Measured Ionization Potentials Research Articles

Surface ionization of the lanthanides

The surface ionization of Lanthanides: Ce, Pr, Nd, Sm and Eu, and its fluctuations (noises) have been studied using a double filament ion source in a 90° magnetic mass spectrometer. Using the Saha-Langmuir equation there were measured ionization potentials of LnO molecules as the dissociation products of the oxides Ln 2O 3 (the Ln symbol denotes the arbitrary rare earth elements: Ce, Pr, Nd, Sm, Eu). Ionic desorption energies of LnO molecules were determined using the fluctuation (noise) method.

Accurate ionization potentials for UO and UO2: a rigorous test of relativistic quantum chemistry calculations.

Accurate ionization potential (IP) measurements provide essential thermodynamic information and benchmark data that can be used to evaluate the validity of electronic structure models. Calculations of the first IP of UO2 using relativistic methods consistently predict values that are approximately 0.7 eV higher than the accepted experimental value. The present measurements validate the theoretical calculations and show that the previous determinations corresponded to the ionization of thermally excited molecules. Similarly, new measurements of the IP for UO show that the currently accepted value is too low by 0.4 eV.

Predicted infrared and Raman spectra for neutralTi8C12isomers

Using a density-functional based algorithm, the full IR and Raman spectra are calculated for the neutral Ti_8C_12 cluster assuming geometries of Th, Td, D2d and C3v symmetry. The Th pentagonal dodecahedron is found to be dynamically unstable. The calculated properties of the relaxed structure having C3v symmetry are found to be in excellent agreement with experimental gas phase infrared results, ionization potential and electron affinity measurements. Consequently, the results presented may be used as a reference for further experimental characterization using vibrational spectroscopy.

Preparation of New Hole Transport Polymers via Copolymerization ofN,N ′-Diphenyl-N,N′-bis(4-alkylphenyl)benzidine (TPD) Derivatives with 1,4-Divinylbenzene

Monomers containing N,N ′,N,N ′-tetraphenylbenzidine (TPD) were prepared by introducing two alkyl or alkoxy substituents into the p-position of two of the four phenyl groups in the triphenylamine unit. The monomers were copolymerized with 1,4-divinylbenzene (DVB) in the presence of p-toluenesulfonic acid and Lewis acids, such as SnCl4 and BF3. The polymerization conditions and the catalyst types showed a marked influence on yield, molecular weight and even polymer structure. 1H NMR measurements revealed that, by using p-toluenesulfonic acid as a catalyst, the linkage between the TPD unit and DVB occurs at the p-position of the phenyl group and at the m-position of the tolyl group in the TPD unit, whereas, with SnCl4 or BF3 catalyst, substitution occurs exclusively at the p-position of TPD. All polymers are soluble in common organic solvents, which allows the preparation of thin films of good quality. The rigid structure of the polymeric backbone results in a high thermal stability up to 400°C. The results of ionization potential measurements and the redox behavior showed that the introduction of the TPD unit into this kind of polymer backbone does not change its electronic structure. On the basis of high electroactivity, the polymers were successfully used as hole transport layers in two-layer electroluminescence devices.

Preparation and characterization of poly(4-alkyltriphenylamine) by chemical oxidative polymerization

High molecular weight poly(4-alkyltriphenylamine) was synthesized by chemical oxidation polymerization. 13 C NMR spectra of the obtained polymers revealed that coupling reaction occurred exclusively at the para-position of unsubstituted N-phenyl rings. Poly(4- n-butyltriphenylamine) (P- nBTPA) showed better solubility and processability than polymers prepared from 4-methyltriphenylamine (MTPA) and 4- t-butyltriphenylamine ( tBTPA) because of the high molecular weight and the flexible long n-butyl group in the side chain. Thus, P- nBTPA allows us to prepare the film from its solution in organic solvent with ease. The rigid structure of polymeric backbone affords high glass transition temperature of 182 °C, indicating high thermal stability. Extended conjugation over polymer chain results in red shift of UV absorption up to 375 nm and low ionization potential compared to monomer. The results of the ionization potential measurement and the redox behavior suggest that the polymer has great potential as a hole-transporting material (HTM). The drift mobility of P- nBTPA measured by a standard time-of-flight (TOF) method was found to be of the order of 10 −5 cm 2/(V s). The results of the photoconductivity measurement revealed that the photoconductivity of P- nBTPA showed much higher than that of poly( N-vinylcarbazole).

