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ELECTRON-POPULATION ANALYSIS AND THE DIPOLE MOMENT OF THE LiH$sup 1$$Sigma$$sup +$ EXCITED STATE

Description: Detailed consideration is given to some properties of the first excited electronic state. Following the analysis proposed by Mulliken, SCF LCAO-MO coefficients are used to obtain qualitative information about the electron distribution. Comparison is made with earlier work with respect to the description of the excited state in terms of the interacting configurations. In addition to the excited-state dipole moment, the effective electric moment between the ground and excited states is also calculated. From this, the oscillator strength for the first allowed transition is computed as a function of the internuclear distance. (auth)
Date: July 1, 1959
Creator: Karo, A.M.
Partner: UNT Libraries Government Documents Department

Role of molecular dynamics on descriptions of shock-front processes

Description: By means of a computational approach based on classical molecular dynamics, we can begin to form a realistic picture of shock-induced processes occurring at the shock front and resulting from the detailed, violent motion associated with shock motion on an atomic scale. Prototype studies of phase transitions will be discussed. We will also examine the interaction of the shock front with defects, surfaces, voids, and inclusions, and across grain boundaries. We will focus on the critical question of how mechanical energy imparted to a condensed material by shock loading is converted to the activation energy required to overcome some initial energy barrier in an initiation process.
Date: July 22, 1981
Creator: Karo, A.M.
Partner: UNT Libraries Government Documents Department

Molecular potentials and relaxation dynamics

Description: The use of empirical pseudopotentials, in evaluating interatomic potentials, provides an inexpensive and convenient method for obtaining highly accurate potential curves and permits the modeling of core-valence correlation, and the inclusion of relativistic effects when these are significant. Recent calculations of the X/sup 1/..sigma../sup +/ and a/sup 3/..sigma../sup +/ states of LiH, NaH, KH, RbH, and CsH and the X/sup 2/..sigma../sup +/ states of their anions are discussed. Pseudopotentials, including core polarization terms, have been used to replace the core electrons, and this has been coupled with the development of compact, higly-optimized basis sets for the corresponding one- and two-electron atoms. Comparisons of the neutral potential curves with experiment and other ab initio calculations show good agreement (within 1000 cm/sup -1/ over most of the potential curves) with the difference curves being considerably more accurate. In the method of computer molecular dynamics, the force acting on each particle is the resultant of all interactions with other atoms in the neighborhood and is obtained as the derivative of an effective many-body potential. Exploiting the pseudopotential approach, in obtaining the appropriate potentials may be very fruitful in the future. In the molecular dynamics example considered here, the conventional sum-of-pairwise-interatomic-potentials (SPP) approximation is used with the potentials derived either from experimental spectroscopic data or from Hartree-Fock calculations. The problem is the collisional de-excitation of vibrationally excited molecular hydrogen at an Fe surface. The calculations have been carried out for an initial vibrotational state v = 8, J = 1 and a translational temperature corresponding to a gas temperature of 500/sup 0/K. Different angles of approach and different initial random impact points on the surface have been selected. For any given collision with the wall, the molecule may pick up or lose vibrotatonal and translational energy.
Date: May 18, 1981
Creator: Karo, A.M.
Partner: UNT Libraries Government Documents Department

Molecular potentials and relaxation dynamics

Description: The use of empirical pseudopotentials, in evaluating interatomic potentials, provides an inexpensive and convenient method for obtaining highly accurate potential curves and permits the modeling of core-valence correlation, and the inclusion of relativistic effects when these are significant. As an example, recent calculations of the chi/sup 1/..sigma../sup +/ and a/sup 3/..sigma../sup +/ states of LiH, NaH, KH, RbH, and CsH and the chi/sup 2/..sigma../sup +/ states of their anions are discussed. Pseudopotentials, including core polarization terms, have been used to replace the core electrons, and this has been coupled with the development of compact, highly-optimized basis sets for the corresponding one- and two-electron atoms. Comparisons of the neutral potential curves with experiment and other ab initio calculations show good agreement (within 1000 cm/sup -1/ over most of the potential curves) with the difference curves being considerably more accurate.
Date: March 27, 1981
Creator: Karo, A.M.
Partner: UNT Libraries Government Documents Department

