Higher Electronic States Research Articles

A theoretical study on excited-state dynamical properties and laser cooling of zinc monohydride including spin-orbit couplings.

By means of highly accurate ab initio and dynamical calculations, we identify a suitable laser cooling candidate that contains a transition metal element, namely zinc monohydride (ZnH). The internally contracted multireference configuration interaction method is employed to compute the five lowest-lying Λ-S states of ZnH with the spin-orbit coupling effects included, and very good agreement is obtained between our calculated and experimental spectroscopic data. Our findings show that the position of crossing point of the A2Π and B2Σ+ states of ZnH is above the v' = 2 vibrational level of the A2Π state indicating that the crossings with higher electronic states will have no effect on laser cooling. Hence, we construct a feasible laser-cooling scheme for ZnH using five lasers based on the A2Π1/2 → X2Σ+ 1/2 transition, which features a large vibrational branching ratio R 00 (0.8458), a large number of scattered photons (9.8 × 103) and an extremely short radiative lifetime (64ns). The present work demonstrates the importance of electronic state crossings and spin-orbit couplings in the study of molecular laser cooling.

Addressing electronic and dynamical evolution of molecules and molecular clusters: DFTB simulations of energy relaxation in polycyclic aromatic hydrocarbons.

We present a review of the capabilities of the density functional based Tight Binding (DFTB) scheme to address the electronic relaxation and dynamical evolution of molecules and molecular clusters following energy deposition via either collision or photoabsorption. The basics and extensions of DFTB for addressing these systems and in particular their electronic states and their dynamical evolution are reviewed. Applications to PAH molecules and clusters, carbonaceous systems of major interest in astrochemical/astrophysical context, are reported. A variety of processes are examined and discussed such as collisional hydrogenation, fast collisional processes and induced electronic and charge dynamics, collision-induced fragmentation, photo-induced fragmentation, relaxation in high electronic states, electronic-to-vibrational energy conversion and statistical versus non-statistical fragmentation. This review illustrates how simulations may help to unravel different relaxation mechanisms depending on various factors such as the system size, specific electronic structure or excitation conditions, in close connection with experiments.

Insights into electronic and transport properties of phosphorene nanorings in two perpendicular directions: Effects of circular and elliptical external potentials

In this work, we study the electronic and transport properties of phosphorene nanorings in two perpendicular directions (zigzag and armchair directions) in the presence of a zigzag metallic source and drain leads. Our results are based on the non-equilibrium Green’s function method and a five-parameter tight-binding approach. We investigate how system parameters affect the electronic transport. These parameters include the radius of the rings, the width of the leads, and the external potential. Our results show that for all configurations studied, a transport energy gap exists whose size can be tuned by the width of the leads and the radius of the nanoring. The transmission function of wider leads shows more sensitivity to the variation of the inner radius due to higher electronic states that can respond to smaller changes in the scattering region. In addition, the transport along the armchair direction is more susceptible to tuning than the transport along the zigzag direction. The effects of external potentials on the conductance are more pronounced than the geometrical parameters. In particular, the circular potential of the amplitude of 0.1 eV can widen the transport gap by about ∼0.35 eV.

Insight into selenium vacancies enhanced CoSe2/MoSe2 heterojunction nanosheets for hydrazine-assisted electrocatalytic water splitting

