Distorted-wave Born approximation calculations of (e, 2e) reactions

The distorted-wave Born approximation (DW-BA) is discussed and samples of calculations are presented for ionization of the 2p inner shell of Ar, the 5p, 5s and 4d shells of Xe, and for ionization of He in asymmetric perpendicular plane geometry. Agreement with measurements of inner shell ionization of Ar is excellent. It is pointed out that triple differential cross sections for ionization of heavy atoms can exhibit much structure, which presents a challenge to both theory and experiment. Particular cases of 5s and 4d ionization of Xe are given as examples of situations worthy of experimental investigation. Comparison is made with very recent measurements of ionization of He in asymmetric perpendicular plane geometry. In agreement with experiment, DWBA shows at all incident energies a single main peak at φ=180°, where φ is the angle between the outgoing electrons. It is demonstrated that at high energies this peak arises from a double collison mechanism. This contrasts with energy-sharing ionization into the perpendicular plane where the double collision mechanism, dominant at high energies, gives a peak at φ=90°, and where with reducing energy this peak is replaced by one at φ=180° coming from the single collision mechanism. Suggestions are made for further experiments.     

Gas-phase conformations and exciton couplings in 5,6,11,12-tetrahydrodibenzo[a,e]cyclooctene

The conformations and exciton couplings in 5,6,11,12-tetrahydrodibenzo[a,e]cyclooctene (THDC) have been studied using resonance-enhanced two-photon ionization spectroscopy in a supersonic jet expansion. It has been estimated from the spectral analysis that 90% of THDC exists in the twist-boat (TB) conformation; the chair (C) conformer constitutes the remaining 10%. Most of the vibronic activity in the spectrum of THDC is associated with the symmetric flapping of the aromatic rings of the TB conformer. The observed S₁/S₂ exciton splitting of the TB conformer is 100 cm^?1. The S₁/S₂ transition of the C conformer is found to be forbidden. The exciton splittings of the C and TB conformers were estimated by the spectral analysis of two deuterated isotopomers of THDC. The estimated exciton splittings of the C and TB conformers are 14.7 and 101.9 cm^?1, respectively. The supramolecular model of bichromophores with identical chromophores at the CIS/6-31+G(d)//HF/6-31+G(d) level of theory predicted electronic coupling energies that are very close to the experimental exciton coupling energies.     

Recoil-ion momentum distribution for He(e,2e)He+-and He(e,3e)He++-reactions

Using Recoil-Ion Momentum Spectroscopy (RIMS) we have measured the momenta of recoiling target ions in He(e,2e)He⁺- and He(e,3e)He⁺⁺-reactions at impact energies between 270 ev and 3200 eV. The recoil-ion momentum reflecting the sum momentum of all outgoing electrons was determined for the first time in all three spatial dimensions separately for single and double ionization by electron impact. The data are compared to results of a nCTMC-calculation.     

Vertical ionization energies of halogen anions in solution

Based on the constrained equilibrium state theory, the nonequilibrium solvation energy is derived in the framework of the continuum model. The formula for spectral shift and vertical ionization energy are deduced for a single sphere cavity with the point charge assumption. The new model is adopted to investigate the vertical ionization for halogen atomic and molecular anions X^? (X = Cl, Br, I, Cl₂, Br₂, I₂) in aqueous solution. According to the calculation using the CCSD-t/aug-cc-pVQZ method in vacuum, our final estimated vertical ionization energies in solution are very close to the experimental observations, while the traditional nonequilibrium solvation theory overestimates these vertical ionization energies.     

Tuning Charge Transfer in Ion–Surface Collisions at Hyperthermal Energies

Charge exchange in ion–surface collisions may be influenced by surface adsorbates to alter the charge state of the scattered projectiles. We show here that the positive‐ion yield, observed during ion scattering on metal surfaces at low incident energies, is greatly enhanced by adsorbing electronegative species onto the surface. Specifically, when beams of N⁺ and O⁺ ions are scattered off of clean Au surfaces at hyperthermal energies, no positive ions are observed exiting. Partial adsorption of F atoms on the Au surface, however, leads to the appearance of positively charged primary ions scattering off of Au, a direct result of the increase in the Au work function. The inelastic energy losses for positive‐ion exits are slightly larger than the corresponding ionization energies of the respective N and O atoms, which suggest that the detected positive ions are formed by surface reionization during the hard collision event.     

Total ionization and electron attachment cross sections of CCl2F2 by electron impact

The absolute total ionization cross sections from threshold to 250 eV and dissociative attachment cross sections from zero to 10 eV have been measured for the CCl₂F₂ (dichloro-difluoro-methane) molecule by using a parallel plate condenser type ionization chamber. The maximum of the ionization cross-section curve was found to be at an energy of about 90 eV with a cross section of 1.44 × 10^?19 m². The attachment cross-section curve shows three peaks, the most intense being at zero electron energy with a cross-section value of 1.80 × 10^?20 m², and the other two at energies of 0.6 eV and 3.5 eV, respectively. The maximal relative error in cross-section values is 0.08, for electron energies larger than 0.4 eV.     

