of a much longer range than in the case when the molecules are pointlike and nonconducting. These are well known results found earlier by Davies, Ninham, and Richmond, and later, using a different formalism, by Rubio and co-workers. We show how the theory can be extended to excited state interactions. A characteristic property observed from our calculation is that the interaction energy dependence on separation (R) follows f(R)/R2 for both resonance and van der Waals cases in the long-range limit. Under some limits, f(R) has a logarithmic dependency, while under others, it has constant values. We predict an unusual slow decay rate for the energy transfer between conducting molecules.

Tools to Simulate Resonance Raman Spectra for in-Situ Studies of Electron Transport Mediators and Bioelectrocatalysts

Raman spectroscopy is widely applied as an in-situ probe of electrochemical processes. When the excitation light is resonant with an electronic transition, the detected signal can be enhanced by more than 1,000-fold. Resonance Raman (RR) spectroscopy has broad application in the study of common organic electron transport mediators (i.e., viologens, phenazines, photosynthetic pigments) and active sites of redox proteins (i.e., laccases, peroxidases, flavin-containing enzymes). To support investigations of bioelectrocatalytic membranes, we have been adapting RR spectroscopy to gain insights into the properties of immobilized redox mediators and biocatalyst active sites.1,2 Computational methods can be useful for guiding spectral interpretation. This poster reports on applications of recently advanced time-dependent density functional theory (TDDFT) codes that enable the simulation of RR spectra for small electron transport mediators (e.g., methyl viologen radical anion) and redox protein active site models (e.g., laccase T1 site mimics). The ORCA program suite3 was adapted. A series of Cu-centered compounds that model the T1 active site in laccases was studied along with the methyl viologen radical cation. The T1 site in laccases facilitates direct electron transport to an electrode and is a target for in-situ RR measurements. The reported work demonstrates careful selection of the functional (B3-LYP versus the long-range corrected hybrid exchange functional, ωB97X-D3) results in TDDFT-calculated electronic absorption and RR spectra that have features in excellent agreement with experiment. The results provide a foundation to build from in advancing models of small redox mediators and laccase catalytic active sites in support of sustainable technologies. Ozuguzel, U.; Aquino, A.J.A.; Nieman, R.; Minteer, S.D.; Korzeniewski, C. “Calculation of Resonance Raman Spectra and Excited State Properties for Blue Copper Protein Model Complexes” ACS Sustainable Chem. Eng. 2022, 10, 14614. Xu, J.; Koh, M.; Minteer, S. D.; Korzeniewski, C. “In Situ Confocal Raman Microscopy of Redox Polymer Films on Bulk Electrode Supports” ACS Measurement Science Au 2023 (DOI: 10.1021/acsmeasuresciau.2c00064). Neese, F.; Wennmohs, F.; Becker, U.; Riplinger, C. “ORCA Quantum Chemistry Program Package” J. Chem. Phys. 2020, 152, 224108.

Resonance Raman spectra and excited state properties of methyl viologen and its radical cation from time-dependent density functional theory.

Time-dependent density functional theory (TDDFT) was applied to gain insights into the electronic and vibrational spectroscopic properties of an important electron transport mediator, methyl viologen (MV2+ ). An organic dication, MV2+ has numerous applications in electrochemistry that include energy conversion and storage, environmental remediation, and chemical sensing and electrosynthesis. MV2+ is easily reduced by a single electron transfer to form a radical cation species (MV•+ ), which has an intense UV-visible absorption near 600 nm. The redox properties of the MV2+ /MV•+ couple and light-sensitivity of MV•+ have made the system appealing for photo-electrochemical energy conversion (e.g., solar hydrogen generation from water) and the study of photo-induced charge transfer processes through electronic absorption and resonance Raman spectroscopic measurements. The reported work applies leading TDDFT approaches to investigate the electronic and vibrational spectroscopic properties of MV2+ and MV•+ . Using a conventional hybrid exchange functional (B3-LYP) and a long-range corrected hybrid exchange functional (ωB97X-D3), including with a conductor-like polarizable continuum model to account for solvation, the electronic absorption and resonance Raman spectra predicted are in good agreement with experiment. Also analyzed are the charge transfer character and natural transition orbitals derived from the TDDFT vertical excitations calculated. The findings and models developed further the understanding of the electronic properties of viologens and related organic redox mediators important in renewable energy applications and serve as a reference for guiding the interpretation of electronic absorption and Raman spectra of the ions.