The integration of interface engineering and vacancy engineering was a feasible way to develop highly efficient electrocatalysts toward water electrolysis. Herein, we designed CoSe2/MoSe2 heterojunction nanosheets with abundant Se vacancies (VSe-CoSe2/MoSe2) for electrocatalytic water splitting. In the VSe-CoSe2/MoSe2 electrocatalyst, the electrons more easily transferred from CoSe2 to MoSe2, and interface engineering not only modulated the electronic structure, but also supplied more heterointerfaces and catalytic sites. After chemical etching, partial Se atoms were eliminated, which further activated the inert plane of the VSe-CoSe2/MoSe2 electrocatalyst and induced electron redistribution. The removal of surface Se atoms was also beneficial to expose inner reactive sites, which promoted adsorption toward reaction intermediates. Density functional theory calculations revealed that interface engineering and vacancy engineering collaboratively optimized the adsorption energy of the VSe-CoSe2/MoSe2 electrocatalyst toward the intermediate H* during the hydrogen evolution reaction process, leading to better electrocatalytic activity. The density of state diagram manifested the refined electronic structure of the VSe-CoSe2/MoSe2 electrocatalyst, and it exhibited a higher electronic state near the Fermi level, which indicated superior electronic conductivity, facilitating electron transport during the catalytic process. In alkaline media, the VSe-CoSe2/MoSe2 electrocatalyst delivered low overpotentials of merely 74 and 242 mV to obtain 10 mA cm−2 toward hydrogen evolution reaction and oxygen evolution reaction. This work illustrated the feasibility of combining two or more strategies to develop high-performance catalysts for water electrolysis.

Synergistic Enhancement Mechanism of High Electron Density and Localized Surface Plasmons for Strong Light‐Matter Interactions

Abstract2D layered semiconductor materials hold the potential to address the crucial technical challenges of poor signal uniformity and low repeatability of traditional metal plasmonic nanostructures in sensing, which hinder quantitative detection and analysis in molecular detection. However, the ultra‐low light absorption efficiency and electronic state density intrinsically result in their inferior sensitivity compared with those of conventional metal plasmonic nanostructures. Therefore, the integration of the classical back‐gate modulation strategy and surface plasmon resonance into 2D heterostructures is proposed. This integration aims to control the interaction between the surface sensing material of the heterostructure and target molecules from the perspective of surface electronic state density and optical absorption efficiency. The sensitivity (≈10−12 m) of as‐designed MoS2/graphene/Ag heterojunction is comparable to that of advanced metal plasmonic nanostructures, and the corresponding enhancement factor (EF, 1.71 × 104) is much higher than that of most heterojunction‐based and metal‐semiconductor coupled devices. This study uncovers the internal mechanism of the surface plasmon effect and high electronic state density in enhancing light‐matter interactions, and provides an alternative avenue for high‐performance surface‐enhanced devices.

Quantifying the Relaxation Dynamics of Higher Electronic Excited States in Perylene.

Gating logical operations through high-lying electronic excited states presents opportunities for developing ultrafast, subnanometer computational devices. A lack of molecular systems with sufficiently long-lived higher excited states has hindered practical realization of such devices, but recent studies have reported intriguing photophysics from high-lying excited states of perylene. In this work, we use femtosecond spectroscopy supported by quantum chemical calculations to identify and quantify the relaxation dynamics of monomeric perylene's higher electronic excited states. The 21B2u state is accessed through single-photon absorption at 250 nm, while the optically dark 21Ag state is excited via the 11B3u state. Population of either state results in subpicosecond relaxation to the 11B3u state, and we quantify 21Ag and 21B2u state lifetimes of 340 and 530 fs, respectively. These lifetimes are significantly longer than the singlet fission time constant from the perylene 21B2u state, suggesting that the higher electronic states of perylene may be useful for gating logical operations.

Coexistence of Zero-Dimensional Electride State and Superconductivity in AlH2 Monolayer

