Intermediate Electronic States Research Articles

Non-adiabatic electronic relaxation of tetracene from its brightest singlet excited state.

The ultrafast relaxation dynamics of tetracene following UV excitation to the bright singlet state S6 has been studied with time-resolved photoelectron spectroscopy. With the help of high-level abinitio multireference perturbation theory calculations, we assign photoelectron signals to intermediate dark electronic states S3, S4, and S5 as well as to a low-lying electronic state S2. The energetic structure of these dark states has not been determined experimentally previously. The time-dependent photoelectron yields assigned to the states S6, S5, and S4 have been analyzed and reveal the depopulation of S6 within 60fs, while S5 and S4 are populated with delays of about 50 and 80fs. The dynamics of the lower-lying states S3 and S2 seem to agree with a delayed population coinciding with the depopulation of the higher-lying states S4-S6 but could not be elucidated in full detail due to the low signal levels of the corresponding two-photon ionization probe processes.

Much more to explore with an oxidation state of nearly four: Pr valence instability in intermetallic m-Pr2Co3Ge5.

For some intermetallic compounds containing lanthanides, structural transitions can result in intermediate electronic states between trivalency and tetravalency; however, this is rarely observed for praseodymium compounds. The dominant trivalency of praseodymium limits potential discoveries of emergent quantum states in itinerant 4f1 systems accessible using Pr4+-based compounds. Here, we use in situ powder x-ray diffraction and in situ electron energy-loss spectroscopy (EELS) to identify an intermetallic example of a dominantly Pr4+ state in the polymorphic system Pr2Co3Ge5. The structure-valence transition from a nearly full Pr4+ electronic state to a typical Pr3+ state shows the potential of Pr-based intermetallic compounds to host valence-unstable states and provides an opportunity to discover previously unknown quantum phenomena. In addition, this work emphasizes the need for complementary techniques like EELS when evaluating the magnetic and electronic properties of Pr intermetallic systems to reveal details easily overlooked when relying on bulk magnetic measurements alone.

Dynamics of photoexcited 5-bromouracil and 5-bromo-2'-deoxyuridine studied by extreme ultraviolet time-resolved photoelectron spectroscopy in liquid flat jets.

The UV-induced photo-relaxation dynamics of 5-bromouracil (BrU) and 5-bromo-2'-deoxyuridine (BrUrd) in aqueous solution were investigated using femtosecond time-resolved photoelectron spectroscopy with an extreme ultraviolet (XUV) probe in a flat liquid jet. Upon excitation to the 1ππ* state by 4.66 eV UV photons, both molecules exhibited rapid relaxation into lower-lying electronic states followed by decay to the S0 ground state. By employing a 21.7 eV XUV probe pulse, we were able to differentiate the relaxation of the excited state population from the initially excited 1ππ* state to an intermediate electronic state with 100 fs. Computational results identify this intermediate as the 1πσ* excited state, accessed by a 1ππ*/1πσ* conical intersection, and the signal from this intermediate state disappears within ∼200 fs. In contrast to thymine, formation of neither the 1nπ* state nor a long-lived triplet state was observed. Although the 1πσ* state is largely repulsive, prior studies have reported a low quantum yield for dissociation, and we observe weak signals that are consistent with production of hot S0 ground state (for BrUrd) on a time scale of 1.5-2 ps. It thus appears that solvent caging effects limit the dissociation yield in solution.

An Ab Initio Correction Vector Restricted Active Space Approach to the L-Edge XAS and 2p3d RIXS Spectra of Transition Metal Complexes.

We describe an ab initio approach to simulate L-edge X-ray absorption (XAS) and 2p3d resonant inelastic X-ray scattering (RIXS) spectroscopies. We model the strongly correlated electronic structure within a restricted active space and employ a correction vector formulation instead of sum-over-state expressions for the spectra, thus eliminating the need to calculate a large number of intermediate and final electronic states. We present benchmark simulations of the XAS and RIXS spectra of the iron complexes [FeCl4]1-/2- and [Fe(SCH3)4]1-/2- and interpret the spectra by deconvolving the correction vectors. Our approach represents a step toward simulating the X-ray spectroscopies of larger metal cluster systems that play a pivotal role in biology.