Double-resonant photoionization efficiency spectroscopy: A precise determination of the adiabatic ionization potential of DCO

We report the first high-resolution measurement of the adiabatic ionization potential of DCO and the fundamental bending frequency of DCO+. Fixing a first-laser frequency on selected ultraviolet transitions to individual rotational levels in the (000) band of the 3pπ 2Π intermediate Rydberg state of DCO, we scan a second visible laser over the range from 20 000 to 20 300 cm−1 to record double resonance photoionization efficiency (DR/PIE) spectra. Intermediate resonance with this Rydberg state facilitates transitions to the threshold for producing ground-state cations by bridging the Franck–Condon gap between the bent neutral radical and linear cation. By selecting a single rotational state for ionization, double-resonant excitation eliminates thermal congestion. Spectroscopic features for first-photon resonance are identified by reference to a complete assignment of the 3pπ 2Π(000)−X 2A′(000) band system of DCO. Calibration with HCO, for which the adiabatic ionization threshold is accurately known, establishes an experimental instrument function that accounts for collisional effects on the shape of the photoionization efficiency spectrum near threshold. Analysis of the DR/PIE threshold for DCO yields an adiabatic ionization threshold of 65 616±3 cm−1. By extrapolation of vibrationally autoionizing Rydberg series accessed from the Σ+ component of the 3pπ 2Π(010) intermediate state, we determine an accurate rotationally state-resolved threshold for producing DCO+(010). This energy, together with the threshold determined for the vibrational ground state of the cation provides a first estimate of the bending frequency for DCO+ as 666±3 cm−1. Assignment of the (010) autoionization spectrum further yields a measurement of an energy of 4.83±0.01 cm−1 for the (2-1) rotational transition in the Σ+1(0110) state of DCO+.

Electric dipole polarizability of one excess-electron alkali–halide cluster

In this communication, we present the first measurement of the electric dipole polarizability of alkali–halide clusters with one excess electron. The polarizability is strongly size dependent. Very large values are observed for certain sizes. Ionization potential measurement is also presented for comparison.

Photoabsorption and ionization energies of nonstoichiometric CsI clusters: Metallization of a salt

Cs and CsI vapors were mixed to produce clusters having compositions varying from pure metal to the ionic salt. Ionization potential measurements on these clusters were performed using photoionization and a time-of-flight mass spectrometer. Results are reported for two metallization sequences, Cs14In for n=1–13 and CsmIn for m+n=26, 27 and m>n. The ionization potentials show a good qualitative agreement with recent computations and experiments carried out on nonstoichiometric NaF. Photoabsorption spectra are presented for CsmIn+ clusters for m+n=26, 27. The spectra were obtained by heating mass selected clusters in a beam by means of photoabsorption to induce evaporation of atoms. The resulting mass loss was observed in a time-of-flight mass spectrometer. The spectra of metal rich clusters with m−n>8 are very similar, showing one broad absorption peak around 1.42 eV, the energy of the plasmon resonance of pure Cs clusters of this size. CsmIn+ clusters with less than 7 excess valence electrons show a clear peak splitting. The shape and width of the peaks is not consistent with the picture of a homogeneous metallic cluster. In this range, a “demetallization” seems to take place.

The generalized parameterization procedure for the semiempirical MO method and calculation of ionization potentials for the third row element molecules

The valence electron ionization potentials ( I l ) were collected from a unique resource for the elements from H to Xe. The electron affinities ( A 1) of halogen atoms in each period were obtained from the same resource of I l . The A l values of the remaining elements were obtained by a combined formulation of Slater-Condon integrals and Pariser-Parr approximation. The generalized bond parameter proportionality constant K nl of each period was also evaluated. Using these parameters, twenty-four molecules consisting of only third row elements were selected for the s-p-d INDO-MO calculations. In order to check the reliability of the energy calculations, similar calculations of ab initio type STO-3G ∗, along with semiempirical-type PM3 and AM1 methods were also carried out for comparison. The Koopmans' theorem was applied to compare the experimentally measurement and calculation of the first ionization potentials in all the cases. The results of the present study indicated that the s-p-d INDO MO computed with the new parameterization procedure which given here is the best method among all the methods used in the present work.