Electrides, which confine “excess anionic electrons” in subnanometer-sized cavities of a lattice, are exotic ionic crystals. We propose a non-stoichiometric strategy to realize intrinsic two-dimensional (2D) superconducting electride. AlH2 monolayer, which is structurally identical to 1H-MoS2, possesses zero-dimensionally confined anionic electrons in the interstitial sites of Al triangles, corresponding to a chemical formula of [AlH2]+e−. The interaction between interstitial anionic electrons (IAEs) and host cation lattice mainly accounts for stabilization of 1H-AlH2 electride. Impressively, 1H-AlH2 monolayer is an intrinsic Bardeen–Cooper–Schrieffer superconductor with T c = 38 K, which is the direct consequence of strong coupling of the H-dominated high electronic states with Al acoustic branch vibrations and mid-frequency H-derived phonon softening modes caused by Kohn anomalies. Under tensile strain, IAEs transform into itinerant electrons, favoring the formation of stable Cooper pairs. Therefore, T c reaches up to 53 K at a biaxial fracture strain of 5%. Our findings provide valuable insights into the correlation between non-stoichiometric electrides and superconducting microscopic mechanisms at the 2D limit.

Two-photon excitation with finite pulses unlocks pure dephasing-induced degradation of entangled photons emitted by quantum dots

Semiconductor quantum dots have emerged as an especially promising platform for the generation of polarization-entangled photon pairs. However, it was demonstrated recently that the two-photon excitation scheme employed in state-of-the-art experiments limits the achievable degree of entanglement by introducing which-path information. In this work, the combined impact of two-photon excitation and longitudinal acoustic phonons on photon pairs emitted by strongly-confining quantum dots is investigated. It is found that phonons further reduce the achievable degree of entanglement even in the limit of vanishing temperature due to phonon-induced pure dephasing and phonon-assisted one-photon processes, which increase the reexcitation probability. In addition, the degree of entanglement, as measured by the concurrence, decreases with rising temperature and/or pulse duration, even if the excitonic fine-structure splitting is absent and when higher electronic states are out of reach. Furthermore, in the case of finite fine-structure splittings, phonons enlarge the discrepancy in concurrence for different laser polarizations.

Excited Electronic States of Sr2: Ab Initio Predictions and Experimental Observation of the 21Σu+ State.

Despite its apparently simple nature with four valence electrons, the strontium dimer constitutes a challenge for modern electronic structure theory. Here we focus on excited electronic states of Sr2, which we investigate theoretically up to 25000 cm-1 above the ground state, to guide and explain new spectroscopic measurements. In particular, we focus on potential energy curves for the 11Σu+, 21Σu+, 11Πu, 21Πu, and 11Δu states computed using several variants of ab initio coupled-cluster and configuration-interaction methods to benchmark them. In addition, a new experimental study of the excited 21Σu+ state using polarization labeling spectroscopy is presented, which extends knowledge of this state to high vibrational levels, where perturbation by higher electronic states is observed. The available experimental observations are compared with the theoretical predictions and help to assess the accuracy and limitations of employed theoretical models. The present results pave the way for future more accurate theoretical and experimental spectroscopic studies.

A beginners guide to understanding the mechanisms of photochemical reactions: things you should know if light is one of your reagents.

The increasing popularity of applied photochemistry has changed the composition of the practitioners of photochemistry, from traditional specialists, to users whose expertise lies elsewhere, yet they find light as a useful and powerful reagent. I introduce Kasha's rule very early in this tutorial; this unconventional approach allows me to bypass information about high electronic states in favor of the lowest singlet and triplet excited states. Doing this I try to provide a fast entry enabling newcomers in the field of applied photochemistry to have a taste of what the field has to offer, in the hope that they will like what they see, and venture further into the many resources available to go deeper into the fascinating field of organic photochemistry.

Highly Reversible Solid‐State Lithium‐Oxygen Batteries by Size‐Matching Between Fe‐Fe Cluster and Li2‐xO2