Relationship between the contact angle of pure Cu and its alloys owing to liquid Na and electronic states at the interface

Contact angle is an indicator of the wettability between liquid Na and pure metals. This has been evaluated using the atomic interactions obtained from the calculations of the electronic structure of the interface. This study aims to investigate the applicability of the atomic interactions of the interface to the alloys. An interface model between Cu and Na with an alloying element was constructed, and the electronic states of the interface were calculated by the molecular orbital calculation. The bond order, which indicates the strength of the covalent bonding at the interface, and ionicity, which indicates the amount of charge transfer, were obtained as theoretical parameters from the calculation. The contact angles between the Cu or Cu alloys and liquid Na were measured using a droplet of liquid Na at 423 K in a high-purity Ar atmosphere. The contact angles of the Cu alloys were evaluated using these theoretical parameters. As a result, a correlation was obtained between the ratio of the bond order between the substrate metal atoms to the bond order between the Na atom and the substrate metal atoms and the contact angle, which is consistent with previous studies. Furthermore, for the first time, the correlation between the ionicity or difference in the ionicity and contact angle was clarified. The difference in ionicity is the difference between the ionicity of Na atoms and that of the alloying element, indicating the strength of the ionic bonding. It was suggested that Cu and Cu alloys should consider covalent and ionic bonding when evaluating wettability, because Cu has an intermediate electronic state between transition and nontransition metals. Further, it became clear that the evaluation of the contact angle using the atomic interactions at the interface are applicable not only to pure metals but also to alloys.

Excitation and probing of low-energy nuclear states at high-energy storage rings

$^{229}\mathrm{Th}$ with a low-lying nuclear isomeric state is an essential candidate for a nuclear clock as well as many other applications. Laser excitation of the isomeric state has been a long-standing goal. With relativistic $^{229}\mathrm{Th}$ ions in storage rings, high-power lasers with wavelengths in the visible range or longer can be used to achieve high excitation rates of $^{229}\mathrm{Th}$ isomers. This can be realized through direct resonant excitation or excitation via an intermediate nuclear or electronic state, facilitated by the tunability of both the laser-beam and ion-bunch parameters. Unique opportunities are offered by highly charged $^{229}\mathrm{Th}$ ions due to the nuclear-state mixing. The significantly reduced isomeric-state lifetime corresponds to a much higher excitation rate for direct resonant excitation. Importantly, we propose electric dipole transitions changing both the electronic and nuclear states that are opened by the nuclear hyperfine mixing. We suggest using them for efficient isomer excitation in Li-like $^{229}\mathrm{Th}$ ions, via stimulated Raman adiabatic passage or single-laser excitation. We also propose schemes for probing the isomers, utilizing nuclear radiative decay or laser spectroscopy on electronic transitions, through which the isomeric-state energy can be determined with an orders-of-magnitude higher precision than the current value. The schemes proposed here for $^{229}\mathrm{Th}$ could also be adapted to low-energy nuclear states in other nuclei, such as $^{229}\mathrm{Pa}$.

Intermediate State between MoSe2 and Janus MoSeS during Atomic Substitution Process

Janus transition metal dichalcogenides (TMDCs), with dissimilar chalcogen atoms on each side of TMDCs, have garnered considerable research attention because of the out-of-plane intrinsic polarization in monolayer TMDCs. Although a plasma process has been proposed for synthesizing Janus TMDCs based on the atomic substitution of surface atoms at room temperature, the formation dynamics and intermediate electronic states have not been completely examined. In this study, we investigated the intermediate state between MoSe2 and Janus MoSeS during plasma processing. Atomic composition analysis and atomic-scale structural observations revealed the intermediate partially substituted Janus (PSJ) structure. Combined with theoretical calculations, we successfully clarified the characteristic Raman modes in the intermediate PSJ structure. The PL exhibited discontinuous transitions that could not be explained by the theoretical calculations. These findings will contribute toward understanding the formation process and electronic-state modulation of Janus TMDCs.