Gas-phase ion-molecule bimolecular and clustering reactions in dilute solutions of acetonitrile in xenon,krypton, argon,neon and helium

Gas-phase bimolecular and clustering reactions of acetonitrile in Xe, Kr, Ar, Ne and He were studied at high chemical ionization pressures in the new coaxial ion source at Auburn. With electron energies near the ionization threshold, the mass spectra are exceedingly simple and are comprised of [CH₄CH]⁺ and clusters of [CH₄CN]⁺ with various ligands such as H₂O and CH₃CN. At higher electron energies many other peaks appear. The intensities of the new peaks depend upon the ionization potential of the charge transfer gas, the ionizing electron energy and the ion source conditions, and are due to reactions of fragment ions. Residence time distributions at electron energies above the ionization threshold (∼ 30 eV) demonstrate that two molecular structures are present in the ion beam at m/z 42, one presumably is protonated acetonitrile ([CH₃CNH]⁺) while the evidence indicates that the second species does not contain acidic hydrogens. With ionizing electron energies near threshold (∼ 10. 5 eV) only one structure is observed. Studies with electron energies near the ionization threshold under high-pressure chemical ionization conditions result in greatly simplified mass spectra and are possible only because of the coaxial geometry of the ion source.     

Design and calibration of an electrostatic energy analyzer-time-of-flight mass spectrometer for measurement of laser-desorbed ion kinetic energies

The design of a hybrid electrostatic energy analyzer-time-of-flight mass spectrometer for measurement of ion kinetic energies produced by laser desorption ionization is presented. The need for experimental evaluation of the calibration and performance of the instrument is discussed and a novel laser multiphoton ionization technique, which allows experimental calibration of the energy bandpass of the electrostatic energy analyzer, is described. Laser multiphoton ionization at varying electric field strengths also allows the effects of electric field distortions on energy resolution of the instrument to be probed. Measurement of the translational energies of ions produced by 266-nm laser desorption ionization at 48 mJ/cm² of material adsorbed to a stainless steel probe by using this instrument also is presented. Ion translational energies of +19±5, +10±5, and +10±5 eV are found for adsorbed Na⁺, K⁺, and m-xylene M⁺, respectively.     

Experimentally determined and calculated values of the energies required to form doubly charged ions of the bromomethanes CH3Br,CH2Br2 and CHBr3

Two experimental techniques were used to determine the double ionization energies of CH₃Br, CH₂Br₂ and CHBr₃. In one, these energies were measured directly by double-charge-transfer spectroscopy. In the other, charge stripping of [CH₃Br]⁺, [CH₂Br₂]⁺ and [CHBr₃]⁺ ions was investigated and the ionization energies of the singly charged ions were measured. The double ionization energies of the molecules obtained by adding known single ionization energies of the molecules to the single ionization energies of the ions were in good agreement with those determined by double-charge-transfer spectroscopy. The relevant mean values from the two techniques were 28.9 ± 0.5, 27.5 ± 0.5 and 29.1 ± 0.5 eV for the double ionization energy of CH₃Br, CH₂Br₂ and CHBr₃, respectively. The results of ab initio calculations using second-order Møller-Plesset perturbation theory were in good agreement with the observed double ionization energies; they were consistently slightly lower than the experimental values.     

Spectroscopy of size-selected sodium clusters

The method of angular resolved elastic scattering has been used to achieve a size selection of neutral sodium clusters. After the scattering process the clusters are ionized by single photon ionization with various laser energies and detected by a time-of-flight mass spectrometer. The elastic scattering of Na₂ by Ne serves as a test for the experimental setup. Angular dependence of scattered sodium clusters ionized with 308 nm shows a significant amount of fragmentation of the neutral Na₆ to Na₉ clusters which are observed in the Na ₅ ⁺ and Na ₆ ⁺ ion channels.     

Observation of the atomic Ramsauer—townsend effect in 4He4He scattering

Measurement of the total scattering cross section for ⁴He⁴He at very low energies are presented. The measurements have resolved the atomic Ramsauer—Townsend effect.     

On the structure and valence ionization energies of o-benzyne

The valence ionization energies of o-benzyne, computed using a Green function method and by CI calculations at a molecular geometry optimized with the 6-31G^* basis using a two-term GVB wavefunction, suggest an assignment of the photoelectron spectrum of this molecule which differs from that given by a previous MNDO calculation. The first three ionization energies are found to be nearly degenerate.     

The Electronic States of the Pentatetraene Radical Cation

HeIα excited photoelectron spectra of pentatetraene and the inferred ionization energies are reported. The first band has a characteristic Franck-Condon envelope similar to the first photoelectron bands of allene and butatriene. The four bands found below 16 eV ionization energy have been assigned to the X?²E, òE, B?²E and C?²B₂ states of the radical cation of pentatetraene by comparison with STO-3G and SPINDO calculations on the cumulene series. The correlation scheme includes the 2s shell ionization energies of ethylene, allene and butatriene. The π-orbital ionization trends of the cumulenes are discussed in the framework of localized orbitals calculated with the STO-3G basis set.     