AbstractSolid‐state lithium‐oxygen batteries (SSLOBs) offer high energy density with enhanced safety. However, the challenge of energy loss induced by the high polarization of the solid‐state air electrode has become a major bottleneck for its further development. Here, a homonuclear Fe‐Fe catalyst (Fe2‐N‐C) matched to reactants (Li2‐xO2, 0 ≤ x ≤ 2) size to alleviate the redox polarization in solid‐state electrodes is designed. Atomically resolved transmission electron microscopy reveals that the spatial size of Fe‐Fe clusters (1.5–2 Å) is commensurate with Li2‐xO2 (1.2–2.2 Å), which can overcome the challenges associated with catalyst/reactant size mismatch. In‐depth theoretical analyses show that orbital coupling and spin state jumping between Fe‐Fe sites allows the Fe centers to exhibit d‐orbital delocalization and high electronic spin states. These features enhance the activation of paramagnetic oxygen species, optimize the binding strength to LiO2 and reduce the charge/discharge voltage gap. Meanwhile, the size‐matching effect induces the ductile growth of the discharge product into a highly reversible film‐like morphology. The Fe2‐N‐C cathode with low overpotential exhibits significantly improved round‐trip efficiency (84.9%). The concept of this work opens up new opportunities for designing high‐performance SSLOBs.

Ab initio calculation of the adsorption of As, Cd, Cr, and Hg heavy metal atoms onto the illite(001) surface: Implications for soil pollution and reclamation

Elucidating the mechanisms of heavy metal (HM) adsorption on clay minerals is key to solving HM pollution in soil. In this study, the adsorption of four HM atoms (As, Cd, Cr, and Hg) on the illite(001) surface was investigated using density functional theory calculations. Different adsorption configurations were investigated and the electronic properties (i.e., adsorption energy (Ead) and electron transfer) were analyzed. The Ead values of the four HM atoms on the illite(001) surface were found to be As > Cr > Cd > Hg. The Ead values for the most stable adsorption configurations of As, Cr, Cd, and Hg were −1.8554, −0.7982, −0.3358, and −0.2678 eV, respectively. The As atoms show effective chemisorption at all six adsorption sites, while Cd, Cr, and Hg atoms mainly exhibited physisorption. The hollow and top (O) sites were more favorable than the top (K) sites for the adsorption of HM atoms. The Gibbs free energy results show that the illite(001) surface was energetically favorable for the adsorption of As and Cr atoms under the influence of 298 K and 1 atm. After adsorption, there was a redistribution of positions and reconfiguration of the chemical bonding of the surface atoms, with a non-negligible influence around the upper surface atoms. Bader charge analysis shows electrons were transferred from the surface to the HM atoms, and a strong correlation between the valence electron variations and the adsorption energy was observed. HM atoms had a high electronic state overlap with the surface O atoms near the Fermi energy level, indicating that the surface O atoms, though not the topmost atoms around the surface, significantly influence HM adsorption. The above results show illite(001) preferentially adsorbed As among all four investigated HM atoms, indicating that soils containing a high proportion of illite might be more prone to As pollution.

Coherent Intramolecular Excimer Formation in Solid [2,2]‐Paracyclophane: Time‐Resolved Springing of a Molecular “Trap”

AbstractImpulsive UV excitation unleashes violent wave packet motions in [2,2]‐paracyclophane (2PC) due to relief of ground state inter‐ring repulsion. Contrary to theoretical predictions for the benzene excimer, transient absorption spectroscopy in oriented single crystals and in liquid THF demonstrates that excited 2PC absorbs visible light polarized along all three molecular axes. Absorption polarized perpendicular to the aromatic rings is deeply modulated in time by the ∼0.5 Å of light induced inter‐ring breathing providing changes in potential energy gaps and dipole strengths during coherent intramolecular excimer formation. Overlapping transitions to several higher electronic states and strong non‐Condon effects contribute to the modulating spectra. These insights prove current understanding of the simplest archetypal organic excimer is fundamentally flawed and provide the basis for testing improved electronic structure models. It is hard to overestimate the significance of achieving such improved understanding given the impact of excimers and exciplexes on all aspects of organic molecular photophysics.

Conformer-Specific Dissociation Dynamics in Dimethyl Methylphosphonate Radical Cation.