New assignments in the UV spectroscopy of the small benzene–argonn clusters: The effects of a structure-selective vibrational predissociation

In this paper, new experimental results using UV spectroscopy of the small benzene–Arn clusters are presented. We have found evidence for the vibrational predissociation of the S1 state of some of these species on a nanosecond time scale and we propose a new assignment for the UV spectral features of this system. This assignment is consistent with other experimental data and it accounts additionally for the previously reported spectral anomalies, in particular the ionization potential measurements of these species. The two-color R2PI spectra performed can thus be considered as nanosecond pump–probe experiments that allow us to estimate the relaxation rate of the S1state of these clusters. The vibrational relaxation is found to be strongly size and structure dependent: The two isomers [(1/1) and (2/0)] of the n=2 species exhibit different lifetimes differing by at least one order of magnitude. The size dependence of the relaxation process within the one-sided isomer series appears also paradoxical since the relaxation rates of the (3/0) and (4/0) species are found to be smaller than that of the (2/0). These properties are discussed in relationship with the symmetry of these species.

Experimental and theoretical characterization of the S1–S transition of benzo[a]pyrene

We use fluorescence excitation and two-color resonance-enhanced multiphoton ionization (REMPI) spectroscopy to characterize the S1–S0 transition of benzo[a]pyrene, B[a]P. In addition, two-color REMPI with a tunable ionization laser provides an improved measurement of the molecular ionization potential, 7.103±0.004 eV. Ab initio calculations are performed to determine the vibrational frequencies for the 90 vibrational modes in the S1 and S0 electronic states. Our results indicate that the single-excitation configuration interaction (CIS) level of theory provides an excellent representation of excited-state frequencies for this large molecule. Virtually all of the vibronic bands between ∼380 nm and 397 nm, as well as numerous bands between 373 nm and 380 nm, are assigned with the aid of the calculated vibrational frequencies. In total, 27 of the 61 a′ symmetry vibrational modes and 5 of the 29 a″ vibrational modes in the S1 state have now been assigned.

Ionization potential measurement of Na nF n- p for p up to 5

Na n F n- p clusters were produced in a laser vaporization source. They were ionized by one photon and detected in a high-resolution time of flight mass spectrometer. Only odd values of p were observed, apart from Na 4F 2, Na 8F 6, Na 14F 12 and Na 23F 21. Ionization potentials (IP) were measured for n up to 30 and p up to 5. The IPs of the Na 14F 14- p sequence are in good agreement with existing theoretical predictions. Na 15F 12 and Na 24F 21 have a surprisingly low IP, which indicates that the metallization sequence is of a different nature in this case.

PHOTOIONIZATION AND ELECTRONIC STRUCTURE OF HYDROGENATED LITHIUM CLUSTERS

The ionization-potential measurements of Li n H m clusters with 3≤n≤11 and 0≤m≤6 are presented. Their evolution is dominated by the number of electrons which are delocalized in the cluster. The analysis is in favor of a segregation between a metallic part and an insulating part in the mixed cluster. By comparison with alkali-halide clusters results, the existence of possible cuboid structures is discussed.

Cluster Beam Chemistry: Hydration of Nucleic Acid Bases; Ionization Potentials of Hydrated Adenine and Thymine

Gas phase hydration of adenine, cytosine, thymine, and uracil in supersonic molecular beams is examined by mass spectra and ionization potential measurements. In the beam source, designed for substances of low volatility that readily decompose on heating, a carrier gas (moist argon or pure water vapor) flows through a porous filter packed with the solid sample before exiting via a supersonic nozzle. With the electron impact energy below the ionization potential of water, progressions of cluster ions of the form Bm+(H2O)n with m = 1−3 and n as high as 15 are obtained. Cytosine, even with modest heating, loses two hydrogens and also undergoes tautomeric conversion to uracil. From appearance potential measurements and calibrations, the ionization potentials for B = adenine or thymine clusters (with m = 1, n = 0−3; m = 1 or 2, n = 0−1 or 2) were determined; relative to the neutral base monomer, hydration by three water molecules decreases the ionization potential by about 8%, or 0.7 eV.

Metal−Metal Coordination Chemistry: Free Clusters of Group 11 Elements with Sodium

Mixed metal clusters of sodium with copper, silver, and gold were produced by supersonic expansions at well-defined thermodynamic conditions. Differences in the behavior toward binary alloying, cluster formation, and relative thermodynamic stabilities are discussed within a structureless, delocalized, electronic shell model and using molecular orbital arguments and the different strengths of the relativistic effects of the coinage metals. Free-energy changes (ΔG) of selected exchange reactions are determined, showing the substitution of gold atoms to be thermodynamically the most favored. This is due to the preferred s−d hybridization in gold atoms, leading to a stronger metal−metal interaction. The measured ionization potentials reveal a distinct shell closing at the octamer for all three heterosystems. The drop of the ionization potential going from the octamer to the nonamer is most pronounced for the sodium−silver heteroclusters and is almost as large as for pure sodium clusters.

Measurement of two-electron contributions to the ground-state energy of heliumlike ions.