Improved charge-transfer resonance in graphene oxide/ZrO2 substrates for plasmonic-free SERS determination of methyl parathion

This work reports a sensitive SERS substrate based on graphene oxide (GO) and quantum-sized ZrO2 nanoparticles (GO/ZrO2) for label-free determination of the organophosphate pesticide methyl parathion (MP). The enhanced light-matter interactions and the consequent SERS effect in these substrates resulted from the effective charge transfer (CT) mechanism attributed to synergistic contributions of three main factors: i) the strong molecular adherence of the MP molecules and the ZrO2 surface which allows the first layer-effect, ii) the relatively abundant surface defects in low dimensional ZrO2 semiconductor NPs, which act as intermediate electronic states that reduce the large bandgap barrier, and iii) the hindered charge recombination derived from the transference of the photoinduced holes to the GO layer. This mechanism allowed an enhancement factor of 8.78 × 104 for GO/ZrO2-based substrates, which is more than 5-fold higher than the enhancement observed for platforms without GO. A detection limit of 0.12 μM was achieved with an outstanding repeatability (variation ≤4.5%) and a linear range up to 10 μM, which is sensitive enough to determine the maximal MP concentration permissible in drinking water according to international regulations. Furthermore, recovery rates between 97.4 and 102.1% were determined in irrigation water runoffs, strawberry and black tea extracts, demonstrating the reliability of the hybrid GO/ZrO2 substrate for the organophosphate pesticides quantification in samples related to agri-food sectors and environmental monitoring.

General theory of light propagation and triplet generation for studies of spin dynamics and triplet dynamic nuclear polarisation

A general theory is presented describing the photo-excitation dynamics of transient, paramagnetic triplet states of aromatic molecules including their absorption properties in a molecular host crystal when irradiated by a pulsed laser beam. It is applied to the system of a pentacene doped naphthalene single crystal, where the photo-excitation of pentacene results in a highly non-equilibrium population of intermediate electronic triplet states. These paramagnetic states can be addressed to align the proton spins of the naphthalene host at moderate experimental conditions using dynamic nuclear polarisation (DNP). The photo-excitation profile and the resulting triplet state distribution, which is determined by the material and the laser parameters, leads to a corresponding proton polarisation profile. Measurements of the resulting polarisation inhomogeneity by spatially resolved neutron transmission confirm the theory. The theoretical model then allows the extraction of crucial parameters that enable quantitative investigations of the dynamics of spin systems. For applications of triplet DNP with pentacene-doped naphthalene crystals, polarisation maximum, homogeneity and build-up rate are key factors, and the theory allows the identification of optimal experimental parameters depending on the priorities placed on these factors.

Strongly coupled intermediate electronic states in one-color two-photon single valence ionization of O2.

We present an experimental and theoretical energy- and angle-resolved investigation on the non-dissociative photoionization dynamics of near-resonant, one-color, two-photon, single valence ionization of neutral O2 molecules. Using 9.3eV femtosecond pulses produced via high harmonic generation and a 3-D momentum imaging spectrometer, we detect the photoelectrons and O2 + cations produced from one-color, two-photon ionization in coincidence. The measured and calculated photoelectron angular distributions show agreement, which indicates that a superposition of two intermediate electronic states is dominantly involved and that wavepacket motion on those near-resonantly populated intermediate states does not play a significant role in the measured two-photon ionization dynamics. Here, we find greater utility in the diabatic representation compared to the adiabatic representation, where invoking a single valence-character diabat is sufficient to describe the underlying two-photon ionization mechanism.

Sign switching of superexchange mediated by a few electrons in a nonuniform magnetic field

Long-range interaction between distant spins is an important building block for the realization of a large quantum-dot network in which couplings between pairs of spins can be selectively addressed. Recent experiments on the coherent oscillation of logical states between remote spins facilitated by intermediate electron states were the first step for large-scale quantum information processing. Reaching this ultimate goal requires extensive studies on the superexchange interaction in different quantum-dot spatial arrangements and electron configurations. Here, we consider a linear triple quantum dot with two antiparallel spins in the outer dots forming the logical states while varying numbers of electrons in the middle dot form a mediator, which facilitates the superexchange interaction. We show that the superexchange is enhanced when the number of mediating electrons increases. In addition, we show that forming a four-electron triplet in the mediator dot further enhances the superexchange strength. Our work can be a guide to scale up the quantum-dot array with controllable and dense connectivity.