The dynamics of the dimethyl methylphosphonate (DMMP) radical cation after production by strong field adiabatic ionization have been investigated. Pump-probe experiments using strong field 1300 nm pulses to adiabatically ionize DMMP and a 800 nm non-ionizing probe induce coherent oscillations of the parent ion yield with a period of about 45 fs. The yields of two fragments, PO2C2H7+ and PO2CH4+, oscillate approximately out of phase with the parent ion, but with a slight phase shift relative to each other. We use electronic structure theory and nonadiabatic surface hopping dynamics to understand the underlying dynamics. The results show that while the cation oscillates on the ground state along the P=O bond stretch coordinate, the probe excites population to higher electronic states that can lead to fragments PO2C2H7+ and PO2CH4+. The computational results combined with the experimental observations indicate that the two conformers of DMMP that are populated under experimental conditions exhibit different dynamics after being excited to the higher electronic states of the cation leading to different dissociation products. These results highlight the potential usefulness of these pump-probe measurements as a tool to study conformer-specific dynamics in molecules of biological interest.

Time-Dependent Quantum Dynamics Study of the Interaction between Helium Atom and Synchrotron Radiation

For the first time, excitation and ionization of helium atom (He) induced by synchrotron radiation(SR) is simulated in the present work. It is found that electrons can tunnel through the ground state of helium atom and leave from the atomic region caused by subsequent SR field. Therefore, the ionization of He atom can happen. Additionally, some electrons can be captured by a higher electronic state, which leads to the tunneling excitation from the ground state to a specific excited state.

Excellent Ultracold Molecular Candidates From Group VA Hydrides: Whether Do Nearby Electronic States Interfere?

By means of highly accurate ab initio calculations, we identify two excellent ultracold molecular candidates from group VA hydrides. We find that NH and PH are suitable for the production of ultracold molecules, and the feasibility and advantage of two laser cooling schemes are demonstrated, which involve different spin-orbit states ( and ). The internally contracted multireference configuration interaction method is applied in calculations of the six low-lying Λ-S states of NH and PH with the spin-orbit coupling effects included, and excellent agreement is achieved between the computed and experimental spectroscopic data. We find that the locations of crossing point between the and states of NH and PH are higher than the corresponding v′ = 2 vibrational levels of the state indicating that the crossings with higher electronic states would not affect laser cooling. Meanwhile, the extremely small vibrational branching loss ratios of the → transition for NH and PH (NH: 1.81 × 10–8; PH: 1.08 × 10–6) indicate that the intermediate electronic state will not interfere with the laser cooling. Consequently, we construct feasible laser-cooling schemes for NH and PH using three lasers based on the → transition, which feature highly diagonal vibrational branching ratio (NH: 0.9952; PH: 0.9977), the large number of scattered photons (NH: 1.04×105; PH: 8.32×106) and very short radiative lifetimes (NH: 474 ns; PH: 526 ns). Our work suggests that feasible laser-cooling schemes could be established for a molecular system with extra electronic states close to those chosen for laser-cooling.

Computational evaluation of FeMo heteroatom coeffect induced high electroreduction activity of N2-to-NH3

Fe-Mo system are recognized as the most successful nitrogenases system towards the synthesis of ammonia. At the same time, Fe or Mo plays a key role in nitrogenase enzymes together with the most common catalyst for nitrogen reduction reaction (NRR). Herein, we designed and explored NRR catalysts of bimetallic Fe-Mo atoms incorporating on three 2D materials, including graphdiyne (GD), Nitrogen atoms substituted graphene (G-N6) and C2N, by means of density functional theory. Fe-Mo atoms supported on C2N is calculated to the most promising one owing to its lowest limiting potential (−0.17 V) and excellent selectivity against hydrogen evolution reaction. The superior performance of FeMo dual-atom NRR catalysts has also been verified by comparing to individual Fe or Mo involved catalysts on account of synergistic effects. Besides, the high NRR activity of FeMo@C2N could be ascribed to the high localized electronic states of 3d/4d and 2p orbitals near the Fermi level, providing the Fe-Mo active sites with optimal electronic state. Our studies would further inspire some experimental and theoretical researchers to find out the efficient catalysts with high activity for the synthesis of ammonia at room temperature.

