Higher Electronic States Research Articles

Parametric dependence of hot electron relaxation timescales on electron-electron and electron-phonon interaction strengths

Understanding how photoexcited electron dynamics depend on electron-electron (e-e) and electron-phonon (e-p) interaction strengths is important for many fields, e.g. ultrafast magnetism, photocatalysis, plasmonics, and others. Here, we report simple expressions that capture the interplay of e-e and e-p interactions on electron distribution relaxation times. We observe a dependence of the dynamics on e-e and e-p interaction strengths that is universal to most metals and is also counterintuitive. While only e-p interactions reduce the total energy stored by excited electrons, the time for energy to leave the electronic subsystem also depends on e-e interaction strengths because e-e interactions increase the number of electrons emitting phonons. The effect of e-e interactions on energy-relaxation is largest in metals with strong e-p interactions. Finally, the time high energy electron states remain occupied depends only on the strength of e-e interactions, even if e-p scattering rates are much greater than e-e scattering rates.

Ultraviolet photodissociation of gas-phase transition metal complexes: dicarbonylcyclopentadienyliodoiron(II)

The ultraviolet photodissociation of the prototypical organometallic half-sandwich compound dicarbonylcyclopentadienyliodoiron(II) [η 5-CpFe(CO)2I] has been studied in the gas phase across the wavelength range 260 ≤ λ ≤ 310 nm using multi-mass velocity-map ion imaging with photoproducts detected using both resonance enhanced multiphoton and vacuum ultraviolet (λ = 118.2 nm) single photon ionisation methods. Ion images recorded for the atomic iodine and the cyclopentadienyl photoproducts reveal fast, anisotropic components to their recoil velocity distributions. The experimental work is supported by multi-reference (spin–orbit averaged) electronic structure calculations that suffice to illustrate the high electronic state density in such transition metal complexes and provide insights into the rival fragmentation dynamics. The ground state parent molecule has singlet spin multiplicity, but the product energy disposal measured following Fe–Cp bond fission shows the involvement of nominally spin-forbidden transitions. The Fe–I and Fe–Cp bond fissions should both be viewed as homolytic and occurring on excited state potentials that are dissociative in the relevant ligand elimination coordinate.

Lanthanide (III) ions as multichannel acceptors for bimolecular photoinduced electron transfer reactions with coumarin dyes

Understanding the kinetics and energetics of photoinduced electron transfer (PET) reactions is of considerable research interest in photochemical sciences. In this study, interactions between trivalent lanthanide ions (Ln(III)) as electron acceptors and coumarin dyes as electron donors have been investigated in aqueous media using ground state absorption, steady-state (SS) emission, time-resolved (TR) fluorescence and laser flash photolysis (LFP) techniques. Among different Ln(III) ions, only Eu(III), Yb(III) and Sm(III) are found to display substantial interaction with the excited singlet (S1) state of the coumarin dyes, which correspond well with the favorable free energy changes (negative ΔG0) associated with the PET reactions in these coumarin-Ln(III) systems. For other Ln(III) ions, as ΔG0 is not favorable, they do not show any interaction with the excited coumarin dyes. Among the interacting coumarin-Ln(III) systems, bimolecular quenching rate constant (kq) increases consistently with the increasing exergonicity (−ΔG0), when each of the Ln(III) ions are considered independently, except for Eu(III), for which the kq values have already reached the diffusion controlled limit. However, considering all the Ln(III) ions together, the kq values do not show any common correlation with the changing ΔG0 values. The kq values are in general much lower for Yb(III) quencher as compared to Sm(III), even though the exergonicity values for coumarin-Yb(III) systems are more favorable than the coumarin-Sm(III) systems. Correlating the observed results based on the diffusion mediated bimolecular reaction model, we infer that depending on the ΔG0 values, some Ln(III) quenchers possibly also engage their upper electronic energy states as well to assist multichannel ET with the excited coumarin donors, and thereby modulating the kq values very substantially for different coumarin-Ln(III) systems. Apparently, the involvement of multichannel ET causes the total reorganization energy (λ) for ET reactions to differ largely for different Ln(III) ions, which would otherwise be very similar for all the coumarin-Ln(III) pairs. Observed results suggest that the intramolecular reorganization energy (λi) associated with lower electronic states of the hydrated Ln(III) ions is much larger than that associated with higher electronic states of these ions. While the involvement of multichannel ET is itself an intriguing inference, modulation in the ET kinetics and total λ values for different Ln(III) quenchers through the differential participation of the multichannel PET is also expected to find potential uses in the controlled utilization of such PET reactions in many practical areas.