In Situ Evolution of Secondary Metallic Phases in Off-Stoichiometric ZrNiSn for Enhanced Thermoelectric Performance.

The full-Heusler (FH) inclusions in the half-Heusler (HH) matrix is a well-studied approach to reduce the lattice thermal conductivity of ZrNiSn HH alloy. However, excess Ni in ZrNiSn may lead to the in situ formation of FH and/or HH alloys with interstitial Ni defects. The excess Ni develops intermediate electronic states in the band gap of ZrNiSn and also generates defects to scatter phonons, thus providing additional control to tailor electronic and phonon transport properties synergistically. In this work, we present the implication of isoelectronic Ge-doping and excess Ni on the thermoelectric transport of ZrNiSn. The synthesized ZrNi1.04Sn1-xGex (x = 0-0.04) samples were prepared by arc-melting and spark plasma sintering, and were extensively probed for microstructural analysis. The in situ evolution of minor secondary phases, i.e., FH, Ni-Sn, and Sn-Zr, primarily observed post sintering resulted in simultaneous optimization of the electrical power factor and lattice thermal conductivity. A ZT of ∼1.06 at ∼873 K was attained, which is among the highest for Hf-free ZrNiSn-based HH alloys. Additionally, ab initio calculations based on density functional theory (DFT) were performed to provide comparative insights into experimentally measured properties and understand underlying physics. Further, mechanical properties were experimentally extracted to determine the usability of synthesized alloys for device fabrication.

Enhanced photoelectrochemical aptasensing triggered by nitrogen deficiency and cyano group simultaneously engineered 2D carbon nitride for sensitively monitoring atrazine

Determination of atrazine (ATZ) residues in aquatic environment has important theoretical and practical significance for protecting the ecological environment and ensuring human health. A photoelectrochemical (PEC) aptasensing based on nitrogen deficiency and cyano group simultaneously engineered two-dimensional (2D) carbon nitride (named as DCN) has been proposed for sensitively detecting ATZ. The introduction of nitrogen deficiency can narrow band gap of DCN, leading to positive position of intermediate electronic state, and improving the absorption of visible light. The intermediate electronic state can function as an electron trap to promote the separation and migration of photogenerated carriers. Cyano groups can trap photoinduced electrons and suppress charge recombination. Gas produced by thermal decomposition has a tailoring effect, endowing DCN with 2D ultra-thin structure, which shortens transmission path of carriers. The synergistic effect of the above enhances PEC performance of DCN. The PEC aptasensing exhibited a wide linear range of 1.0 × 10−4 to 1.0 × 103 pM and a low detection limit of 3.33 × 10−5 pM with excellent selectivity, stability, and satisfactory accuracy during detecting real samples. The proposed design of carbon nitride-based materials may provide a better understanding of the relationship between kind of photoactive materials and PEC performance.

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.

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 Extreme Ultraviolet Photoelectron Spectroscopy of Nonadiabatic Photodissociation of CS2 from 1B2 (1Σu+) State: Product Formation via an Intermediate Electronic State.

We studied nonadiabatic dissociation of CS2 from the 1B2 (1Σu+) state using ultrafast extreme ultraviolet photoelectron spectroscopy. A deep UV (200 nm) laser using the filamentation four-wave mixing method and an extreme UV (21.7 eV) laser using the high-order harmonic generation method were employed to achieve the pump-probe laser cross-correlation time of 48 fs. Spectra measured with a high signal-to-noise ratio revealed clear dynamical features of vibrational wave packet motion in the 1B2 state; its electronic decay to lower electronic state(s) within 630 fs; and dissociation into S(1D2), S(3PJ), and CS fragments within 300 fs. The results suggest that both singlet and triplet dissociation occur via intermediate electronic state(s) produced by electronic relaxation from the 1B2 (1Σu+) state.