On the importance of excited state species in low pressure capacitively coupled plasma argon discharges

In the past three decades, first principles-based fully kinetic particle-in-cell Monte Carlo collision (PIC/MCC) simulations have been proven to be an important tool for the understanding of the physics of low pressure capacitive discharges. However, there is a long-standing issue that the plasma density determined by PIC/MCC simulations shows quantitative deviations from experimental measurements, even in argon discharges, indicating that certain physics may be missing in previous modeling of the low pressure radio frequency (rf) driven capacitive discharges. In this work, we report that the energetic electron-induced secondary electron emission (SEE) and excited state atoms play an important role in low pressure rf capacitive argon plasma discharges. The ion-induced secondary electrons are accelerated by the high sheath field to strike the opposite electrode and produce a considerable number of secondary electrons that lead to additional ionizing impacts and further increase of the plasma density. Importantly, the presence of excited state species even further enhances the plasma density via excited state neutral and resonant state photon-induced SEE on the electrode surface. The PIC/MCC simulation results show good agreement with the recent experimental measurements in the low pressure range (1–10 Pa) that is commonly used for etching in the semiconductor industry. At the highest pressure (20 Pa) and driving voltage amplitudes 250 and 350 V explored here, the plasma densities from PIC/MCC simulations considering excited state neutrals and resonant photon-induced SEE are quantitatively higher than observed in the experiments, requiring further investigation on high pressure discharges.

Detailed study of quark-hadron duality in spin structure functions of the proton and neutron

Background: The response of hadrons, the bound states of the strong force (QCD), to external probes can be described in two different, complementary frameworks: as direct interactions with their fundamental constituents, quarks and gluons, or alternatively as elastic or inelastic coherent scattering that leaves the hadrons in their ground state or in one of their excited (resonance) states. The former picture emerges most clearly in hard processes with high momentum transfer, where the hadron response can be described by the perturbative expansion of QCD, while at lower energy and momentum transfers, the resonant excitations of the hadrons dominate the cross section. The overlap region between these two pictures, where both yield similar predictions, is referred to as quark-hadron duality and has been extensively studied in reactions involving unpolarized hadrons. Some limited information on this phenomenon also exists for polarized protons, deuterons, and $^{3}\mathrm{He}$ nuclei, but not yet for neutrons.Purpose: In this paper, we present comprehensive and detailed results on the correspondence between the extrapolated deep inelastic structure function ${g}_{1}$ of both the proton and the neutron with the same quantity measured in the nucleon resonance region. Thanks to the fine binning and high precision of our data, and using a well-controlled perturbative QCD (pQCD) fit for the partonic prediction, we can make quantitative statements about the kinematic range of applicability of both local duality and global duality.Method: We use the most updated QCD global analysis results at high $x$ from the Jefferson Lab Angular Momentum Collaboration to extrapolate the spin structure function ${g}_{1}$ into the nucleon resonance region and then integrate over various intervals in the scaling variable $x$. We compare the results with the large data set collected in the quark-hadron transition region by the CLAS Collaboration, including, for the first time, deconvoluted neutron data, integrated over the same intervals. We present this comparison as a function of the momentum transfer ${Q}^{2}$.Results: We find that, depending on the integration interval and the minimum momentum transfer chosen, a clear transition to quark-hadron duality can be observed in both nucleon species. Furthermore, we show, for the first time, the approach to scaling behavior for ${g}_{1}$ measured in the resonance region at sufficiently high momentum transfer.Conclusions: Our results can be used to quantify the deviations from the applicability of pQCD for data taken at moderate energies and can help with extraction of quark distribution functions from such data.

Excited-state resonance Raman spectroscopy probes the sequential two-photon excitation mechanism of a photochromic molecular switch.

