Measured Ionization Potentials Research Articles

Characterization of the field ionization extension for the laser ion source and trap: Measurement of the ionization potential of ytterbium

We report on the development and application of the Field Ionization Laser Ion Source and Trap (FI-LIST) at the RISIKO mass separator at Mainz University. The FI-LIST is an adaptation of the well-established LIST unit developed at Mainz and successfully adapted to CERN-ISOLDE. It is specifically designed for field ionization of highly excited atoms in a homogeneous electric field. To assess the performance of the device for future use on radioactive species of e.g. actinides, we conducted ionization potential (IP) measurements on ytterbium, for which the IP is precisely known. The IP value was derived by applying the saddle-point model with a relative precision of 3⋅10−6 and in very good agreement with the literature value.

Bond dissociation energies for Fe2+, Fe2O+, and Fe2O2+ clusters determined through threshold photodissociation in a cryogenic ion trap.

Understanding and controlling the chemical behavior of iron and iron oxide clusters requires accurate thermochemical data, which, because of the complex electronic structure of transition metal clusters, can be difficult to calculate reliably. Here, dissociation energies for Fe2+, Fe2O+, and Fe2O2+ are measured using resonance enhanced photodissociation of clusters contained in a cryogenically cooled ion trap. The photodissociation action spectrum of each species exhibits an abrupt onset for the production of Fe+ photofragments from which bond dissociation energies are deduced for Fe2+ (2.529 ± 0.006eV), Fe2O+ (3.503 ± 0.006eV), and Fe2O2+ (4.104 ± 0.006eV). Using previously measured ionization potentials and electron affinities for Fe and Fe2, bond dissociation energies are determined for Fe2 (0.93 ± 0.01eV) and Fe2- (1.68 ± 0.01eV). Measured dissociation energies are used to derive heats of formation ΔfH0(Fe2+) = 1344 ± 2 kJ/mol, ΔfH0(Fe2) = 737 ± 2 kJ/mol, ΔfH0(Fe2-) = 649 ± 2 kJ/mol, ΔfH0(Fe2O+) = 1094 ± 2 kJ/mol, and ΔfH0(Fe2O2+) = 853 ± 21 kJ/mol. The Fe2O2+ ions studied here are determined to have a ring structure based on drift tube ion mobility measurements prior to their confinement in the cryogenic ion trap. The photodissociation measurements significantly improve the accuracy of basic thermochemical data for these small, fundamental iron and iron oxide clusters.

Chemistry of the elements at the end of the actinide series using their low-energy ion-beams

Abstract Studies of the chemical properties of the elements at the uppermost end of the Periodic Table are extremely challenging both experimentally and theoretically. One of the most important and interesting subjects is to clarify the basic chemical properties of these elements as well as to elucidate the influence of relativistic effects on their electronic configuration. Isotopes of these elements produced at accelerators, however, are short-lived, and the number of produced atoms is so small; any chemistry to be performed must be done on an atom-at-a-time basis that imposes stringent limits on experimental procedures. Here we describe our recent achievements in the effective production of low-energy ion-beams of the elements at the end of the actinide series, fermium (Fm, atomic number Z = 100), mendelevium (Md, Z = 101), nobelium (No, Z = 102), and lawrencium (Lr, Z = 103), using a surface ionization ion-source installed in the ISOL (Isotope Separator On-Line) at the Tandem accelerator facility of JAEA (Japan Atomic Energy Agency). Then the successful measurements of the first ionization potentials (IP1) of these elements with the ISOL setup are reviewed. The measured IP1 values increased up to No via Fm and Md, while that of Lr was the lowest among the actinides. Based on the variation of the IP1 values of the heavy actinides with the atomic number in comparison with those of the heavy lanthanides, the results clearly demonstrated that the 5f orbitals are fully filled at No, and the actinide series ends with Lr. Furthermore, the IP1 value of Lr provoked controversy over its position in the Periodic Table, so a short introduction to this issue is presented. The feasibility of the extension of chemical studies to still heavier elements with their ion-beams generated by ISOL is briefly discussed.

