Measured Ionization Potentials Research Articles

Measurement of the first ionization potential of lawrencium, element 103.

The chemical properties of an element are primarily governed by the configuration of electrons in the valence shell. Relativistic effects influence the electronic structure of heavy elements in the sixth row of the periodic table, and these effects increase dramatically in the seventh row--including the actinides--even affecting ground-state configurations. Atomic s and p1/2 orbitals are stabilized by relativistic effects, whereas p3/2, d and f orbitals are destabilized, so that ground-state configurations of heavy elements may differ from those of lighter elements in the same group. The first ionization potential (IP1) is a measure of the energy required to remove one valence electron from a neutral atom, and is an atomic property that reflects the outermost electronic configuration. Precise and accurate experimental determination of IP1 gives information on the binding energy of valence electrons, and also, therefore, on the degree of relativistic stabilization. However, such measurements are hampered by the difficulty in obtaining the heaviest elements on scales of more than one atom at a time. Here we report that the experimentally obtained IP1 of the heaviest actinide, lawrencium (Lr, atomic number 103), is 4.96(+0.08)(-0.07) electronvolts. The IP1 of Lr was measured with (256)Lr (half-life 27 seconds) using an efficient surface ion-source and a radioisotope detection system coupled to a mass separator. The measured IP1 is in excellent agreement with the value of 4.963(15) electronvolts predicted here by state-of-the-art relativistic calculations. The present work provides a reliable benchmark for theoretical calculations and also opens the way for IP1 measurements of superheavy elements (that is, transactinides) on an atom-at-a-time scale.

A plea for a reexamination of ionization potential depression measurements

Experiments at the Linac Coherent Light Source determined the ionization potential depression (IPD) in dense plasmas by measuring the Kα fluorescence associated with K-shell holes created by the X-ray free-electron laser. The analysis of the experimental spectrum found a significantly larger IPD than predicted by the widely used Stewart–Pyatt model. It is shown, however, that a more accurate treatment of atomic levels than used in the analysis has additional channels reducing the threshold laser energy for creating Kα photons without invoking an increase in the IPD. Thus, it is argued that a simulation of the Kα fluorescence using improved atomic data could impact the interpretation of the experimental results.

Measurement of the first ionization potential of astatine by laser ionization spectroscopy

The radioactive element astatine exists only in trace amounts in nature. Its properties can therefore only be explored by study of the minute quantities of artificially produced isotopes or by performing theoretical calculations. One of the most important properties influencing the chemical behaviour is the energy required to remove one electron from the valence shell, referred to as the ionization potential. Here we use laser spectroscopy to probe the optical spectrum of astatine near the ionization threshold. The observed series of Rydberg states enabled the first determination of the ionization potential of the astatine atom, 9.31751(8) eV. New ab initio calculations are performed to support the experimental result. The measured value serves as a benchmark for quantum chemistry calculations of the properties of astatine as well as for the theoretical prediction of the ionization potential of superheavy element 117, the heaviest homologue of astatine.

PbTiO3/SrTiO3interface: Energy band alignment and its relation to the limits of Fermi level variation