''Hot spots'': Subnanometer femtosecond energy localization

Description: In a condensed energetic material an understanding of the dynamics and microscopic mechanisms underlying energy transfer between a shock front and various defects is of prime importance for a realistic description of ''hot spot'' formation and explosives initiation. A wide variety of simulations using the well-established technique of computer molecular dynamics has enabled us to obtain a general and very useful microscopic description of the processes beneath the macroscopic behavior of shocked systems. The calculation of the influence of heterogeneities such as point and line defects, voids, and grain boundaries is made possible by computer codes that can handle totally heterogeneous dynamical systems and track the dynamics of energy concentration and partitioning among the molecular bonds in the defect and the nearby region. One and two-dimensional calculations will be discussed in which the spatial and temporal dependence of the energy flux in a general lattice-defect system is calculated quantitatively as a function of shock strength, initial temperature, and initial parameters defining the lattice and defect.
Date: December 1, 1986
Creator: Karo, A.M. & Hardy, J.R.
Partner: UNT Libraries Government Documents Department

Electron energy distributions, vibrational population distributions, and negative-ion concentrations in hydrogen discharges

Description: We consider the negative ion concentrations in hydrogen discharges caused by electron excitation and dissociative attachment processes. The principal formation and destruction processes are discussed for electron densities in the range 10/sup 8/ to 10/sup 13/ electrons cm/sup -3/. Expressions are developed for calculating the high energy portion of the electron energy distribution in the discharge; using these energy distributions the electron excitation rates are evaluated. At low densities, the vibrational distribution arises from singlet electronic excitations and triplet excitations through the /sup 3/..pi../sub u/ state, in equilibrium with wall de-excitation processes. At high densities singlet excitations predominate in equilibrium with atom-molecule de-excitation processes. Possibilities for negative ion generation in a two-chamber tandem system are discussed in which the vibrational excitation occurs in a high power, high electron temperature discharge, kT/sub e/ = 5 eV, and dissociative attachment occurs in a low temperature kT/sub e/ = 1 eV, plasma chamber.
Date: June 28, 1982
Creator: Hiskes, J.R. & Karo, A.M.
Partner: UNT Libraries Government Documents Department

Volume generation of negative ions in high density hydrogen discharges. Revision 1

Description: An optimized tandem two-chamber negative-ion source system is discussed. In the first chamber high energy (E > 20 eV) electron collisions provide for H/sub 2/ 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 near nR = 10/sup 13/ electrons cm/sup -2/ and for gas densities, N/sub 2/R, in the range 10/sup 14/ to 10/sup 15/ molecules cm/sup -2/. The extracted current densities are sensitive to the atomic concentration in the discharge. The atomic concentration is parametrized by the wall recombination coefficient, ..gamma.., and scale length, R. As ..gamma.. ranges from 0.1 to 1.0 and for system scale lengths of one centimeter, extracted current densities range from 8.0 to 80. mA cm/sup -2/.
Date: November 11, 1983
Creator: Hiskes, J.R. & Karo, A.M.
Partner: UNT Libraries Government Documents Department

Volume generation of negative ions in high density hydrogen discharges

Description: A parametric survey is made of a high-density tandem two-chamber hydrogen negative ion system. The optimum extracted negative ion current densities are sensitive to the atom concentration in the discharge and to the system scale length. For scale lengths ranging from 10 cm to 0.1 cm optimum current densities range from of order 1 to 100 mA cm/sup -2/, respectively.
Date: July 1, 1983
Creator: Hiskes, J.R. & Karo, A.M.
Partner: UNT Libraries Government Documents Department

Atomic and molecular surface and volume processes in the analysis of negative hydrogen discharges

Description: The principal source of negative ion generation in hydrogen discharges is now recognized to be low-energy electron attachment to H/sub 2/(/nu//prime//prime/) molecules excited to the middle portion of the vibrational spectrum. Electron excitation processes are generally taken to be the principal source of H/sub 2/(/nu//prime//prime/) generation, with high-energy excitations through the singlet spectrum being the principal excitation process populating the active portion of the vibrational spectrum. A description of the collisional re-excitation from level /nu//prime//prime/, to level /nu//prime//prime/, requires 15 /times/ 15 matrix of cross sections linking all initial and final levels. These cross sections have been evaluated and incorporated into the modelling code. An additional source of vibrational excitation may be derived from recombination of H/sub 2//sup +/ and H/sub 3//sup +/ ions on the surfaces of the discharge. In this case the molecular ions will impinge with kinetic energies given by the plasma potential, 1--10 eV. In this paper we report the evaluation of H/sub 2/(/nu//prime//prime/) resulting from the surface recombination process. The use of low-work-function materials for the discharge surfaces makes possible two additional source of negative ions: the direct formation of negative ions by hydrogen atoms rebounding from the surface, and the dissociation of H/sub 2//sup /minus// ions formed in the surface selvage. A recent paper has demonstrated the efficacy of Ba surfaces as an active medium for H/sup /minus// formation. Here we shall extend our discussion to H/sub 2//sup +/, H/sub 3//sup +/ collisions on Ba surfaces and inventory the generation of H, H/sub 2/(/nu//prime//prime/), and H/sup /minus// rebounding from these surfaces. 16 refs., 4 figs.
Date: July 3, 1989
Creator: Hiskes, J.R. & Karo, A.M.
Partner: UNT Libraries Government Documents Department