Cationic Axial Ligand Effects on Sulfur-Substituted Subphthalocyanines.

Herein, we report the synthesis of sulfur-substituted boron(III) subphthalocyanines (SubPcs) with cationic axial ligands. Subphthalocyanines were synthesized by a condensation reaction using the corresponding phthalonitriles and boron trichloride as a template. An aminoalkyl group was introduced on the central boron atom; this process was followed by N-methylation to introduce a cationic axial ligand. The peripheral sulfur groups shifted the Q band of SubPcs to a longer wavelength. The cationic axial ligands increased the polarity and enhanced the hydrophilicity of SubPcs. The effect of axial ligands on absorption and fluorescence properties is generally small. However, a further red shift was observed by introducing cationic axial ligands into the sulfur-substituted SubPcs. This change is similar to that in sulfur-substituted silicon(IV) phthalocyanines. The unique effect of the cationic axial ligand was extensively investigated by theoretical calculations and electrochemistry. In particular, the precise oxidation potential was determined using ionization potential measurements. Thus, the results of the present study provide a novel strategy for developing functional dyes and pigments based on SubPcs.

Pyrolysis-Free Oxygen Reduction Reaction (ORR) Electrocatalysts Composed of Unimolecular Layer Metal Azaphthalocyanines Adsorbed onto Carbon Materials

The oxygen reduction reaction (ORR) at the cathode is one of the critical bottlenecks in the energy conversion process in polymer electrolyte fuel cells (PEFCs) and metal–air batteries. An iron azaphthalocyanine unimolecular layer (Fe-AZUL) adsorbed onto a carbon substrate exhibited high ORR activity as an alternative of Pt supported on carbon black (Pt/C) electrocatalyst. We herein report the synthesis of various AzPc molecules with different peripheral structures and complex metals, as well as measurements of the ORR activities of catalysts containing the synthesized molecules and carbons. The catalytic activities were characterized by electrochemical analysis, density functional theory (DFT) calculations, and measurements of ionization potential (IP) values. From the results, the IP values on Ketjen Black (KB) were found to be one of the significant indicators for high ORR activities, providing a guideline for the development of high-performance electrocatalysts.

Interfacial Charge-Transfer Transitions in TiO2 Nanoparticles Adsorbed with 4-Mercaptobenzenoic Acid: Carboxy versus Thiol Anchor and Adsorbate-to-TiO2 versus TiO2-to-Adsorbate Charge Transfer

Interfacial charge-transfer transitions (ICTTs) in surface coordination complexes of organic compounds with inorganic semiconductors enable the absorption of visible light with colorless organic molecules and direct photoinduced charge separation for applications in photoenergy conversions and biosensing. The surface complex of 4-mercaptobenzenoic acid (4-MBA) with TiO2 nanoparticles has attracted much attention for the surface-enhanced Raman scattering (SERS). However, the adsorption and electronic structures and optical properties of the TiO2–4-MBA surface complex remain to be clarified. Here, we examine the TiO2–4-MBA surface complex in detail experimentally and computationally. Upon adsorption of 4-MBA, TiO2 nanoparticles show a broad absorption band in the visible region. The IR vibrational analysis clearly indicates that 4-MBA adsorbs on TiO2 via the carboxylate group, contrary to the reported thiol linkage. Ionization potential measurements and time-dependent density functional theory (TD-DFT) calculations reveal that the absorption band is attributed to ICTT from the highest occupied molecular orbital (HOMO) of the adsorbed 4-MBA to the conduction band of TiO2, contrary to the reported TiO2-to-adsorbate photoinduced charge-transfer scheme. Our research clarifies the predominant adsorption mode of 4-MBA via the carboxylate linkage and organic-to-inorganic ICTT and suggests that the reported SERS originates from the ICTT.

Paradigms and paradoxes: the ionization potential of atomic astatine (Z =85), polonium (Z = 84), and some other elements—what does this value tell us about the energetics of atomic and diatomic halogens

The newly measured ionization potential of atomic astatine is discussed and compared with that of the recently determined value for polonium and for the other atomic halogens. Regularities in these atomic values are discussed and applied to the understanding of the energetics of diatomic halogens. Some surprises remain.

