Arnold M. Karo

Theoretical determination of bound–free absorption cross sections in Ar+2

Ab initio calculations have been carried out for the potential energy curves and transition moments of the 2Σ+u, 2Πg, 2Πu, and 2Σ+g states of Ar+2 which arise from the 2P+1S ion–atom asymptote. These data have been used in a theoretical calculation of the dissociative absorption cross sections from the bound 2Σ+u state to the repulsive 2Πg and 2Σ+g states. The 2Σ+u→2Πg transition, which is dominated by spin–orbit effects, has a maximum absorption cross section of 2.6×10−19 cm2 centered at 716 nm with a full width at half‐maximum of 185 nm at room temperature. The 2Σ+u→2Σ+g transition is found to be much stronger with a maximum cross section of 0.5×10−16 cm2 centered at 300 nm with a full width at half‐maximum of 75 nm at room temperature.

Generation of negative ions in tandem high-density hydrogen discharges

An optimized tandem two‐chamber negative‐ion source system is discussed. In the first chamber high‐energy (E>20 eV) electron collisions provide for H2 vibrational excitation, while in the second chamber negative ions are formed by dissociative attachment. The gas density, electron density, and system scale length are varied as independent parameters. The extracted negative ion current density passes through a maximum as electron and gas densities are varied. This maximum scales inversely with system scale length R. The optimum extracted current densities occur for electron densities nR=1013 electrons cm−2 and gas densities N2R in the range 1014–1015 molecules cm−2. The extracted current densities are sensitive to the atomic concentration in the discharge. The atomic concentration is parametrized by the wall recombination coefficient γ and scale length R. As γ ranges from 0.1 to 1.0 and for system scale lengths of 1 cm, extracted current densities range from 8.0 to 80 mA cm−2. The relative negative‐ion yie...

The lattice dynamics and statics of alkali halide crystals

I. Introduction.- 1. Historical Background.- II. General Theory.- 2. Introduction of Normal Coordinates.- 3. The Adiabatic Approximation.- III. Dipolar Models.- 4. Long-Wave Optical Vibrations of Cubic Ionic Lattices...- 4.1. Macroscopic Theory.- 4.2. Microscopic Theory.- 5. Description and Justification of the Various Dipolar Models.- 6. Derivation of the Dipolar Coupling Coefficients.- IV. Theoretical and Experimental Single-Phonon Data.- 7. Comparison of Theoretical and Experimental Debye-Waller and Specific-Heat Data.- 7.1. Debye-Waller Factors.- 7.2. Specific-Heat Data.- 8. Direct Measurement of Phonon-Dispersion Curves.- V. Two-Phonon Data.- 9. Interpretation of the Infrared Spectra of Perfect Alkali Halides.- 10. Second-Order Raman Spectra of Alkali Halide Crystals.- VI. Dynamic and Static Defects.- 11. Theory of Impurity Vibrations.- 12. The Method of Lattice Statics 7..- VII. Conclusions.- References.

Hydrogen vibrational population distributions and negative ion concentrations in a medium density hydrogen discharge

The vibrational population distribution for hydrogen molecules in a hydrogen discharge has been calculated taking into account electron collisional excitation, molecule‐molecule, and wall collisional de‐excitation processes. Electronic excitation processes include vibrational excitation by 1 eV thermal electrons acting through the intermediary of the negative ion resonances, and vibrational excitation caused by the radiative decay of higher singlet electronic states excited by a small population of 60 eV electrons in the discharge. The molecules are de‐excited by molecular collisions transferring vibrational energy into translational energy, and by wall collisions. The distributions exhibit a plateau, or hump, in the central portion of the spectrum. The relative concentration of negative ions is calculated assuming dissociative attachment of the low temperature electrons to vibrationally excited, non‐rotating molecules. The ratio of negative ions to electrons in the discharge is calculated to be of order ...

Lattice Vibrations in Alkali Halide Crystals. I. Lithium and Sodium Halides

Vibrational frequency distributions for the lithium and sodium halides have been evaluated on the basis of the Born lattice theory by the use of Blackmans numerical‐sampling technique. Both room temperature and extrapolated 0°K parameters have been used in the calculation. Specific heats, the corresponding Debye characteristic temperatures, and the moments of the distributions have been evaluated directly from the frequencies. Comparison is made with experimental data and with other theoretical work.