Generation of vibrationally excited H/sub 2/ molecules by H/sub 2//sup +/ wall collisions

Description: The H/sub 2//sup +/ ions from the volume of a hydrogen discharge will strike the discharge chamber walls with a kinetic energy equivalent to the plasma potential. A three-step process is described in which the H/sub 2//sup +/ ions are neutralized in a two-stage Auger process followed by a third stage wall relaxation collision, with the net result that the incident ions are converted to ground state molecules having a broad vibrational excitation spectrum. For kinetic energies ranging from a few electron volts up to twenty electron volts a substantial fraction, approx. = 2/3, of these ions will reflect as molecules, and of this population a fraction as large as twenty percent will have vibrational excitation of v'' greater than or equal to 6. This large vibrational population will provide a contribution to the total excited level distribution that is comparable to the E-V process. Implications for negative ion generation in an optimized tandem configuration are discussed.
Date: November 11, 1983
Creator: Hiskes, J.R. & Karo, A.M.
Partner: UNT Libraries Government Documents Department

Recombination and dissociative recombination of H/sub 2//sup +/ and H/sub 3//sup +/ ions on surfaces with application to hydrogen negative ion sources

Description: A four-step model for recombination and dissociative recombination of H/sub 2//sup +/ and H/sub 3//sup +/ ions on metal surfaces is discussed. Vibrationally excited molecules, H/sub 2/(v''), from H/sub 3//sup +/ recombination are produced in a broad spectrum that enhances the excited level distribution. The application of this latter process to hydrogen negative ion discharges is discussed. 5 refs., 3 figs., 1 tab.
Date: December 1, 1988
Creator: Hiskes, J.R. & Karo, A.M.
Partner: UNT Libraries Government Documents Department

Microscopic simulations of shock propagation in condensed media: comparison between real time and frequency domains

Description: Computer molecular dynamics (CMD) is now recognized as a very powerful technique for examining the microscopic details of a wide variety of chemical and physical phenomena, including the shock-induced fast decomposition processes that characterize the shock-initiation of energetic materials. The purpose of the present paper is to describe some results obtained by new methods of post processing of CMD data. First we present a pictorial history of a canonical system which is bonded with identical potentials and has identical atomic masses. We then present Fourier transforms of the energy components of different units judiciously chosen to show the ''frequency fingerprint'' of the shock impact and passage through specific units of the system, including, e.g., the behavior of spalled fragments. To complement these studies, we also display the behavior of our canonical system when defect (point or line) are present. In these studies we monitor the motion of diatoms above and below a line defect consisting of heavy masses. The Fourier transform techniques provide optimum compromise histories which present neither too much nor too little detail.
Date: July 1, 1985
Creator: Karo, A.M.; Hardy, J.R. & Mehlman, M.H.
Partner: UNT Libraries Government Documents Department

Hydrogen vibrational population distributions and negative ion generation in tandem high-density hydrogen discharges

Description: The tandem high-density hydrogen negative-ion-source system is optimized to identify the largest possible ion concentrations and extracted ion currents. The optimization includes varying the length of the second chamber, varying neutral gas and electron densities, and varying the ratio of atomic to molecular density. Vibrational excitation occurs via high-energy electron excitation (E-V process) and H/sub 2//sup +/ surface neutralization (s-V process). These processes are considered separately and acting in parallel. The solutions are presented in terms of a dimensional scale factor, R. For a system scale length R = 1 cm, optimum extracted current densities are in the range 50 to 100 mA cm/sup -2/. A single-chamber s-V system with the high-energy electron component suppressed can provide larger ion concentrations than a tandem E-V, s-V system. 14 references, 10 figures.
Date: December 3, 1984
Creator: Hiskes, J.R.; Karo, A.M. & Willmann, P.A.
Partner: UNT Libraries Government Documents Department