Development of photoelectron yield spectrometer and measurement of ionization potential of photochromic materials

The photoelectron yield spectroscope (PYS) applies the photoelectric effect and detects the photocurrent emitted from a sample when light of a specific wavelength is irradiated to a sample. Work function or ionization potential of samples can be measured using PYS. X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy are usually employed as methods for measuring their electronic structures. However, their methods can only be measured in high vacuum. PYS can be used when measuring in the atmosphere. In this work, PYS was developed. Gold, aluminum, ITO, and MTO were used as samples and their work function or ionization potential were measured using PYS.

Nuclear and chemical characterization of heavy actinides

Abstract Recent progress in the production of heavy nuclei far from stability and in the studies of nuclear and chemical properties of heavy actinides is briefly reviewed. Exotic nuclear decay properties including nuclear fission of heavy nuclei, measurements of first ionization potentials of heavy actinides, and redox studies of heavy actinides are described. Brief history of discovery of the transuranium elements is also presented.

Direct mass measurements and ionization potential measurements of the actinides

Abstract The precise determination of atomic and nuclear properties such as masses, differential charge radii, nuclear spins, electromagnetic moments and the ionization potential of the actinides has been extended to the late actinides in recent years. In particular, laser spectroscopy and mass spectrometry have reached the region of heavy actinides that can only be produced only at accelerator facilities. The new results provide deeper insight into the impact of relativistic effects on the atomic structure and the evolution of nuclear shell effects around the deformed neutron shell closure at N = 152. All these experimental activities have also opened the door to extend such measurements to the transactinide elements in the near future. This contribution summarizes recent achievements in Penning trap mass spectrometry and laser spectroscopy of the late actinides and addresses future perspectives.

Carbon-chain length substituent effects on Cu(II) phthalocyanines as dopant-free hole-transport materials for perovskite solar cells

Owing to their excellent thermal and chemical stability properties, low cost, and easy purification processes, metal phthalocyanines have received extensive research attention. To reveal the influence of alkyl-chain length of substituents on phthalocyanines rings, we prepared three small organic molecules based on copper phthalocyanine rings substituted with end-capping ethyl, butyl, and hexyl, alkyl groups. The molecular orientation and intermolecular π-π stacking of these materials were affected by the alkyl chains, resulting in different hole mobilities. The crystallization of the phthalocyanines improved as the carbon chain length was extended, which we attributed to increased intermolecular π-π stacking. We used density functional theory calculations and solution cyclic voltammetry and ionization potential measurements to confirm a small difference in the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. The morphology, molecular orientation and π-stacking were affected by the introduction of alkyl chains and resulted in differences in the photovoltaic performance of solution processed PSC based on these copper phthalocyanines. The long-term stability of devices based on CuEtPc, CuBuPc, and CuHePc was improved by elongating the carbon chains because of the better hydrophobicity of the phthalocyanines.

A bulk adjusted linear combination of atomic orbitals (BA-LCAO) approach for nanoparticles.

We describe a bulk adjusted linear combination of atomic orbitals (BA-LCAO) approach for nanoparticles. In this method, we apply a many-body scaling function (in similar manner as in the environment-modified total energy based tight-binding method) to the DFT-derived diatomic AO interaction potentials (like in the conventional orbital-based density-functional tight binding approach) strictly according to atomic valences acquired naturally in a bulk structure. This modification, (a) facilitates all atom orbital-based electronic structure calculations of charge carrier dynamics in nanoscale structures with a molecular acceptor, and (b) allows to closely match high-level density functional calculation data (previously adjusted to the available experimental findings) for bulk structures. To advance practical application of the BA-LCAO approach we parameterize the Hamiltonian of wurtzite CdSe by fitting its band structure to a high-level DFT reference, corrected for experimentally measured band edges. Here, unlike in conventional DFTB approach, we: (1) use hydrogen-like AOs for the basis as exact atomic eigenfunctions, while orbital energies of which are taken from experimentally measured ionization potentials, and (2) parameterize the many-body scaling functions rather than the atomic wavefunctions. Development of this approach and parameters is guided by our goals to devise a method capable of simultaneously treating the problems of (i) interfacial electron/hole transfer between finite, variable size nanoparticles and electron scavenging molecules, and (ii) high-energy electronic transitions (Auger transitions) that mediate multi-exciton decay in quantum dots. Electronic structure results are described for CdSe quantum dots of various sizes. © 2018 Wiley Periodicals, Inc.