We report direct measurements of two-electron contributions to the ground-state energy of high-{ital Z} heliumlike ions. The difference in radiative-recombination x-ray energy (i.e., ionization potential of the recombined ion) was measured for bare and hydrogenlike target ions trapped in an electron-beam ion trap for six elements ranging from {ital Z}=32 to {ital Z}=83. The achieved uncertainties (as small as 1.6 eV) test the second-order many-body contributions to the energy and approach the size of the two-electron (screened) Lamb shift. We also report a measurement of the ground-state ionization potential of heliumlike bismuth relative to heliumlike xenon. All results are in agreement with available theories.

Systematic LSD investigation on cationic boron clusters: B(n  2–14)

AbstractThe geometrical structures and properties of small cationic boron clusters B(n = 2–14) have been investigated using the local spin density (LSD) formalism. The nonlocal correction has also been calculated. The linear search for minima on the potential energy surface has been performed using analytical gradients of the LSD total energy. Most of the final structures of the cationic boron clusters prefer planar or quasi‐planar nuclear arrangements and can be considered as fragments of a planar surface or as segments of a sphere. The calculated adiabatic ionization potentials of Bn exhibit features that are analogous to those of measured ionization potentials for boron clusters. Most of the calculated normal modes of the cationic clusters have frequencies that are around 1000 cm−1 and have strong infrared intensities, and they correspond quite well with analogous properties of solid boron. © 1994 John Wiley & Sons, Inc.

Photoionization of gas-phase bromotrifluoromethane and its complexes with methanol: State dependence of intracluster reactions

Weakly bound complexes of CF3Br molecules or of CF3Br and CH3OH were prepared in supersonic jet expansions and studied by photoionization mass spectrometry. Argon was present in the jet expansion to promote cooling of the complexes. Measured ionization potentials are CF3Br, 11.404±0.014 eV; (CF3Br)2, 11.10±0.02 eV; (CF3Br⋅CH3OH), 10.76±0.05 eV. The appearance potential of CF+3 from CF3Br was found to be 11.56±0.02 eV, and evidence is presented that this value is adiabatic. From this appearance potential, ΔH0f0(CF+3)=86.6±1.1 kcal mol−1. The magnitude of the spin–orbit splitting in X̃ 2E CF3Br+ into E1/2 and E3/2 states dominates any Jahn–Teller distortion. The effects of these phenomena on the stability of X̃ 2E CF3Br+ are discussed. The following dissociation channels were observed: CF+3 from X̃ 2E CF3Br+, CF2Br+ from à 2A1 CF3Br+, Br+ from B̃ 2A2 CF3Br+, and CF+2 from D̃ 2E CF3Br+. (CF3⋅CH3OH)+ is not produced from neutral dimers, but is created when a CF3Br moiety embedded in a trimer or larger cluster is ionized to à 2A1 CF3Br+. In contrast, (Br⋅CH3OH)+ does come from dimers as well as larger clusters. The cross sections for the production of (CF3Br)+2 and (CF3Br⋅CH3OH)+ from trimers or larger progenitors are enhanced when intracluster CF3Br is ionized to CF3Br+ in the B̃ or higher electronic states. Possible reasons for these state-specific reactions are given. Finally, a new method is presented for determining neutral cluster distributions in a molecular beam, and the method is shown to be in quantitative agreement with the near-threshold technique previously published by Grover et al. [J. Phys. Chem. 95, 6473 (1991)].

Ionization potentials of CaF and BaF

Although the alkaline earth–monohalide compounds, MX, have been extensively studied, their ionization potentials to MX+ are only known to within a few tenths of an eV. Using an improved electron impact ionization potential measurement to assist in arranging spectroscopic vibronic band origins into Rydberg series, we report high precision ionization potential measurements for CaF and BaF to be IP(CaF)=5.8270±0.0006 eV and IP(BaF)=4.8034±0.0004 eV. The synergism between the electron impact method and the spectroscopic method is detailed, and the minimum accuracy required of the electron impact data is assessed.

Ground-state properties of small iron clusters

Ground-state structures of small iron clusters, N\ensuremath{\le}18, are determined from simulated-annealing calculations. Total energies are calculated using the effective-medium theory, and the d-band energy is included using the tight-binding approximation. The main effects of the partially filled d band are a higher symmetry of the ground-state structures than those assumed by simple metals such as Na and Cu and a tendency to produce hollow structures. We find a correlation between the presence of unscreened atoms in the iron-cluster ground states and measured ionization potentials and reactivities with ${\mathrm{H}}_{2}$.