Change In Ionization Energy Research Articles

(Invited) Inductive Effects to Control the Operating Voltage of Fast-Charging Molybdenum Oxide Anodes

Batteries are essential to most modern technologies, as they provide a sustainable alternative to traditional energy sources. One area of active research is in identifying new battery materials made from plentiful elements to produce stable, high power and fast-charging batteries. A candidate for the next-generation anode material is molybdenum oxide, as it shows fast-charging capabilities, stability, and high capacity. The disadvantage of this material is that the expected power is relatively low, as the redox voltage for molybdenum is not optimized to the stability window of the liquid electrolyte. Using the inductive effect as a guiding principle, we suggest lowering the Mo+6/+5 redox voltage by adding a cation to the structure. The structural modification will influence the covalency of the metal-oxygen bond and will therefore allow lower ionization energy (IE) for the d-orbital electrons. To this aim, we compare the electrochemistry of Li2Mo4O13 and MoO3 as well as the IE and the Li diffusivity. The results show that the addition of Li lowers the voltage by 300 mV for Mo+5/+6 compared to MoO3 but does not reduce the Li diffusivity. The change in IE also supports our hypothesis that the Li reduces the polarity of the M-O bond. Additional electrochemical studies indicate limited capacity for Li2Mo4O13 compound, which is related to structural changes after the first lithiation process. Our results demonstrate the utility of exploring new systems with elevated redox voltage to better fit the electrolyte stability window and yield higher power.

Structurally induced large changes of the energy level alignment in CuPc on Cu(110)−(2×1)O

While it is becoming apparent that organic semiconductor/metal interfaces may exhibit a variety of different structural phases, it is at present unclear to what extent these different thin-film structures determine the interfacial electronic structure. Here, we observe large changes in the interfacial electronic structure for the case of copper(II) phthalocyanine (CuPc) on $\mathrm{Cu}(110)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1)\mathrm{O}$. This striking evolution of the interfacial electronic structure occurs beyond the first monolayer of CuPc and is particularly evident in the frontier orbital region. Using scanning tunneling microscopy in conjunction with photoemission spectroscopy, we characterize ultrathin films of CuPc grown on oxygen reconstructed Cu(110). We propose that the observed unique changes to the electronic structure result from an abrupt transition in film structure between the first and second layers: An interface layer of ordered, face-on molecules templates a largely vertical, edge-on orientation of molecules in the subsequent layer. The quadrupole moment of the molecule accounts for the sizable and unusual change in ionization energy between molecules in the two layers. Our results demonstrate that the precise structure of the organic semiconductor film exerts an important role in determining the interfacial electronic structure that must be considered and may be harnessed for tailoring energy level alignment at such interfaces.

A redshift mechanism of high-order harmonics: Change of ionization energy

We theoretically study the high-order harmonic generation of H2+ and its isotopes beyond the Born-Oppenheimer dynamics. It is surprising that the spectral redshift can still be observed in high harmonic spectra of H2+ driven by a sinusoidal laser pulse in which the trailing (leading) edge of the laser pulse is nonexistent. The results confirm that this spectral redshift originates from the reduction in ionization energy between recombination time and ionization time, which is obviously different from the nonadiabatic spectral redshift induced by the falling edge of the laser pulse. Additionally, the improved instantaneous frequency of harmonics by considering the changeable ionization energy can deeply verify our results. Therefore, this new mechanism must be taken into account when one uses the nonadiabatic spectral redshift to retrieve the nuclear motion.

Unveiling the Electronic Structure of ZnO–C60 Core–Shell Quantum Dots: The Origin of Efficient Electron Transport

The electronic structure of ZnO core–C60 shell (ZnO@C60) quantum dots (QDs) was investigated with the help of model interface analysis between ZnO QDs and C60 using in situ ultraviolet and X-ray photoelectron spectroscopy measurements. To form the ZnO QDs/C60 interface in situ, a vacuum-integrated electrospray deposition technique was employed to simulate the electronic interactions between ZnO QDs and C60 upon the formation of a ZnO@C60 core–shell structure. Photoelectron spectra of ZnO QDs/C60 interface formation were compared with those of ZnO@C60 QD and pristine ZnO QD films. The results revealed that ZnO QDs and C60 interacted via electron transfer leading to the change in ionization energy of the surface C60. This induced a negligible energy barrier between the lowest unoccupied molecular orbital level of C60 and the conduction band minimum of ZnO, which led to efficient electron transport through the ZnO@C60 QDs.