Some diarylethene molecular switches have a low quantum yield for cycloreversion when excited by a single photon, but react more efficiently following sequential two-photon excitation. The increase in reaction efficiency depends on both the relative time delay and the wavelength of the second photon. This paper examines the wavelength-dependent mechanism for sequential excitation using excited-state resonance Raman spectroscopy to probe the ultrafast (sub-30fs) dynamics on the upper electronic state following secondary excitation. The approach uses femtosecond stimulated Raman scattering (FSRS) to measure the time-gated, excited-state resonance Raman spectrum in resonance with two different excited-state absorption bands. The relative intensities of the Raman bands reveal the initial dynamics in the higher-lying states, Sn, by providing information on the relative gradients of the potential energy surfaces that are accessed via secondary excitation. The excited-state resonance Raman spectra reveal specific modes that become enhanced depending on the Raman excitation wavelength, 750 or 400nm. Many of the modes that become enhanced in the 750nm FSRS spectrum are assigned as vibrational motions localized on the central cyclohexadiene ring. Many of the modes that become enhanced in the 400nm FSRS spectrum are assigned as motions along the conjugated backbone and peripheral phenyl rings. These observations are consistent with earlier measurements that showed higher efficiency following secondary excitation into the lower excited-state absorption band and illustrate a powerful new way to probe the ultrafast dynamics of higher-lying excited states immediately following sequential two-photon excitation.

Three-Electron Dynamics of the Interparticle Coulombic Decay in Doubly Excited Clusters with One-Dimensional Continuum Confinement.

A detailed analysis of the electronic structure and decay dynamics in a symmetric system with three electrons in three linearly aligned binding sites representing quantum dots (QDs) is given. The two outer A QDs are two-level potentials and can act as (virtual) photon emitters, whereas the central B QD can be ionized from its one level into a continuum confined on the QD axis upon absorbing virtual photons in the inter-Coulombic decay (ICD) process. Two scenarios in such an ABA array are explored. One ICD process is from a singly excited resonance state, whose decay releasing one virtual photon we find superimposed with resonance energy transfer among both A QDs. Moreover, the decay-process manifold for a doubly excited (DE) resonance is explored, in which collective ICD among all three sites and excited ICD among the outer QDs engage. Rates for all processes are found to be extremely low, although ICD rates with two neighbors are predicted to double compared to ICD among two sites only. The slowing is caused by Coulomb barriers imposed from ground or excited state electrons in the A sites. Outliers occur on the one hand at short distances, where the charge transfer among QDs mixes the possible decay pathways. On the other hand, we discovered a shape resonance-enhanced DE-ICD pathway, in which an excited and localized B* shape resonance state forms, which is able to decay quickly into the final ICD continuum.

Calculation of Resonance Raman Spectra and Excited State Properties for Blue Copper Protein Model Complexes

Laccase enzymes have been investigated for their potential as eco-friendly biofuel cell cathode catalysts. In these applications, electrons transfer from the electrode directly to the so-called Type 1 (T1) redox site within the enzyme. Over the years, resonance Raman (RR) spectroscopy has been utilized to probe the nature of ligand interactions with the Cu2+ center at the T1 site of the resting, oxidized form of laccases and related blue copper proteins. Spectral interpretation has been guided by parallel studies of small molecule T1 site mimics. Reported herein are results of time-dependent density functional theory (TDDFT) calculations performed on a series of experimentally well-studied T1 site mimics based on Cu2+ thiolate complexes stabilized by either a tridentate tris(pyrazolyl)hydroborate ligand or a bidentate β-diketiminate ligand. Vertical excitation energies were computed for the first 10 doublet states of each complex. The electronic absorption and RR spectra derived show excellent agreement with the experiment. RR spectra were dominated by a strong Cu–S stretching transition near 400 cm–1, which is sensitive to molecular conformation within the thiolate ligand. The results provide a foundation to build from in advancing models of laccase catalytic active sites in support of sustainable technologies.

Excited-state resonance tunneling in strong-field ionization

We study the tunneling dynamics of a system with two bound states. We find that the system can be effectively described by a two-level leakage cavity model, with which we can reproduce the dynamics of tunneling ionization. It shows that tunneling via the excited-state resonance dominates over the ground-state tunneling within the field strength considered, where the electron prefers jumping to the excited-state resonance channel first and then tunneling out. Moreover, several key parameters of the system naturally emerge in our two-level model. Especially, one of the parameters is the physical correspondence of the tunneling time obtained from the ground-state tunneling.