Investigation on Photophysical, Solvatochromism and Biological Significance of Substituted 2H‐Indazole Derivatives

AbstractThe electronic properties of substituted 2H‐indazoles were investigated for their photophysical behavior under isolated conditions and in solvent environment, both computationally and experimentally. DFT calculations with CAM−B3LYP functional and 6–311++G** basis set were performed in the ground and excited states. The effect of solvents on the low and high energy electronic states was studied by following the steady state absorption and fluorescence excitation and emission spectra. Solvatochromic interactions including general and specific solvent effects are reported along with the results of Lippert and ET(30) plots. The biological significance of these 2H‐indazole derivatives was also investigated through docking studies. The interactions of 2H‐indazole derivatives with the active site of subdomain II of human casein kinase (C2 K) alpha are reported.

Boosting Catalysis of Pd Nanoparticles in MOFs by Pore Wall Engineering: The Roles of Electron Transfer and Adsorption Energy.

The chemical environment of metal nanoparticles (NPs) possesses significant influence on their catalytic performance yet is far from being well understood. Herein, tiny Pd NPs are encapsulated into the pore space of metal-organic frameworks (MOFs), UiO-66-X (X = H, OMe, NH2 , 2OH, 2OH(Hf)), affording Pd@UiO-66-X composites. The surface microenvironment of the Pd NPs is readily modulated by pore wall engineering, via the functional group and metal substitution in the MOFs. Consequently, the catalytic activity of Pd@UiO-66-X follows the order of Pd@UiO-66-OH > Pd@UiO-66-2OH(Hf) > Pd@UiO-66-NH2 > Pd@UiO-66-OMe > Pd@UiO-66-H toward the hydrogenation of benzoic acid. It is found that the activity difference is not only ascribed to the distinct charge transfer between Pd and the MOF, but is also explained by the discriminated substrate adsorption energy of Pd@UiO-66-X (-OH < -2OH(Hf) < -NH2 < -OMe < -H), based on CO-diffuse reflectance infrared Fourier transform spectra and density-functional theory (DFT) calculations. The Pd@UiO-66-OH, featuring a high Pd electronic state and moderate adsorption energy, displays the highest activity. This work highlights the influence of the surface microenvironment of guest metal NPs, the catalytic activity of which is dominated by electron transfer and the adsorption energy, via the systematic substitution of metal and functional groups in host MOFs.

Formation and Stabilization of Ground and Excited-State Singlet O2 upon Recombination of 3P Oxygen on Amorphous Solid Water.

The recombination dynamics of 3P oxygen atoms on cold amorphous solid water to form triplet and singlet molecular oxygen (O2) is investigated under conditions representative of cold clouds. Reactive molecular dynamics simulations including Landau-Zener-based hopping to account for nonadiabatic transitions find that both ground-state (X3Σg-) O2 and molecular oxygen in the two lowest singlet states (a1Δg and b1Σg+) can be formed and the molecular species stabilize through vibrational relaxation. The relative populations of the species are approximately 1:1:1. These results also agree qualitatively with a kinetic model based on simplified wavepacket simulations. The presence and stabilization of higher electronic states of O2 are expected to modify the chemical evolution of cold interstellar (T ∼ 10-50 K) and warmer noctilucent (T ∼ 100 K) clouds.

Rotatory Response of Molecular Electron Momentum Densities in Linear, Homogeneous Weak Electric Fields: A Topographical Analysis.