Experimental measurements and theoretical model of the cryogenic performance of bialkali photocathode and characterization with Monte Carlo simulation

High-average-current, high-brightness electron sources have important applications, such as in high-repetition-rate free-electron lasers, or in the electron cooling of hadrons. Bialkali photocathodes are promising high-quantum-efficiency (QE) cathode materials, while superconducting rf (SRF) electron guns offer continuous-mode operation at high acceleration, as is needed for high-brightness electron sources. Thus, we must have a comprehensive understanding of the performance of bialkali photocathode at cryogenic temperatures when they are to be used in SRF guns. To remove the heat produced by the radio-frequency field in these guns, the cathode should be cooled to cryogenic temperatures. We recorded an 80% reduction of the QE upon cooling the ${\mathrm{K}}_{2}\mathrm{CsSb}$ cathode from room temperature down to the temperature of liquid nitrogen in Brookhaven National Laboratory (BNL)'s 704 MHz SRF gun. We conducted several experiments to identify the underlying mechanism in this reduction. The change in the spectral response of the bialkali photocathode, when cooled from room temperature (300 K) to 166 K, suggests that a change in the ionization energy (defined as the energy gap from the top of the valence band to vacuum level) is the main reason for this reduction. We developed an analytical model of the process, based on Spicer's three-step model. The change in ionization energy, with falling temperature, gives a simplified description of the QE's temperature dependence. We also developed a 2D Monte Carlo code to simulate photoemission that accounts for the wavelength-dependent photon absorption in the first step, the scattering and diffusion in the second step, and the momentum conservation in the emission step. From this simulation, we established a correlation between ionization energy and reduction in the QE. The simulation yielded results comparable to those from the analytical model. The simulation offers us additional capabilities such as calculation of the intrinsic emittance, the temporal response, and the thickness dependence of the QE for the ${\mathrm{K}}_{2}\mathrm{CsSb}$ photocathode.

Fine Substituent Effects in Sandwich Complexes: A Threshold Ionization Study of Monosubstituted Chromium Bisarene Compounds.

Mass-analyzed threshold ionization spectra of jet-cooled [(η(6) -PhMe)(η(6) -PhH)Cr] and [(η(6) -Ph2 )(η(6) -PhH)Cr] reveal with unprecedented accuracy the effects of methyl and phenyl groups on the electronic structure of bis(η(6) -benzene)chromium. These "pure" substituent effects allow quantitative experimental determination of the ionization energy changes caused by the mutual substituent influence in bisarene systems. Two types of such influence have been revealed for the first time in bis(η(6) -toluene)chromium.

스컬용융법에 의해 성장시킨 Co와 Pr이 첨가된 이트리아안정화큐빅지르코니아(YSZ) 단결정의 연구

Co-(0.7 wt%) and Pr-(2.0, 3.5 or 5.0 wt%) doped cubic zirconia (ZrO :Y O = 50 : 50 wt%) single crystals grown by a skull melting method were heat-treated in N at 1150 C for 5 hrs. The brown colored as-grown single crystals were changed into either dark brownish green, greenish blue and light green color after the heat treatment. Before and after the heat treatment, the YSZ (yttria-stabilized zirconia) single crystals were cut for wafer form (o7.5 mm × t3 mm). The optical and structural properties were examined by UV-VIS spectrophotometer and X-ray diffraction. Absorption by Co (≒589 nm: Γ ( A ( F)) →Γ +Γ ( T ( F)), ≒610 nm: Γ ( A ( F)) →Γ ( T ( F)), ≒661 nm: Γ ( A ( F)) →Γ ( T ( F))) and Pr (≒450 nm: H − P , ≒473 nm: H → P , ≒484 nm: H → P ), change of ionization energy and lattice parameter were confirmed.