The interface formation between PbTiO${}_{3}$ and SrTiO${}_{3}$ has been studied by in situ photoelectron spectroscopy. A valence band offset of $1.1\ifmmode\pm\else\textpm\fi{}0.1 $eV, corresponding to a conduction band offset of $1.3\ifmmode\pm\else\textpm\fi{}0.1 $eV, is determined. These values are in good agreement with the band offsets estimated from measured ionization potentials of SrTiO${}_{3}$ and PbTiO${}_{3}$ surfaces. The observed band offsets are also in line with a $~1.1 $eV difference in barrier heights of PbTiO${}_{3}$ in contact with different electrode materials as compared to barrier heights of SrTiO${}_{3}$ with the same electrode materials. The results indicate that the band alignment is not strongly affected by Fermi level pinning and that the barrier heights are transitive. The limits of Fermi level variation observed from a number of thin films prepared on different substrates with different conditions are the same for both materials when these are aligned following the experimentally determined band offsets. By further comparing electrical conductivities reported for SrTiO${}_{3}$ and PbTiO${}_{3}$, it is suggested that the range of Fermi level position in the bulk of these materials, which corresponds to the range of observed conductivities, is comparable to the range of Fermi level position at interfaces with different contact materials. In particular the possibly low barrier height for electron injection into SrTiO${}_{3}$ is consistent with the metallic conduction of donor doped or reduced SrTiO${}_{3}$, while barrier heights $\ensuremath{\gtrsim}1 $eV for PbTiO${}_{3}$ are consistent with the high resistivity even at high doping concentrations. The variation of barrier heights at interfaces therefore provides access to the range of possible Fermi level positions in the interior of any, including insulating, materials, which is relevant for understanding defect properties.

Impact of Isomeric Structures on Transistor Performances in Naphthodithiophene Semiconducting Polymers

Four isomeric naphthodithiophenes (NDTs) with linear and angular shapes were introduced into the polythiophene semiconductor backbones, and their field-effect transistor performances were characterized. The polymers bearing naphtho[1,2-b:5,6-b']dithiophene (NDT3), an angular-shaped NDT, exhibited the highest mobilities of ∼0.8 cm(2) V(-1) s(-1) among the four NDT-based polymers, which is among the highest reported so far for semiconducting polymers. Interestingly, the trend of the mobility in the NDT-based polymers was contrary to our expectations; the polymers with angular NDTs showed higher mobilities than those with linear NDTs despite the fact that naphtho[2,3-b:6,7-b']dithiophene (NDT1), a linear-shaped NDT, has shown the highest mobility in small-molecule systems. X-ray diffraction studies revealed that angular-NDT-based polymers gave the highly ordered structures with a very close π-stacking distance of 3.6 Å, whereas linear-NDT-based polymers had a very weak or no π-stacking order, which is quite consistent with the trend of the mobility. The nature of such ordering structures can be well understood by considering their molecular shapes. In fact, a linear NDT (NDT1) provides angular backbones and an angular NDT (NDT3) provides a pseudostraight backbone, the latter of which can pack into the highly ordered structure and thus facilitate the charge carrier transport. In addition to the ordering structure, the electronic structures seem to correlate with the carrier transport property. MO calculations, supported by the measurement of ionization potentials, suggested that, while the HOMOs are relatively localized within the NDT cores in the linear-NDT-based polymers, those are apparently delocalized along the backbone in the angular-NDT-based polymers. The latter should promote the efficient HOMO overlaps between the polymer backbones that are the main paths of the charge carrier transport, which also agrees with the trend of the mobility. With these results, we conclude that angular NDTs, in particular NDT3, are promising cores for high-performance semiconducting polymers. We thus propose that both the molecular shapes and the electronic structures are important factors to be considered when designing high performance semiconducting polymers.

Synthesis and Properties of Terthiophene and Bithiophene Derivatives Functionalized by BF2 Chelation: A New Type of Electron Acceptor Based on Quadrupolar Structures

Terthiophene and bithiophene derivatives functionalized by BF(2) chelation were synthesized as a new type of electron acceptor, and their properties were compared to those of bifuran and biphenyl derivatives. These new compounds are characterized by quadrupolar structures due to resonance contributors generated by BF(2) chelation. The bithiophene derivative has a strong quadrupolar character compared with the bifuran and biphenyl derivatives because their hydrolytic analyses indicated that the bithiophene moiety has a larger on-site Coulomb repulsion than the others. The terthiophene derivative has a smaller on-site Coulomb repulsion than the bithiophene derivative due to the addition of a thiophene spacer. These BF(2) complexes exhibit long-wavelength absorptions and according to measurements of ionization potentials and absorption edges they have energetically low-lying HOMOs and LUMOs. The crystal structure of the bithiophene derivative is of the herringbone type, with short F···S and F···C contacts affording dense crystal packing. n-Type semiconducting behaviour was observed in organic field-effect transistors based on these BF(2) complexes.