One-electron properties for molecules such as electron density, electrostatic potential (ESP), and electron momentum density (EMD) are experimentally tractable quantities, useful in understanding chemical characteristics. In this work, effects of a uniform homogeneous external electric field on some characteristic one-electron properties of simple molecules are analyzed. EMDs and ESPs were used to understand the response of water, hydrogen fluoride, carbon monoxide, chloroacetylene, and ammonia in an electric field. Remarkably, the EMD maxima for these molecules get rotated as the electric field strength is varied. A greater order of change in EMD than in ESP with increasing electric field strength brings out the sensitivity of the EMDs, especially for the valence electronic region, which in the momentum space is mapped onto the vicinity of its origin. The eigenvectors of the EMD Hessian maxima at the momentum-space origin are seen to rotate as a function of increasing field strength, with the extra angular momentum imparted by the field manifesting itself as reconfiguration of the EMD distribution. In the presence of the field, valence states may couple with higher electronic states, leading to a mixing of the states resulting in avoided crossings as a function of the field strength. The avoided crossings legitimately estimate maximal field strength limits for the calculation, prior to ionization.

Coupling effects and ultrafast third-order nonlinear optical behavior in ion-implanted silicon quantum dots and platinum nanoclusters

Third-order nonlinear optical properties exhibited by ion-implanted hybrid systems integrated by Silicon quantum dots and Platinum nanoclusters embedded in a Silica host matrix are reported. Femto-, pico- and nano-second pulses were employed as optical sources with excitation wavelengths at 830 nm, 355 nm and 532 nm, respectively. Transmission electron microscopy studies revealed sizes between 3 and 7 nm for Si quantum dots and 0.5–4 nm for Pt nanoclusters. Quantum confinement effects in this nanoscale seem to dominate the physical properties of the nanostructures and they must be taken into account to describe the interaction of intense laser beams with the hybrid nanocomposites. Femtosecond Z-scan studies showed a strong nonlinearity provided by the Si quantum dots influence, with no important contribution from the Pt nanoparticles to the nonlinear absorption and positive refractive index exhibited by the sample at 830 nm wavelength. Picosecond two-wave mixing experiments with self-diffraction indicate an enhancement close to one order of magnitude at 355 nm compared to the 532 nm wavelength of excitation. The picosecond results can be explained from the active participation of Pt nanoclusters together with Si quantum dots which can be efficiently excited at shorter wavelengths to reach higher electronic states above their energy band gaps. This selectable behavior by changing wavelength and pulse duration of excitation can be useful for tuning multifunctional operations performed by all-optical nanodevices.

Concentration-Dependent Impact of Alkali Li Metal Doped Mesoporous TiO2 Electron Transport Layer on the Performance of CH3NH3PbI3 Perovskite Solar Cells

TiO2 is most commonly employed as an electron transport layer (ETL) in mesoscopic n–i–p perovskite solar cells (PSCs). However, the low electron mobility, low electrical conductivity, and high electronic trap states of TiO2 may have negative impacts on further enhancement of PSC performance. Metal doping is an efficient way to improve the electronic properties of TiO2 films. In this work, we investigate the concentration-dependent impact of alkali lithium metal doping of the mesoporous TiO2 ETL on the performance of mesoscopic CH3NH3PbI3 PSCs. It was found that Li doping results in remarkable improvement in electrical conductivity and electron mobility and reduces the number of electronic trap states arising due to the oxygen vacancies within TiO2 lattice. Such enhancements led to an enhanced charge extraction and transport and reduced charge recombination rate at the perovskite/mesoporous TiO2 interface as revealed by steady-state photoluminescence (PL) and time-resolved PL (TRPL) spectra, and resulted i...

Atomic‐Scale Insights into the Low‐Temperature Oxidation of Methanol over a Single‐Atom Pt1‐Co3O4 Catalyst

AbstractHeterogeneous catalysts with single‐atom active sites offer a means of expanding the industrial application of noble metal catalysts. Herein, an atomically dispersed Pt1‐Co3O4 catalyst is presented, which exhibits an exceptionally high efficiency for the total oxidation of methanol. Experimental and theoretical investigations indicate that this catalyst consists of Pt sites with a large proportion of occupied high electronic states. These sites possess a strong affinity for inactive Co2+ sites and anchor over the surface of (111) crystal plane, which increases the metal–support interaction of the Pt1‐Co3O4 material and accelerates the rate of oxygen vacancies regeneration. In turn, this is determined to promote the coadsorption of the probe methanol molecule and O2. Density functional theory calculations confirm that the electron transfer over the oxygen vacancies reduces both the methanol adsorption energy and activation barriers for methanol oxidation, which is proposed to significantly enhance the dissociation of the CH bond in the methanol decomposition reaction. This investigation serves as a solid foundation for characterizing and understanding single‐atom catalysts for heterogeneous oxidation reactions.