ZrO2와 Y2O3조성비에 따른 Co와 Ce 첨가 큐빅지르코니아(YSZ) 단결정의 광학적 및 구조적 특성

Co-(0.8 wt%) and Ce-(0.4 wt%) doped cubic zirconia (ZrO2 :Y 2O3 = 80 : 20, 70 : 30, 60 : 40, 50 : 50 wt%) single crystals grown by a skull melting method were heat-treated in N2 at 1000 o C for 5 hrs. The orange, yellowish brown and brown colored as-grown single crystals were changed into either brownish red, yellow and green color after the heat treatment. Before and after the heat treatment, the YSZ (yttria-stabilized zirconia) single crystals were cut for wafer form (φ6.5 mm × t2 mm). The optical and structural properties were examined by UV-VIS spectrophotometer and X-ray diffraction. Absorption by Ce 3+ ( 2 F5/2, 7/2(4f) → 2 Tg(5d 1 )), Co 2+ ( 4 A2( 4 F) → 4 T1( 4 F) or 4 T1( 4 P)) and Co 3+ , change of ionization energy and lattice

Co와 Ce를 첨가한 큐빅지르코니아(YSZ) 단결정의 이온화에너지 및 격자상수

Co-(0.5 mol%) and Ce-(0~0.3 mol%) doped cubic zirconia (ZrO :Y O = 64 : 36 mol%) single crystals grown by a skull melting method were heat-treated in N at 1200 C for 3 hrs. The brown-colored as-grown single crystals were changed into either green or blue color after the heat treatment. Before and after the heat treatment, the YSZ (yttria- stabilized zirconia) single crystals were cut for wafer form (φ7 mm × t2 mm) and round brilliant cut (φ12 mm). The optical and structural properties were examined by UV-VIS spectrophotometer and X-ray diffraction. Absorption by Ce ( F (4f) → T (5d )), Co ( A ( F) → T ( F) or T ( P)) and Co , change of ionization energy and lattice parameter were confirmed.

The effect of π-stacking, H-bonding, and electrostatic interactions on the ionization energies of nucleic acid bases: adenine–adenine, thymine–thymine and adenine–thymine dimers

A combined theoretical and experimental study of the ionized dimers of thymine and adenine, TT, AA, and AT, is presented. Experimentally observed and computed adiabatic and vertical ionization energies (IEs) for monomers and dimers as well as thresholds for the appearance of the protonated species are reported and analyzed. Non-covalent interactions strongly affect the observed IEs. The magnitude and the nature of the effect is different for different isomers of the dimers. The computations reveal that for TT, the largest changes in vertical IEs (0.4 eV) relative to the monomer occur in asymmetric H-bonded and symmetric pi-stacked isomers, whereas in the lowest-energy symmetric H-bonded dimer the shift in IEs is much smaller (0.2 eV). The origin of the shift and the character of the ionized states is different in asymmetric H-bonded and symmetric stacked isomers. In the former, the initial hole is localized on one of the fragments, and the shift is due to the electrostatic stabilization of the positive charge of the ionized fragment by the dipole moment of the neutral fragment. In the latter, the hole is delocalized, and the change in IE is proportional to the overlap of the fragments' MOs. Relative to TT, the shifts in AA and AT are much smaller due to a less efficient overlap, smaller dipole of A and the large energy gap between ionized states of A and T monomers in the case of AT dimer. The ionization of the H-bonded dimers results in barrierless (or nearly barrierless) proton transfer, whereas the pi-stacked dimers relax to structures with the hole stabilized by the delocalization or electrostatic interactions.