Partially oxidized niobium carbonitride as a non-platinum catalyst for the reduction of oxygen in acidic medium

Partially oxidized NbC 0.5N 0.5 has been evaluated as a non-platinum catalyst for the reduction of oxygen in acidic medium. NbC 0.5N 0.5 powder was partially oxidized in N 2 gas containing O 2 of 10 −4 atm at the temperature range of 700–1000 °C. Partially oxidized NbC 0.5N 0.5 had a definite oxygen reduction reaction (ORR) activity, while as-prepared NbC 0.5N 0.5 and completely oxidized Nb 2O 5 had a poor catalytic activity for ORR. The onset potential of the partially oxidized NbC 0.5N 0.5 for the ORR achieved 0.92 V vs. RHE in 0.1 M H 2SO 4 at 30 °C. The results of X-ray absorption spectroscopy and ionization potential measurements suggested that oxygen-vacancy defects might be responsible for the oxygen reduction capability by creating electronically favorable oxygen adsorption sites.

Molecular double core hole electron spectroscopy for chemical analysis

We explore the potential of double core hole electron spectroscopy for chemical analysis in terms of x-ray two-photon photoelectron spectroscopy. The creation of deep single and double core vacancies induces significant reorganization of valence electrons. The corresponding relaxation energies and the interatomic relaxation energies are evaluated by complete active space self-consistent field (CASSCF) calculations. We propose a method on how to experimentally extract these quantities by the measurement of single ionization potentials (IPs) and double core hole ionization potentials (DIPs). The influence of the chemical environment on these DIPs is also discussed for states with two holes at the same atomic site and states with two holes at two different atomic sites. Electron density difference between the ground and double core hole states clearly shows the relaxations accompanying the double core hole ionization. The effect is also compared to the sensitivity of single core hole IPs arising in single core hole electron spectroscopy. We have demonstrated the method for a representative set of small molecules LiF, BeO, BF, CO, N2, C2H2, C2H4, C2H6, CO2, and N2O. The scalar relativistic effect on IPs and on DIPs are briefly addressed.

Ultraviolet photoelectron studies of 5-trifluoromethyluracil: Electronic susceptibility of substituted uracils to nucleophilic attack

UV photoelectron spectroscopy has been employed to investigate the electronic structure of 5-trifluoromethyluracil. Band assignments have been made by comparison with photoelectron results from uracil, thymine, 5-fluorouracil, and 1-methyl-5-trifluoromethyluracil. Spectroscopically measured ionization potentials have also been compared with energy levels predicted by INDO molecular orbital (MO) calculations. The calculations predict the ordering and the spacing of the four uppermost Orbitals in 5-trifluoromethyluracil with good accuracy. They also give a good representation of the shifts observed in the ionization potentials of these orbitals when spectra of uracil, thymine, 5-fluorouracil, 5-trifluoromethyluracil, and 1-methyl-5-trifluoromethyluracil are compared. Descriptions of electronic structure provided by the photoelectron results and the INDO calculations have been examined in conjunction with results from kinetic and equilibrium studies of sulfite attack at the 6 position of uracil and substituted uracils. Results of the comparison indicate that while resonance effects cause 5-fluorouracil to exhibit greater electron density at the 6 position than thymine or uracil, the order of reactivity is 5-fluorouracil ≥ uracil > thymine. The enhanced reactivity of 5-fluorouracil over that expected from considerations of electron density may be ascribed in part to inductive electron withdrawing effects of the F atom, which oppose resonance effects and which stabilize the transition state involved in the addition of an SO2−3 ion.