Photoelectron imaging spectroscopy and photodynamics of m-xylene cations

The delayed photofragmentation and photoelectron spectroscopy coupled with DFT calculations were employed for the investigation of the relaxation dynamics of m-xylene cations. The multiphoton ionization induced by the pump pulse was used for the preparation of the m-xylene cations, and the population was characterized by the technique of photoelectron imaging. A time-delayed probe pulse was used to excite the prepared m-xylene cations to energetically higher electronic states, which undergo fragmentation. The relaxation dynamics of the m-xylene cations was tracked by monitoring the depletion of excited parent-ion and the formation of the fragment-ion. Two relaxation time constants are determined by monitoring the depletion of the excited parent-ion and the formation of the fragment-ion with the delay time. The first component of τ1 = 764 (±25) fs is attributed to an intramolecular vibrational redistribution process within the D0 state and the slow one of τ2 > 440 ns is ascribed to the long lifetime of the cations.

Solution of the Rovibrational Schrödinger Equation of a Molecule Using the Volterra Integral Equation

By using the Rayleigh-Schrödinger perturbation theory the rovibrational wave function is expanded in terms of the series of functions ϕ0,ϕ1,ϕ2,…ϕn, where ϕ0 is the pure vibrational wave function and ϕι are the rotational harmonics. By replacing the Schrödinger differential equation by the Volterra integral equation the two canonical functions α0 and β0 are well defined for a given potential function. These functions allow the determination of (i) the values of the functions ϕι at any points; (ii) the eigenvalues of the eigenvalue equations of the functions ϕ0,ϕ1,ϕ2,…ϕn which are, respectively, the vibrational energy Ev, the rotational constant Bv, and the large order centrifugal distortion constants Dv,Hv,Lv….. Based on these canonical functions and in the Born-Oppenheimer approximation these constants can be obtained with accurate estimates for the low and high excited electronic state and for any values of the vibrational and rotational quantum numbers v and J even near dissociation. As application, the calculations have been done for the potential energy curves: Morse, Lenard Jones, Reidberg-Klein-Rees (RKR), ab initio, Simon-Parr-Finlin, Kratzer, and Dunhum with a variable step for the empirical potentials. A program is available for these calculations free of charge with the corresponding author.

Tuning the Electronic Spin State of Catalysts by Strain Control for Highly Efficient Water Electrolysis

AbstractThe electronic configuration is crucial in governing the binding strength of intermediates with catalysts, yet it is still challenging to control the catalysts' surface electronic spin state. Here, it is demonstrated that through surface metal–organic framework transformation followed by acid etching, the electronic spin state of surface Co3+ ions on spinel Co3O4 can be transformed from t2g6 to the high electronic spin state of t2g4eg2 by expanding the surface lattice constant, which significantly enhances the overlap of the eg orbital of cobalt with the oxygen adsorbates, and greatly improves the intermediates adsorption and thus the oxygen evolution reaction activity. The high electronic spin rich Co3O4 electrode exhibits an anodic current density of 10 mA cm−2 at an overpotential of 280 mV. The finding offers a rational design strategy to manipulate the electronic spin state of catalyst and the hybridization of molecular orbitals in water electrolysis.