Influence of ionization energy change on valence band offset in organic p-n junction

Valence band offsets at [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)/metallophthalocyanine (MPc, M=Cu and Zn) interfaces have been investigated with ultraviolet photoelectron spectroscopy, which are organic p-n junctions in organic photovoltaics. The highest occupied molecular orbitals of MPcs rise toward the interface, while that of PCBM lowers. This behavior implies that the different energy band offsets from inorganic p-n junction are realized in organic p-n junction. The depletion layers were not observed at the interfaces. Such anomalous energy band offsets are attributed to the interfacial dipole and also ionization energy changes of MPcs and PCBM at the interface.

Photoelectron spectroscopy of natural products: terpenes

HeI photoelectron spectra of three terpenes: α-pinene, pulegone and cembrene have been measured. The analysis of their electronic structure is based on the comparison of measured spectra with those of related compounds and on the comparison with molecular structures of studied compounds. We discuss changes in ionization energies of C–C double bonds which are situated at different positions along the rings.

Metal−Metal Bonding in Rh2(O2CCF3)4: Extensive Metal−Ligand Orbital Mixing Promoted by Filled Fluorine Orbitals

He I and He II gas-phase photoelectron spectra of Rh2(O2CCF3)4 are reported. The electron configuration of the metal−metal bond of Rh2(O2CCF3)4 is determined to be σ2 π4 δ2 δ*2 π*4 with an ionization energy order of σ ≈ π > δ > δ* ≈ π*. The δ* and π* ionization energies are similar within the range of vibrational energy separations. Assignment of the Rh−Rh δ ionization is assisted by previous observations that ionizations from δ orbitals in M2(O2CCF3)4 (M = Mo, W) and Mo2(O2CH)4 show enhanced intensity over ionizations from the σ and π orbitals with He II excitation. Changes in ionization energies from the dimolybdenum molecule to the dirhodium molecule and changes in ionization intensities from He I to He II excitation indicate greater metal−ligand mixing in these molecules than observed in other dimetal tetracarboxylates. Amsterdam density functional calculations agree with the observation that the Rh−Rh δ* and π* energies are similar. The calculations also indicate substantial ligand mixing into the metal−metal σ and π orbitals, which is enhanced by the trifluoroacetate ligand despite the inductive withdrawal of electron density by the electronegative fluorine atoms. It is found that a specific set of ligand orbitals that possess the same symmetries as the Rh−Rh σ and π orbitals are destabilized by overlap interactions with the filled fluorine p orbitals, resulting in greater metal−ligand mixing among these orbitals. The increased mixing explains the long-observed enhancement of the δ and δ* ionizations from He I to He II excitation for a number of M2(O2CCF3)4 (M = Mo, W, Ru) systems.

The photoelectron spectroscopy and electronic structure of metal β-diketonates and their analogs

He(I) photoelectron spectra of transition and non-transition metal bis- and tris-β-diketonate complexes were studied. Spectra have been interpreted based on the study of regularities in spectra of compound rows and results of quantum chemical calculations of ground and ionized states by X α-DV and ab initio methods. In group III d-metal complexes, the bond covalency changes in the sequenceYL 3 < LuL 3 < ScL 3, whereas for complexes of final elements in the transition row, the bond covalency is directly proportional to the period row. Ionization energy changes for O to NR group substitution, γ-H to Cl and CH 3 group, β-CH 3 groups to H, C(CH 3) and C 6H 5, are discussed. It was shown that for high-spin complexes, despite the closeness of the final state energies, ψ-function symmetry prohibits the interaction between states caused by d- and L-electrons removal.

Electronic Structure of the Ultrathin Cs/Si(100)-(2 × 1) and Cs/Si(111)-(7 × 7) Interfaces in the Threshold Energy Region

The electronic band structure of the Cs/Si(100)-(2 × 1) and Cs/Si(111)-(7 × 7) interfaces has been studied near the Fermi level at submonolayer coverages. The technique of threshold photoemission spectroscopy with linearly polarized light excitation has been employed. Surface photoemission spectra reveal on Cs-induced band which can be either below the VBM or at the Fermi level, depending on the substrate. Parameters of the Cs band and the change in ionization energy and work function are obtained as a function of Cs coverage. Experimental data provide direct evidence that the Cs/Si(100)-(2 × 1) interface exhibits semiconducting charcter in a great part of the submonolayer range, in contrast to the metallic-like Cs/Si(111)-(7 × 7) interface.