Spectroscopic and physicochemical studies on the charge transfer complex of dextromethorphan drug with iodine

The drug dextromethorphan had been shown to form a charge transfer (CT) complex of 1 : 1 stoichiometry with iodine in different solvents. The enthalpy and entropy of formation of this complex had been determined by estimating the formation constant spectrophotometrically at three different temperatures. The solvent variation study on the intermolecular CT complex formation revealed that the intensity of the interaction increases with decrease in relative permittivity of the medium. Transition energy, oscillator strength and dipole moments of the complex as well as the ionization potential of the donor have been computed from spectroscopic data and discussed. The experimentally measured ionization potential of the donor drug closely agrees with that computed theoretically using MOPAC (PM3) semi-empirical method.

Excited electronic and ionized states of the nitramide molecule, H2NNO2, studied by the symmetry-adapted-cluster configuration interaction method

Symmetry-adapted-cluster configuration interaction (SAC-CI) wave functions were employed to compute 16 singlet and 13 triplet vertical transitions, and 14 ionized states including relative intensities of the nitramide molecule, H2NNO2. This molecule is the simplest neutral closed-shell molecule which has an N–NO2 bond and is a member of the nitramine family, R,R′N(NO2), an important class of energetic materials with practical applications. The present nitramide results showed strong similarities with the ones of the N, N-dimethylnitramine molecule, which has also an N–NO2 bond and was previously studied using the SAC-CI method. Experimental ultraviolet and photoelectron band spectra of the nitramide molecule could be successfully assigned. All the singlet transitions have valence character. The computed singlet and triplet transitions, excepting a singlet one, result from excitation originating in the four highest occupied molecular orbitals, which have close energies. Most of the singlet and triplet transitions involved mixing of singly excited configurations. The strongest computed transition, at 6.8 eV, is a mixture of two nπNO2 → π* configurations corresponding to excitations from the highest occupied molecular orbital (HOMO) to the first two virtual orbitals and has an optical oscillator strength value of 0.2665. The computed ionized states described the whole measured spectrum, have excellent agreement when compared with the measured ionization potentials and revealed an inversion of the ordering of the first states not expected according to Koopmanns’ theorem, thereby showing the limitations of the latter.

On the Determination of Monomer Dissociation Energies of Small Water Clusters from Photoionization Experiments

Recently, monomer dissociation energies of neutral water clusters were estimated via a thermodynamic cycle that utilized the measured appearance energies of vacuum ultraviolet (VUV) photoionized water clusters and the previously reported dissociation energies of protonated water clusters. The thermodynamic cycle incorrectly assumed that the energy difference between the (H2O)n+ --> (H2O)n-1H+ + OH* asymptotes (the relaxation energy) was zero. We show that these relaxation energies are large and cannot be neglected in the analysis. Thus, the neutral water cluster monomer dissociation energies cannot be directly determined from the measured ionization potentials because they are themselves involved in the appropriate thermodynamic cycle.

Catalytic Activity of Zirconium Oxynitride Prepared by Reactive Sputtering for ORR in Sulfuric Acid

Zirconium oxynitride (ZrO x N y ) prepared by the reactive sputtering method under O 2 and N 2 atmosphere was evaluated as a nonplatinum cathode catalyst for polymer electrolyte fuel cells. The ZrO x N y prepared at p Ar = 0.886 Pa, p N2 = 0.41 Pa, and p O2 = 1.7 mPa with a substrate temperature of 800°C during the film formation had a superior catalytic activity for the oxygen reduction reaction (ORR) in sulfuric acid. The onset potential for the ORR had a maximum of 0.8 V vs a reversible hydrogen electrode. The thin-film X-ray diffraction analysis and ionization potential measurement revealed that the ZrO x N y with a high catalytic activity for the ORR contained the Zr 2 ON 2 crystalline structure.