Autoionization decay and radiative decay for doubly excited 1Po resonance states in H− and Ps− ions

The autoionization decay (AD) and radiative decay (RD) for doubly excited 1Po resonance states in H− and Ps− are studied using correlated exponential wavefunctions based on the variational methods. To trace the resonance poles from Ps− to H−, the nuclear mass is varied from one unit to infinitely heavy. Resonance positions, widths related to AD rates, oscillator strengths, the ratios of oscillator strengths between the Feshbach and shape resonances (FSR), RD rates for the lowest two FSR in the 1Po spectrum, are reported. Few results are new, and others are comparable with the available theoretical and experimental data.

Excited-state spin-resonance spectroscopy of V{}_{{{{{{{{rm{B}}}}}}}}}^{-} defect centers in hexagonal boron nitride

The recently discovered spin-active boron vacancy (V{}_{{{{{{{{rm{B}}}}}}}}}^{-}) defect center in hexagonal boron nitride (hBN) has high contrast optically-detected magnetic resonance (ODMR) at room-temperature, with a spin-triplet ground-state that shows promise as a quantum sensor. Here we report temperature-dependent ODMR spectroscopy to probe spin within the orbital excited-state. Our experiments determine the excited-state spin Hamiltonian, including a room-temperature zero-field splitting of 2.1 GHz and a g-factor similar to that of the ground-state. We confirm that the resonance is associated with spin rotation in the excited-state using pulsed ODMR measurements, and we observe Zeeman-mediated level anti-crossings in both the orbital ground- and excited-state. Our observation of a single set of excited-state spin-triplet resonance from 10 to 300 K is suggestive of symmetry-lowering of the defect system from D3h to C2v. Additionally, the excited-state ODMR has strong temperature dependence of both contrast and transverse anisotropy splitting, enabling promising avenues for quantum sensing.

A Unique QP Partitioning and Siegert Width Using Real-Valued Continuum-Remover Potential.

A simple, practical quantum chemical procedure is presented for computing the energy position and the decay width of autoionization resonances. It combines the L2-stabilized resonance wave function obtained using the real-valued continuum-remover (CR) potential [Y. Sajeev Chem. Phys. Lett. 2013, 587, 105-112] and the Feshbach projection operator (FPO) partitioning technique. Unlike the conventional FPO partitioning of the total wave function into its resonant space and background space components, an explicit partitioning of the total wave function into its interaction region and noninteraction region components is obtained with the help of real-valued continuum-remover potential. The molecular system is initially confined inside a CR potential which removes the electronic continuum of the molecular system in which its resonance state is embedded and, thus, unravels the space component of the resonance wave function as a bound, localized eigenstate of the confined system. The eigenfunctions of the molecular Hamiltonian represented in the space constitute a complementary, orthogonal space. A unique partition is obtained when the level-shift of the space function due to its coupling with the space is zero, and the resonance width is computed using these unique partitioned spaces. This new procedure, which we refer to as CR-FPO formalism, is formally very simple and straightforward to implement, yet its applications to the resonance state of a model Hamiltonian and to the doubly excited resonance states of atomic and molecular systems at the full-CI level are very accurate as compared to the alternative, very precise L2 methods. In addition, the CR-FPO formalism is implemented in the multireference configuration interaction (MRCI) method, and uses it for calculating the energy position and the autionization decay width of 2Πg shape resonance in N2-.

Heralded preparation of spin qubits in droplet-etched GaAs quantum dots using quasiresonant excitation

We present a first comprehensive study on deterministic spin preparation employing excited state resonances of droplet etched GaAs quantum dots. This achievement facilitates future investigations of spin qubit based quantum memories using the GaAs quantum dot material platform. By observation of excitation spectra for a range of fundamental excitonic transitions the properties of different quantum dot energy levels, i.e. shells, are revealed. The innovative use of polarization resolved excitation and detection in quasi-resonant excitation spectroscopy facilitates determination of $85$ $\%$ maximum spin preparation fidelity - irrespective of the relative orientations of lab and quantum dot polarization eigenbases. Additionally, the characteristic non-radiative decay time is investigated as a function of ground state, excitation resonance and excitation power level, yielding decay times as low as $29$ ps for s-p-shell exited state transitions. Finally, by time resolved correlation spectroscopy it is demonstrated that the employed excitation scheme has a significant impact on the electronic environment of quantum dot transitions thereby influencing its charge and coherence.