UV Photoionization of Sodium-Doped Formic Acid Clusters.

The processes involved in the photoionization of sodium-doped clusters are complex, not fully understood for many systems and still strongly debated, especially because of the discrepancy between experimental results and predicted cluster structures. We have performed a study on sodium doped formic acid clusters based on UV photoionization spectroscopy and DFT/TDDFT calculations. Apart from the monomer, all the predicted structures show vertical ionization potential values higher than those obtained by the photoionization measurements. We have calculated the absorption spectra and found many Rydberg-like states near the adiabatic ionization potentials and, crucially, in the UV range where the clusters appearance energies fall. This finding supports the hypothesis of adiabatic contributions in the measured ionization potentials for these clusters.

Comparative study of conduction-band and valence-band edges of TiO2, SrTiO3, and BaTiO3 by ionization potential measurements

Here, we report the energy positions of the conduction-band and valence-band edges of anatase titanium dioxide (TiO2), strontium titanate (SrTiO3), and barium titanate (BaTiO3). From the photoelectron yield spectra, the ionization potentials of anatase TiO2, SrTiO3, and BaTiO3 were estimated to be ca. 7.25, 6.90, 7.05eV, respectively, which correspond to the top of the valence band of each titanium oxide. From these data and their band gaps, it was found that the bottoms of the conduction bands of SrTiO3 and BaTiO3 are positioned above that of anatase TiO2 by 0.40 and 0.23eV, respectively.

First Experimental Evidence for the Diverse Requirements of Excimer vs Hole Stabilization in π-Stacked Assemblies

Exciton formation and charge separation and transport are key dynamical events in a variety of functional polymeric materials and biological systems, including DNA. Beyond the necessary cofacial approach of a pair of aromatic molecules at van der Waals contact, the extent of overlap and necessary geometrical reorganization for optimal stabilization of an excimer vs dimer cation radical remain unresolved. Here, we compare experimentally the dynamics of excimer formation (via emission) and charge stabilization (via threshold ionization) of a novel covalently linked, cofacially stacked fluorene dimer (F2) with the unlinked van der Waals dimer of fluorene, that is, (F)2. Although the measured ionization potentials are identical, the excimeric state is stabilized by up to ∼30 kJ/mol in covalently linked F2. Supported by theory, this work demonstrates for the first time experimentally that optimal stabilization of an excimer requires a perfect sandwich-like geometry with maximal overlap, whereas hole stabilization in π-stacked aggregates is less geometrically restrictive.

Electronic structures of TiO2–TCNE, –TCNQ, and –2,6-TCNAQ surface complexes studied by ionization potential measurements and DFT calculations: Mechanism of the shift of interfacial charge-transfer bands

Interfacial charge-transfer (ICT) transitions between inorganic semiconductors and π-conjugated molecules allow direct charge separation without loss of energy. This feature is potentially useful for efficient photovoltaic conversions. Charge-transferred complexes of TiO2 nanoparticles with 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its analogues (TCNX) show strong ICT absorption in the visible region. The ICT band was reported to be significantly red-shifted with extension of the π-conjugated system of TCNX. In order to clarify the mechanism of the red-shift, in this work, we systematically study electronic structures of the TiO2–TCNX surface complexes (TCNX; TCNE, TCNQ, 2,6-TCNAQ) by ionization potential measurements and density functional theory (DFT) calculations.

Recent great impact by an Isotope Separator On-Line (ISOL) in nuclear and radiochemistry.