Unraveling the Ultrafast Exciton Relaxation and Hidden Energy State in CH3NH3PbBr3 Nanoparticles

Recently organic–inorganic lead-halide perovskite nanoparticles (NPs) have been attractive as low-cost and high-conversion-efficient solar cells and light-emitting diode. The generation of the exciton and dissociation into free carriers are quite important for primary photoelectric conversion processes. In this study, we have examined the initial exciton dynamics of CH3NH3PbBr3 (MAPbBr3) NPs by femtosecond transient absorption spectroscopy and picosecond time-resolved luminescence spectroscopy. The ultrafast exciton quenching with a time constant of 200 fs was observed, which may be related with longitudinal optical phonon and/or lurching motion of MA cation in the excited state. In addition, higher electronic state with a short lifetime was clearly detected by the excitation intensity dependence of time-resolved luminescence and transient absorption spectra. These findings of MAPbBr3 NPs are very important not only for understanding the generation of charge carrier but also for constructing the high-effi...

Bi2OS2: a direct-gap two-dimensional semiconductor with high carrier mobility and surface electron states

Bi2OS2 nanosheets possess tunable anomalous layer-dependent bandgaps, derived from the synergetic effect of the quantum confinement and surface electron states.

Rovibrational spectrum calculations of four electronic states in carbon monoxide molecule: Comparison of two effect correction methods

Accurate calculation of molecular energy is of great significance for studying molecular spectral properties. In this work, the potential energy curve and rovibrational spectrum (Gν) of the ground state X1∑+ and the excited states a3Π, a'3∑+ and A1Π of carbon monoxide molecule are calculated by the multi-reference configuration interaction method. In the calculation, the core-valence correlation correction (CV) effect and scalar relativistic (SR) effect are included.In order to obtain an accurate energy of molecule, two computational schemes are adopted. In the first scheme, i.e. (m MRCI+Q/CBS(TQ5)+CV+SR), the molecular orbital wavefunction is obtained from the Hartree-Fock self-consistent field method by using the basis set aug-cc-pVnZ. The wavefunction is first calculated by the state-averaged complete active space self-consistent field approach. Then the multi-reference configuration interaction method (MRCI) is adopted to calculate the dynamic correlation energy in the potential energy curve. Finally, we use the basis set cc-pCVQZ and aug-cc-pVQZ to calculate the CV effect and SR effect by the MRCI method. In the second scheme (aug-cc-pwCVnZ-DK (n=T, Q, 5)), the potential energy curves (PECs) of these four electronic states are calculated by the MRCI method whose basis set (aug-cc-pwCVnZ-DK) contains the CV effect and SR effect. Finally, in order to reduce the error caused by the basis set, we extrapolate the basis sets of the two computational schemes to the complete basis set. On the basis of the PECs plotted by the different methods, we obtain the spectroscopic parameters of the X1∑+, a3Π, a'3∑+ and A1Π states of the carbon monoxide by solving the internuclear Schrödinger equations through utilizing the numerical integration program “LEVEL”.In this paper, we calculate the SR effect and the CV effect by using different schemes, and the latter is indispensable for accurately calculating the molecular structure. For the lowest two electronic states, we consider the dependence of the two effects on the calculation of the Gaussian basis group (Method B), and find that the accuracy of the rovibrational spectrum is improved. It can be seen that these electronic states have higher requirements for electronic correlation calculation. For higher electronic states, the electron cloud distribution is relatively loose, and the electronic correlation obtained by a single Gaussian basis group can achieve the corresponding calculation accuracy. Of course, since the calculation of the rovibrational spectra is essentially only the relative energy, the offset effect of the electronic correlation effect of different electronic states is also included here in this paper.

Atomic-Scale Insights into the Low-Temperature Oxidation of Methanol Over a Single-Atom Pt1-Co3o4 Catalyst

Heterogeneous catalysts with single-atom active sites offer a means of expanding the industrial application of noble metal catalysts. Herein, we present an atomically dispersed Pt1-Co3O4 catalyst, which exhibits an exceptionally high catalytic activity for the total oxidation of various representative volatile organic compounds. Experimental and theoretical investigations indicated that this catalyst consisted of Pt sites with a large proportion of occupied high electronic states. These sites exhibited a strong affinity for the 3d orbital of Co, which increased electron transfer from Pt to Co3O4 and ultimately, accelerated the rate of oxygen vacancy regeneration. In turn, this was determined to promote the co-adsorption of probe VOC and O2. With methanol, DFT calculations confirmed that the electron transfer over the oxygen vacancy reduces both the methanol adsorption energy and activation barriers for methanol oxidation, which was proposed to significantly enhance the dissociation of the C-H bond in the methanol decomposition reaction. This investigation serves as a solid foundation for characterizing and understanding single-atom catalysts for heterogeneous oxidation reactions.