Mechanism for negative‐ion production in the surface‐plasma negative‐hydrogen‐ion source

The system parameters and surface adsorption conditions of the surface‐plasma negative‐hydrogen‐ion source are reviewed. A mechanism is developed for the production of negative ions by incident energetic hydrogen atoms backscattering from a cesiated tungsten surface. The active electronic level during the course of the collision is approximated as the sum of the negative electron affinity of the cesium hydride negative molecular ion and the image potential. The model predicts negative‐ion formation for atomic collisions with all alkali‐coated surfaces for alkalis from sodium through cesium. In the case of lithium, the model does not apply due to the breakdown of the image potential approximation close to the surface. Negative‐ion formation is also expected to occur for incident positive ions which backscatter from the substrate tungsten as neutrals. The energy thresholds for incident atoms are established.

MCSCF pseudopotential calculations for the alkali hydrides and their anions

Multiconfiguration self‐consistent‐field calculations have been carried out on the X 1S+ and a 3S+ states of LiH, NaH, KH, RbH, and CsH, and on the X 2S+ states of their respective anions. Pseudopotentials, including core polarization terms, have been used to replace the core electrons, resulting in simple two‐ and three‐electron calculations. Comparisons of the neutral potential curves with experiment and other ab initio calculations (where available) show very good agreement. The agreement with ab initio calculations on LiH− and NaH− is also very good. Adiabatic electron affinities have been calculated for LiH (0.293 ev), NaH (0.316 eV), KH (0.437 eV), RbH (0.422 eV), and CsH (0.438 eV).

Abinitio MC–SCF ground‐state potential energy curves for LiH−, NaH−, and CsH−

We have evaluated the 1Σ+ ground states of LiH, NaH, and CsH and the 2Σ+ ground states of LiH−, NaH−, and CsH−, as well as the 2Σ+ ground state of CsH+, over a wide range of internuclear distances. Multiconfiguration‐self‐consistent‐field wavefunctions were obtained with the optimized valence configuration approach to the description of chemical bonding. Four configurations for the neutral molecules and seven for the negative ions provided satisfactory descriptions. All of the negative‐molecular‐ion ground states are attractive and have lower potential energies than the neutral‐parent‐hydride molecules over the internuclear distances studied. Molecular electron affinities at the equilibrium internuclear separations were found to be 0.283 eV (LiH), 0.278 eV (NaH), and 0.357 eV (CsH).

Volume dependence of the Grüneisen γ and other thermodynamic properties of NaCI

Abstract The quasi-harmonic approximation is used to calculate the Helmholtz free energy F of NaCl. The interionic forces are represented by a deformation-dipole model, which is parameterized self-consistently. We give a parameterized expression for F which can be used to predict the temperature and volume dependence of the thermodynamic properties. The expression for F is tested by comparing our predicted values for the heat capacity C p , the thermal expansion α, and the bulk modulus B s with experimental atmospheric-pressure values and by comparing predicted values for the Gruneisen γ at high pressures with the recent measurements of Boehler, Getting and Kennedy. Excellent agreement is obtained. Predictions for the volume dependence of C p , α, and B s and for the isothermal equation of state at room temperature are given. Values for the logarithmic derivative q T = ( ∂ In γ ⧸ ∂ In V) T and for the Anderson-Gruneisen parameters δ S and δ T are also given.

Recombination and dissociation of H2+ and H3+ ions on surfaces to form H2(v‘): Negative‐ion formation on low‐work‐function surfaces

The recombination and dissociation of H+2 and H+3 ions incident upon metal surfaces leads to H, H2(v‘), and H− products rebounding from the surface. A four‐step model for H+2 ‐ion recombination generates H2(v‘) via resonant electron capture through the b 3Σ+u and X 1Σ+g states. A molecular trajectory analysis provides final‐state H2(v‘) distributions for incident energies of 1, 4, 10, and 20 eV. The calculated H2: H+2 yields compare favorably with the observed yields. A similar four‐step model for incident H+3 proceeds via resonant capture to form the H3(2p 2E’→2p 2A1) ground state, in turn dissociating into H+H2(v_‘), with the fragment molecule rebounding to give the final H2(v‘) distribution. Comparing the final populations v‘≥5 for incident H+2 or H+3 shows that the H+3 ion will be more useful than H+2 for H− generation via dissociative attachment. Molecular ions incident upon low‐work‐function surfaces generate additional H2(v‘) via resonant electron capture through excited electronic states and provi...