Correlated variational treatment of ionization coupled to nuclear motion: Ultrafast pump and ionizing probe of electronic and nuclear dynamics in LiH

Author(s): Bello, RY; Lucchese, RR; Rescigno, TN; McCurdy, CW | Abstract: We demonstrate a theoretical treatment of dissociative single ionization of the LiH molecule using two-color UV-UV pulse sequences that makes use of a highly correlated description of both the ionization continuum and target molecular ion and neutral states to which it is coupled. The present results emphasize how the details of the ionization process at various internuclear distances combine to form a lens through which such experiments image the dynamics of intermediate electronic states populated by the pump pulse. While ionization yields (dissociative and nondissociative) provide information about the amplitudes and phases that build up the molecular wave packet in the neutral states, molecular frame photoelectron angular distributions exhibit the changing character of those states, i.e., from ionic to covalent. In addition, the time-dependent mean kinetic energy of the wave packet on neutral states is clearly mapped onto the kinetic energy release of the atomic fragments produced by the probe ionization pulse.

Nonadiabatic contributions to Bragg-regime dynamics in atomic Kapitza-Dirac scattering

Atomic Kapitza-Dirac Bragg regime scattering is a multiphoton process in which a neutral atom undergoes a change of momentum through an interaction with a coherent light source. When the Bragg conditions are met, the outgoing atom beams are spatially quantized. Counterpropagating lasers act as pump and probe scattering from far-off-resonant excited intermediate electronic states, leaving the atoms in the electronic ground state with quantized transverse momentum. Each nontrivial scattering event imparts transverse velocity and therefore kinetic energy to the deflected atoms through recoil. In the Bragg regime, the loss of energy in the light fields is equal to the gain of kinetic energy in the atom. Energy nonconserving intermediate states, which are described by nonadiabatic contributions, are not accounted for by first-order off-resonant states. By comparing the solutions of increasing orders of off-resonant intermediate states, it becomes clear that calculating the correct Pendell\"{o}sung frequencies and phases requires including an ever increasing number of off-resonant states in proportion to the square root of the field strength.

Optically and electrically excited intermediate electronic states in donor:acceptor based OLEDs

Using spin-sensitive techniques, we show that optical excitation and electrical generation in donor:acceptor TADF OLEDs involve different excited state pathways towards light emission.

Role of chemical pressure on optical and electronic structure of Ho2GexTi2−xO7

Chemical pressure plays a crucial role in determining the electronic properties of the quantum materials. Investigation of electronic structure of Ho2GexTi2−xO7 (x = 2, 1.9, 1.75, 1.5 1, 0.5, 0.25, 0.1 and 0) series has been performed. Pyrochlore and Pyrogermanate, Re2B2O7 (Re = Ho3+, B = Ti4+ and Ge4+; rare earth titanates and germanates), substituted with increasing amount of Ge4+ at the Ti4+ site and vice versa develops structural distortions. Distinct shrinkage effect has been established in the Ho2Ti2O7 matrix upon Ge+4 substitutions at B site, resulting in the modification of band gap value. Band gap of 5.24 eV drastically drops to 3.92 eV with immediate Ti4+ substitution in Ho2Ge2O7. Electronic states of Ho3+ (4f forbidden transitions) had also been identified. We observe favored sub level transition (Specific Stark component) corresponding to5F5 to 5I8 electronic transition for Ho3+ at λexc. = 450 nm. The upper valence band consisted of O 2p state hybridized with Ho 5p and Ti and Ge 4p states and conduction band primarily formed by Ho 5d state hybridized with Ti 3d and Ge 4d states as obtained from density of states (DOS) calculations. Strong hybridization between Ho 5p1/2 and Ti 3p orbital upon Ti4+ inclusion in Ho2Ge2O7 has been observed through both theoretical studies using LDA—1/2 and UV–Vis, photoluminescence, ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy. The evolution of total DOS of all studied composition shows that valence band edge is more sensitive than conduction band to composition. These results provide chemical pressure as an excellent tool to tailor the band gap and fine tune the intermediate electronic states in Ho2GexTi2−xO7.