A PbS nanocrystal-C60 photovoltaic device for infrared light harvesting

PbS nanocrystal (nc-PbS)-C60 photovoltaic devices are demonstrated, in which nc-PbS function as electron donors, showing infrared photosensitivity up to 1600nm. Annealing nc-PbS is proved to remove capping oleic acid ligands, studied using x-ray photoelectron spectroscopy, significantly improving the short circuit current, open circuit voltage, and fill factor. The device performance is rationalized by quantum confinement in nc-PbS and energy level alignment at the heterojunction based on direct measurements of nc-PbS ionization potential using ultraviolet photoelectron spectroscopy.

Ionization Thresholds of Small Carbon Clusters: Tunable VUV Experiments and Theory

Small carbon clusters (Cn, n = 2-15) are produced in a molecular beam by pulsed laser vaporization and studied with vacuum ultraviolet (VUV) photoionization mass spectrometry. The required VUV radiation in the 8-12 eV range is provided by the Advanced Light Source (ALS) at the Lawrence Berkeley National Laboratory. Mass spectra at various ionization energies reveal the qualitative relative abundances of the neutral carbon clusters produced. By far the most abundant species is C3. Using the tunability of the ALS, ionization threshold spectra are recorded for the clusters up to 15 atoms in size. The ionization thresholds are compared to those measured previously with charge-transfer bracketing methods. To interpret the ionization thresholds for different cluster sizes, new ab initio calculations are carried out on the clusters for n = 4-10. Geometric structures are optimized at the CCSD(T) level with cc-pVTZ (or cc-pVDZ) basis sets, and focal point extrapolations are applied to both neutral and cation species to determine adiabatic and vertical ionization potentials. The comparison of computed and measured ionization potentials makes it possible to investigate the isomeric structures of the neutral clusters produced in this experiment. The measurements are inconclusive for the n = 4-6 species because of unquenched excited electronic states. However, the data provide evidence for the prominence of linear structures for the n = 7, 9, 11, 13 species and the presence of cyclic C10.

Ionization potentials of neutral atoms and positive ions in the limit of large atomic number

The many-electron Schr\"odinger equation allows one to make predictions for the nonrelativistic ionization potentials of both highly charged positive atomic ions and neutral atoms in the limit of large atomic number $Z$. Beginning with theoretical configuration interaction data on both Li- and Be-like positive atomic ions by K.T. Chung et al. [Phys. Rev. A 45, 7766 (1992); Phys. Rev. A 47, 1740 (1993)], their nonrelativistic first ionization potentials $I$ are plotted in the form of $I∕{Z}^{2}$ versus $1∕Z$. In these two sequences, it is then clear that ${\mathrm{lim}}_{Z\ensuremath{\rightarrow}\ensuremath{\infty}}(I∕{Z}^{2})$ remains finite and that this limit is approached essentially linearly. Interpretation is afforded by means of the so-called $1∕Z$ expansion of atomic theory. The question of the behavior of the nonrelativistic ionization potentials for large $Z$ is then addressed for neutral atoms. We perform density functional theory (DFT) calculations using the local density approximation with Dirac exchange and neglecting Coulomb correlation for the heavy actinides as well as for the rare gas and alkali atoms from $Z=10$ up to $Z=291$. Our results suggest that there is a nonzero value for ${\mathrm{lim}}_{Z\ensuremath{\rightarrow}\ensuremath{\infty}}I$ for both the noble gases and the alkalis, the second one implying that there is a lower bound of about $2.56\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ to the ionization potential of any nonrelativistic neutral atom. We also explore an alternative approach via the one-body potential $V(r)$ of DFT by calculating the eigenenergies of the highest occupied atomic orbital in neutral atoms. Finally, relativistic corrections needed to compare measured ionization potentials to the predictions from the Schr\"odinger equation are briefly considered.