On April 9 2015, the Letter article titled "Measurement of the first ionization potential of lawrencium, element 103" is now published at News and Views on Nature (2015) which has been performed by our remarkably Japanese colleagues of nuclear and radiochemistry at JAEA (Japan Atomic Energy Agency). In this review, the author will state that the isotope separator on-line (ISOL) our regularly used, one of mass separation techniques, with a thermal surface ionization makes possible for determining the ionization potential of lawrencium based on the fruitful fundations of developing the ISOL system until now and also ever studying searches for unknown nuclei and these nuclear decay properties around actinide region in the past 20 years.

Titanium-Niobium Oxides as Non-Noble Metal Cathodes for Polymer Electrolyte Fuel Cells

In order to develop noble-metal- and carbon-free cathodes, titanium-niobium oxides were prepared as active materials for oxide-based cathodes and the factors affecting the oxygen reduction reaction (ORR) activity were evaluated. The high concentration sol-gel method was employed to prepare the precursor. Heat treatment in Ar containing 4% H2 at 700–900 °C was effective for conferring ORR activity to the oxide. Notably, the onset potential for the ORR of the catalyst prepared at 700 °C was approximately 1.0 V vs. RHE, resulting in high quality active sites for the ORR. X-ray (diffraction and photoelectron spectroscopic) analyses and ionization potential measurements suggested that localized electronic energy levels were produced via heat treatment under reductive atmosphere. Adsorption of oxygen molecules on the oxide may be governed by the localized electronic energy levels produced by the valence changes induced by substitutional metal ions and/or oxygen vacancies.

EQeq+C: An Empirical Bond-Order-Corrected Extended Charge Equilibration Method.

The extended charge equilibration (EQeq) scheme computes atomic partial charges using the experimentally measured ionization potentials and electron affinities of atoms. However, EQeq erroneously predicts constant (environment independent) charges for high-oxidation-state transition metals in amine-templated metal oxide (ATMO) compounds, contrary to the variation observed in iterative Hirshfeld (Hirshfeld-I) charges, bond-valence sum calculations, and formal oxidation state calculations. To fix this problem, we present a simple, noniterative empirical pairwise correction based on the Pauling bond-order/distance relationship, which we denote EQeq+C. We parametrized the corrections to reproduce the Hirshfeld-I charges of ATMO compounds and REPEAT charges of metal organic framework (MOF) compounds. The EQeq+C correction fixes the metal charge problem and significantly improves the partial atomic charges compared to EQeq. We demonstrate the transferability of the parametrization by applying it to a set of unrelated dipeptides. After an initial parametrization, the EQeq+C correction requires minimal computational overhead, making it suitable for treating large unit cell solids and performing large-scale computational materials screening.

The design of double electrostatic-lens optics for resonance enhanced multiphoton ionization and photoelectron imaging experiments.

Compared to single ion/electron-optics for velocity-map imaging, a double-focusing lens assembly designed not only allows for mapping velocity imaging of photoelectrons but also allows for investigating the vibrational structure of the intermediate states of neutral species in resonance enhanced multiphoton ionization (REMPI) spectra. In this presentation, in order to record REMPI and photoelectron spectra separately, we have constructed a compact photoelectron velocity-map imaging (VMI) apparatus combined with an opposite linear Wiley-Mclaren time-of-flight mass spectrometer (TOFMS). A mass resolution (m/Δm) of ∼1300 for TOFMS and electron energy resolution (ΔE/E) of 2.4% for VMI have been achieved upon three-photon ionization of Xe atom at 258.00 nm laser wavelength. As a benchmark, in combination of one-color (1 + 1) REMPI and photoelectron imaging of benzene via 6(1) and 6(1)1(1) vibronic levels in the S1 state, the vibrational structures of the cation and photoelectron angular anisotropy are unraveled. In addition, two-color (1 + 1') REMPI and photoelectron imaging of aniline was used to complete the accurate measurement of ionization potential (62,271 ± 3 cm(-1)). The results suggest that the apparatus is a powerful tool for studying photoionization dynamics in the photoelectron imaging using vibrational-state selected excitation to the intermediate states of neutrals based on REMPI technique.