A study of measured neutron elastic differential neutron cross sections for 23Na

Elastic neutron scattering angular distributions from ²³Na have been measured for incident neutron energies between 1.0 and 4.0 MeV at the University of Kentucky Accelerator Laboratory using neutron time-of-flight techniques for the scattered neutrons. This is an energy region in which existing data are very sparse. Measurements are compared with the predictions of the light particle-induced reaction code TALYS. The calculations reproduce forward angle scattering but have difficulty with relative minima in the differential cross section and large-angle scattering.     

Ionization energies of the diatomic halogens and interhalogens studied with relativistic hartree-fock-slater calculations

Vertical ionization energies from the outermost two π and two σ molecular orbitals in the diatomic halogen and interhalogen molecules have been calculated using a relativistic Hartree-Fock-Slater (HFS) method. The results obtained are in good agreement with the corresponding experimental ionization energies obtained from photoelectron spectroscopy. The spin-orbit splittings of the ²Π cationic states are rationalized in terms of the degree of localization of the partially filled π orbital in the cation on the header atom.     

Molecular Structure, Vibrational Spectrum and Ionization Energy of m-Dimethoxybenzene

The optimized molecular geometries of the three rotamers of m-dimethoxybenzene in the ground So and electronically excited Sl states were predicted by ab initio and density functional theory （DFF） calculations. Their vibrational spectra in the St state were studied by one color resonant two photon ionization （1C-R2PI） method, and their ionization energies were measured by two color resonant two photon ionization （2C-R2PI） experiment. The optimized molecular geometries showed that the total energy of conformer a was the lowest in the So state. Most of the active vibrations assigned from the 1C-R2PI spectrum were found to be of the in-plane ring modes. The ionization energies （IE） of conformers a, b and c were determined to be 63521, 64487 and 63755 cm^-1, respectively.     

Note on the atomic correlation energy

In the introductory section, we compare the total, kinetic, nuclear-electron, Coulomb, exchange, and correlation energies of ground-state atoms. From the analyses of the data, one can conclude that the Hartree-Fock (HF) model is notably good and might require only a small perturbation to become essentially an “accurate” model. For this reason and considering past literature, we present a semiempirical extension of the HF model. We start with a calibration of three independent models, each one with an effective Hamiltonian, which introduces a small perturbation on the kinetic, the nuclear-electron, or the Coulomb HF operators. The perturbations are expressed as very simple functions of products of orbital probability density. The three perturbations yield very equivalent results and the computed ground-state energies are reasonably near to the accurate nonrelativistic energies recently provided by E. Davidson and his collaborators for the 2–18 electron systems and the estimates by Clementi and his collaborators for the 19–54 electron systems. The first ionization potentials from He to Cs, the second ionization potentials from Li to Zn, and excitation energies for npⁿ, 3dⁿ, and 4s¹3dⁿ configurations are used as additional verification and validation. The above three effective Hamiltonians are then combined in order to redistribute the correlation energy correction in a way which exactly satisfies the virial theorem and maintains the HF energy ratios between kinetic, nuclear-electron, and electron-electron interaction energies; the resulting effective Hamiltonian, named “virial constrained,” yields good quality data comparable to those obtained from the three independent effective operators. Concerning excitation energies, these effective Hamiltonians yield values only in modest agreement with experimental data, even if definitively superior to HF computations. To further improve the computed excitation energies, we applied an empirical scaling in the vector coupling coefficient; this correction yields very reasonable excitations for all the configurations that we have considered. We conclude that the use of effective potentials to introduce small perturbations density-dependent onto the HF model constitutes a broad class of practical and reliable semiempirical solutions to atomic many-electron problems, can provide an alternative to popular proposals from density functional theory, and should prepare the ground for “generalized HF models.” © 1997 John Wiley & Sons, Inc. Int J Quant Chem 62: 571–591, 1997     

He-I and He-II excited photoelectron spectra of cyclohepttatrienetricarbonyl complexes of group via metals

Photoelectron spectra of Group VIA metal complexes M(CO)₃(η⁶-C₇H₈) have been assigned using experimental criteria along with quantum-mechanical calculations. A general agreement between computed and measured ionization energies has been found for molecular orbitas mainly ligand in character. Similar correlations do not hold for the highest orbitals, mainly metal d based. The energies associated with ionization processes are, in this case, largely dominated by the relaxation terms. The variations of intensities of these bands on changing the energy of ionizing radiation were of crucial significance in the assignment     

On the relation between core electron binding energies of ions in solution and their solvation energies

Experimental and computational results from the study of positive and negative ions in solution are presented. The importance of short-range interactions between ion and solvent is studied with regard to core ionization of the ion. Exchange repulsion is found to be a significant factor in the interpretation of data for both cations and anions. Experimental results are presented for the core ionization of the OH^? ion in solution. The data show a strong similarity with corresponding data for the F^? ion, resulting in a large negative solvation energy for the final core hole state. The Be²⁺ ion shows large solvation energies for both ground- and core-ionized states, which is interpreted as due to charge transfer effects between solvent and ion.