Observation of giant conductance fluctuations in a protein

Proteins are insulating molecular solids, yet even those containing easily reduced or oxidized centers can have single-molecule electronic conductances that are too large to account for with conventional transport theories. Here, we report the observation of remarkably high electronic conductance states in an electrochemically inactive protein, the ∼200 kD αVβ3 extracellular domain of human integrin. Large current pulses (up to nA) were observed for long durations (many ms, corresponding to many pC of charge transfer) at large gap (>5 nm) distances in an STM when the protein was bound specifically by a small peptide ligand attached to the electrodes. The effect is greatly reduced when a homologous, weakly binding protein (α4β1) is used as a control. In order to overcome the limitations of the STM, the time- and voltage-dependence of the conductance were further explored using a fixed-gap (5 nm) tunneling junction device that was small enough to trap a single protein molecule at any one time. Transitions to a high conductance (∼nS) state were observed, the protein being ‘on’ for times from ms to tenths of a second. The high-conductance states only occur above ∼100 mV applied bias, and thus are not an equilibrium property of the protein. Nanoamp two-level signals indicate the specific capture of a single molecule in an electrode gap functionalized with the ligand. This offers a new approach to label-free electronic detection of single protein molecules. Electronic structure calculations yield a distribution of energy level spacings that is consistent with a recently proposed quantum-critical state for proteins, in which small fluctuations can drive transitions between localized and band-like electronic states.

Effect of conduction band non-parabolicity on the optical gain of quantum cascade lasers based on the effective two-band finite difference method

We theoretically investigate the effect of conduction band non-parabolicity (NPB) on the optical gain spectrum of quantum cascade lasers (QCLs) using the effective two-band finite difference method. Based on the effective two-band model to consider the NPB effect in the multiple quantum wells (QWs), the wave functions and confined energies of electron states are calculated in two different active-region structures, which correspond to three-QW single-phonon and four-QW double-phonon resonance designs. In addition, intersubband optical dipole moments and polar-optical-phonon scattering times are calculated and compared without and with the conduction band NPB effect. Finally, the calculation results of optical gain spectra are compared in the two QCL structures having the same peak gain wavelength of 8.55 μm. The gain peaks are greatly shifted to longer wavelengths and the overall gain magnitudes are slightly reduced when the NPB effect is considered. Compared with the three-QW active-region design, the redshift of the peak gain is more prominent in the four-QW active-region design, which makes use of higher electronic states for the lasing transition.

Energy partitioning and its influence on surface scatter coefficients within fluorinated inductively coupled plasmas

Energy partitioning for molecules formed from fluorinated plasma systems has been measured using laser-induced fluorescence, optical emission, and broadband absorption spectroscopies. For the two radicals discussed here, SiF in tetrafluorosilane plasmas, and CF in CxFy-type fluorocarbon plasmas, high electronic excited state vibrational temperatures, TV, suggest that vibrational modes are preferentially excited over other degrees of freedom. In CxFy plasma systems, rotational temperatures, TR, for the radicals equilibrate to the plasma gas temperature and remain independent of changing plasma parameters. TR for ground state CF2 molecules are elevated in comparison to the excited state radical. Translational temperatures (TT) and TR for SiF increase concomitantly with increasing vibrational temperatures, suggesting that a vibrational-translational energy exchange mechanism is the preferred pathway for vibrational relaxation in these molecules and rotational degrees of freedom are more easily thermalized compared to vibrational modes. Using the imaging of radicals interacting with surfaces technique, surface scattering coefficients measured for each radical show a strong correlation with the associated TV, with little dependence upon TR or TT. Ultimately, this work provides fundamental, molecular-level understanding of fluorinated plasma systems and helps to reveal the complex interactions between the distribution of energy within the plasma and its influence at a surface.