Excited State Resonance Raman of Flavin Mononucleotide: Comparison of Theory and Experiment.

Blue light absorbing flavoproteins play important roles in a variety of photobiological processes. Consequently, there have been numerous investigations of their excited state structure and dynamics, in particular by time-resolved vibrational spectroscopy. The isoalloxazine chromophore of the flavoprotein cofactors has been studied in detail by time-resolved Raman, lending it a benchmark status for mode assignments in excited electronic states of large molecules. However, detailed comparisons of calculated and measured spectra have proven challenging, as there are many more modes calculated than are observed, and the role of resonance enhancement is difficult to characterize in excited electronic states. Here we employ a recently developed approach due to Elles and co-workers ( J. Phys. Chem. A 2018, 122, 8308-8319) for the calculation of resonance-enhanced Raman spectra of excited states and apply it to the lowest singlet and triplet excited states of the isoalloxazine chromophore. There is generally good agreement between calculated and observed enhancements, which allows assignment of vibrational bands of the flavoprotein cofactors to be refined. However, some prominently enhanced bands are found to be absent from the calculations, suggesting the need for further development of the theory.

Chemical Modification of Polaronic States in Anatase TiO2(101).

Two polymorphs of TiO2, anatase and rutile, are employed in photocatalytic applications. It is broadly accepted that anatase is the more catalytically active and subsequently finds wider commercial use. In this work, we focus on the Ti3+ polaronic states of anatase TiO2(101), which lie at ∼1.0 eV binding energy and are known to increase catalytic performance. Using UV-photoemission and two-photon photoemission spectroscopies, we demonstrate the capability to tune the excited state resonance of polarons by controlling the chemical environment. Anatase TiO2(101) contains subsurface polarons which undergo sub-band-gap photoexcitation to states ∼2.0 eV above the Fermi level. Formic acid adsorption dramatically influences the polaronic states, increasing the binding energy by ∼0.3 eV. Moreover, the photoexcitation oscillator strength changes significantly, resonating with states ∼3.0 eV above the Fermi level. We show that this behavior is likely due to the surface migration of subsurface oxygen vacancies.

Tailored Engineering of Novel Xanthonium Polymethine Dyes for Synergetic PDT and PTT Triggered by 1064 nm Laser toward Deep-Seated Tumors.

Small molecular dye that simultaneously exerts dual PDT/PTT effects as well as florescence imaging triggered by a single NIR-II light has never been reported to date. Apart from the huge challenge in pushing absorption profile into NIR-II region, fine-tuning dyes' excited state via rational structure design to meet all three functions, especially oxygen photosensitization, remains the most prominent throttle. Herein, five novel NIR-II dyes (BHs) are productively developed by strategically conjugating dyad innovative xanthonium with sequentially extended polymethine bridges, enabling intense absorption from 890 to 1206 nm, significantly 400 nm longer than conventional cyanine dyes with same polymethines. More importantly, owning to high resonance and favorable excited state energy population induced by greater rigidity via ring-fused amino, BH 1024 exhibits best singlet oxygen generation capability, moderate photothermal heating, and considerable fluorescence under 1064 nm laser irradiation. Furthermore, BH 1024 is encapsulated into folate-functionalized polymer, which demonstrated a synergetic PDT/PTT effect in vitro and in vivo, eventually achieving solid tumors elimination under NIR-II fluorescence guide. As far as it is known, this is the first time small molecular dyes for NIR-II PDT or NIR-II PDT/PTT are explored and designed.

Unbound spectra of neutron-rich oxygen isotopes predicted by the Gamow shell model

The Gamow shell model has shown to efficiently describe weakly bound and unbound nuclear systems, as internucleon correlations and continuum coupling are both taken into account in this model. In the present work, we study neutron-dripline oxygen isotopes. It is hereby demonstrated that the presence of continuum coupling is important for the description of oxygen isotopes at dripline, and especially to assess the eventual bound or unbound character of $^{28}$O. Our results suggest that the ground state of $^{28}$O is weakly unbound and is similar to the narrow resonant $^{26}$O ground state. Predictions of weakly bound and resonance excited states in $^{24\text-26}$O are also provided. The asymptotes of the studied many-body states are analyzed via one-body densities, whereby the different radial properties of well bound, loosely bound, resonance states are clearly depicted.