Effect of adsorbed potassium on the electrostatic potential on Rh clusters in relation with photoemission of adsorbed noble gases

A locally lowered surface potential develops around potassium atoms on a transition metal surface. These potentials are accessible experimentally by measuring the ultraviolet photoemission spectra of adsorbed noble gases. We determine the potassium-induced change in electrostatic potential on K/Rh clusters on the basis of calculated electronic structures using density functional theory. It appears that the calculated potentials depend critically on the geometry of the cluster and may differ substantially from the experimentally observed values. Clusters that allow a substantial charge redistribution around the potassium atoms give the best results. Another improvement is obtained from adding the contribution of potassium atoms further away from the potential. We also present theoretical evidence that the potassium-induced change in ionization energy of Xe, Kr, and Ar on a Rh15K3 cluster is mainly determined by the change in electrostatic potential. This provides support for the use of photoemission of adsorbed noble gases as a probe for changes in local surface potential induced by adsorbed species.

The photoelectron spectra and electronic structure of 2-carbonyl furans

The He(I), λ = 58.4 nm, photoelectron spectra of a series of six 2-carbonyl furans and 2-furonitrile have been measured in the gas phase. The low ionization energy bands are assigned to photoionizations from π orbitals of the furan ring, orbitals localized on the carbonyl group, and orbitals of the carbonyl substituents. Changes in ionization energies within the series can be rationalized in terms of the relative conjugative and inductive effects of the substituents. Substituent inductive effects were found to be dominant. The relatively broad vibrational fine structure on the low IE bands of the carbonyl furans could result from the presence of rotational isomers of the species. Keywords: photoelectron spectra, 2-carbonyl furans, molecular orbitals.

Ionization-energy dependence on GaAs(001) surface superstructure measured by photoemission-yield spectroscopy.

The ionization energies for GaAs(001) are determined by photoemission-yield spectroscopy as a function of surface superstructure. The ionization energy changes by as much as 0.5 eV in accordance with the surface superstructure.

Electrical measurements of the electron irradiation induced E- and H-traps in GaAs under hydrostatic pressure

The change of resistivity of the 2.3 MeV-electron-irradiated bulk n- and p-GaAs have been measured at hydrostatic pressure up to 5 kbar at RT. Corrections for the changes in free electron and hole mobilities with pressure have been neglected. The resistivity changes are explained by a dependence on pressure of the ionisation energy of the radiation-induced E- and H-traps. The results indicate that most from these radiation- induced levels moves away from the conduction-band edge (γ c -point) at a rate approximately (0.8−1.0) γ G , here γ G=11.6×10 −6 eV bar −1 is the energy gap pressure coefficient for GaAs at RT. The high changes in ionization energies of E2 to E5-traps upon pressure are to be compared with the lower changes in ionization energies found for the deep-lying impurity levels. In accordance with the theoretical investigation it was suggested that most of the investigated radiation-induced levels in GaAs are t 2-states of Ga- and As-vacancies.

Relativistic contributions to ionization energies and bond lengths

The interchange theorem of double perturbation theory permits two equivalent approaches for the determination and discussion of relativistic changes of ionization energies and bond lengths. Electron density differences due to ionization and bond length variation, respectively, at the nonrelativistic level, or alternatively density differences due to relativity, have to be taken into account ; either may be used for explaining the changes in ionization energies or bond lengths. Relativistic bond length contractions and relativistic orbital contractions are related to each other. But one effect is not the cause of the other. The relation, although more complicated than originally anticipated, offers a simple explanation for relativistic bond length changes within both the energy and the force concepts of the chemical bond. Corresponding model calculations at the semiempirical as well as at the ab initio Dirac-Fock level are presented and analysed.