Accurate ab Initio Binding Energies of the Benzene Dimer

Accurate binding energies of the benzene dimer at the T and parallel displaced (PD) configurations were determined using the single- and double-coupled cluster method with perturbative triple correction (CCSD(T)) with correlation-consistent basis sets and an effective basis set extrapolation scheme recently devised. The difference between the estimated CCSD(T) basis set limit electronic binding energies for the T and PD shapes appears to amount to more than 0.3 kcal/mol, indicating the PD shape is a more stable configuration than the T shape for this dimer in the gas phase. This conclusion is further strengthened when a vibrational zero-point correction to the electronic binding energies of this dimer is made, which increases the difference between the two configurations to 0.4-0.5 kcal/mol. The binding energies of 2.4 and 2.8 kcal/mol for the T and PD configurations are in good accord with the previous experimental result from ionization potential measurement.

A history of mass spectrometry in Australia

An interest in mass spectrometry in Australia can be traced back to the 1920s with an early correspondence with Francis Aston who first visited these shores a decade earlier. The region has a rich tradition in both the development of the field and its application, from early measurements of ionization and appearance potentials by Jim Morrison at the Council for Scientific and Industrial Research (CSIR) around 1950 to the design and construction of instrumentation including the first use of a triple quadrupole mass spectrometer for tandem mass spectrometry, the first suite of programs to simulate ion optics (SIMION), the development of early TOF/TOF instruments and orthogonal acceleration and the local design and construction of several generations of a sensitive high-resolution ion microprobe (SHRIMP) instrument. Mass spectrometry has been exploited in the study and characterization of the constituents of this nation's unique flora and fauna from Australian apples, honey, tea plant and eucalyptus oil, snake, spider, fish and frog venoms, coal, oil, sediments and shale, environmental studies of groundwater to geochronological dating of limestone and granite, other terrestrial and meteoritic rocks and coral from the Great Barrier Reef. Peter Jeffery's establishment of geochronological dating techniques in Western Australia in the early 1950s led to the establishment of geochronology research both at the Australian National University and at what is now the Curtin Institute of Technology in the 1960s. This article traces the history of mass spectrometry in its many guises and applications in the island continent of Australia. An article such as this can never be complete. It instead focuses on contributions of scientists who played a major role in the early establishment of mass spectrometry in Australia. In general, those who are presently active in the field, and whose histories are incomplete, have been mentioned at best only briefly despite their important contributions to the field.

Ab initio calculations for the photoelectron spectra of vanadium clusters.

We report ab initio calculations for the electronic and structural properties of V(n), V(n) (-), and V(n) (+) clusters up to n=8. We performed the calculations using a real-space pseudopotential method based on the local spin density approximation for exchange and correlation. This method assumes no explicit basis. Wave functions are evaluated on a uniform grid; only one parameter, the grid spacing, is used to control convergence of the electronic properties. Charged states are easily handled in real space, in contrast to methods based on supercells where Coulombic divergences require special handling. For each size and charge state, we find the lowest energy structure. Our results for the photoelectron spectra, using the optimized structure, agree well with those obtained by experiment. We also obtain satisfactory agreement with the measured ionization potential and electron affinity, and compare our results to calculations using an explicit basis.

Synthesis and characterization of novel light-emitting copolymers containing triphenylamine derivatives

Novel light-emitting copolymers containing 4- n-butyltriphenylamine and either anthracene, diphenylanthracene or perylene units were synthesized by polycondensation of two types of dibromides in the presence of bis(1,5-cyclooctadiene)nickel(0). The structure and incorporation ratio of the copolymers were determined by 1 H NMR analysis. From the results of yield and molecular weight measured by gel permeation chromatography, the reactivity of arene units with the 4- n-butyltriphenylamine unit shows decreasing tendency in the order diphenylanthracene>anthracene>perylene. The results of ionization potential measurement reveal that the copolymers have not only hole transporting ability but also electron transporting ability. Single-layer electroluminescent devices were fabricated using these copolymers as a charge transporting and light emitting layer.