Production of ultracold CaCCH and SrCCH molecules by direct laser cooling: A theoretical study based on accurate ab initio calculations.

Laser cooling of polyatomic molecules to the ultracold regime may enable some new science and technology applications; however, the related study is still at its very early stage. Here, by means of accurate ab initio and dynamical calculations, we identify two new candidate tetratomic molecules that are suitable for laser cooling and demonstrate the feasibility and advantage of two laser cooling schemes that are able to produce ultracold CaCCH and SrCCH molecules. The internally contracted multiconfiguration reference configuration interaction method is applied, and excellent agreement is achieved between the computed and experimental spectroscopic data. We find that the X2Σ1/2 +→A2Π1/2 transitions for both candidates feature diagonal Franck-Condon factors, short radiative lifetimes, and no interference from intermediate electronic states. In addition, the crossings with higher electronic states do not interfere. We further construct feasible laser cooling schemes for CaCCH and SrCCH, each of which allows scattering 104 photons for direct laser cooling. The estimated Doppler temperatures for both CaCCH and SrCCH are on the order of μK.

Ultrafast electron energy-dependent delocalization dynamics in germanium selenide

Ultrafast scattering process of high-energy carriers plays a key role in the performance of electronics and optoelectronics, and have been studied in several semiconductors. Core-hole clock spectroscopy is a unique technique for providing ultrafast charge transfer information with sub-femtosecond timescale. Here we demonstrate that germanium selenide (GeSe) semiconductor exhibits electronic states-dependent charge delocalization time by resonant photo exciting the core electrons to different final states using hard-x-ray photoemission spectroscopy. Thanks to the experiment geometry and the different orbital polarizations in the conduction band, the delocalization time of electron in high energy electronic state probed from Se 1s is ~470 as, which is three times longer than the delocalization time of electrons located in lower energy electronic state probed from Ge 1s. Our demonstration in GeSe offers an opportunity to precisely distinguish the energy-dependent dynamics in layered semiconductor, and will pave the way to design the ultrafast devices in the future.

Fermionic criticality is shaped by Fermi surface topology: a case study of the Tomonaga-Luttinger liquid

We perform a unitary renormalization group (URG) study of the 1D fermionic Hubbard model. The formalism generates a family of effective Hamiltonians and many-body eigenstates arranged holographically across the tensor network from UV to IR. The URG is realized as a quantum circuit, leading to the entanglement holographic mapping (EHM) tensor network description. A topological Θ-term of the projected Hilbert space of the degrees of freedom at the Fermi surface are shown to govern the nature of RG flow towards either the gapless Tomonaga-Luttinger liquid or gapped quantum liquid phases. This results in a nonperturbative version of the Berezenskii-Kosterlitz-Thouless (BKT) RG phase diagram, revealing a line of intermediate coupling stable fixed points, while the nature of RG flow around the critical point is identical to that obtained from the weak-coupling RG analysis. This coincides with a phase transition in the many-particle entanglement, as the entanglement entropy RG flow shows distinct features for the critical and gapped phases depending on the value of the topological Θ-term. We demonstrate the Ryu-Takyanagi entropy bound for the many-body eigenstates comprising the EHM network, concretizing the relation to the holographic duality principle. The scaling of the entropy bound also distinguishes the gapped and gapless phases, implying the generation of very different holographic spacetimes across the critical point. Finally, we treat the Fermi surface as a quantum impurity coupled to the high energy electronic states. A thought-experiment is devised in order to study entanglement entropy generated by isolating the impurity, and propose ways by which to measure it by studying the quantum noise and higher order cumulants of the full counting statistics.