Maximum current densities from tandem high density systems

Description: The tandem high-density negative-ion-source system is optimized to identify the largest possible ion concentrations and extracted ion currents. The optimization includes varying the length of the second chamber, varying neutral gas and electron densities, and varying the ratio of atomic to molecular density. The electron excitation processes, e-V and E-V, are taken into account in the first chamber. Negative ions are formed through dissociative attachment in the second chamber.
Date: March 14, 1985
Creator: Hiskes, J.R.; Karo, A.M. & Willmann, P.A.
Partner: UNT Libraries Government Documents Department

Vibrational relaxation in H/sub 2/ molecules by wall collisions: applications to negative ion source processes

Description: In the volume of a hydrogen discharge, H/sub 2/ molecules, excited to high vibrational levels (v'' > 6), are formed either by fast-electron collisions or from H/sub 2//sup +/ ions that are accelerated across the discharge-wall potential that undergo Auger neutralization prior to impact with the discharge chamber wall. We have used computer molecular dynamics to study the de-excitation and re-excitation of vibrationally-excited H/sub 2/ molecules undergoing repeated wall collisions. The initial translational energies range from thermal to 100 eV and the initial vibrational states range from v'' = 2 to v'' = 12. The average loss or gain of vibrational, rotational, translational, and total molecular energies and the survival rates of the molecules have been evaluated. At thermal energies vibrational de-excitation is the predominant process, and a consistent picture emerges of rapid energy redistribution into all the molecular degrees of freedom and a slower rate of loss of total molecular energy to the wall. At higher translational energies (1 to 100 eV) a substantial fraction of the molecules survive with large (v'' > 6) vibrational energy. This vibrational population provides a contribution to the total excited vibrational population comparable to that from the fast-electron collision process.
Date: October 1, 1984
Creator: Karo, A.M.; Hiskes, J.R. & Hardy, R.J.
Partner: UNT Libraries Government Documents Department

Hydrogen vibrational population distributions and negative ion generation in tandem high density hydrogen discharges

Description: The tandem high-density hydrogen negative-ion-source system is optimized to identify the largest possible ion concentrations and extracted ion currents. The optimization includes varying the length of the second chamber, varying neutral gas and electron densities, and varying the ratio of atomic to molecular density. Vibrational excitation occurs via high-energy electron excitation (E-V process) and H/sub 2//sup +/ surface neutralization (s-V process). These processes are considered separately and acting in parallel. The solutions are presented in terms of a dimensional scale factor, R. For a system scale length R = 1 cm, optimum extracted current densities are in the range 50 to 100 mA cm/sup -2/. A single-chamber s-V system with the high-energy electron component suppressed can provide larger ion concentrations than a tandem E-V, s-V system. 14 references, 4 figures.
Date: October 1, 1984
Creator: Hiskes, J. R.; Karo, A. M. & Willmann, P. A.
Partner: UNT Libraries Government Documents Department

Relaxation of H/sub 2/ vibrational motion by wall collisions

Description: The method of computer molecular dynamics was applied to an important technological problem where experimental data may be extremely difficult to obtain: collisional de-excitation at surfaces of vibrationally highly excited hydrogen molecules. The calculations have been carried out primarily for vibrational states about the v = 6 level. The technique is illustrated by means of a specific example and the detail that can be obtained from such calculations concerning the dynamics of the interaction is shown. Molecular states excited to the v = 4, v = 6, and v = 11 vibrational levels and the J = 1 rotational level have been examined. The translational temperature of the molecules is chosen to be 500/sup 0/K, and the wall is given thermal motion characteristic of a temperature of 500/sup 0/K. Different angles of approach have been studied, and different impact points on the surface have been selected. After the first collision, the impact point becomes random because of the stochastic nature of subsequent molecular trajectories. For any given collision with the wall, the molecule may pick up or lose vibrational, rotational, and translational energy. Careful sampling and averaging of the results of many trajectory runs is needed in order to arrive at meaningful conclusions about the probability of de-excitation as a function of such parameters as the number of wall collisions, the residence time, the wall temperature, and the microscopic nature of the wall itself.
Date: January 1, 1980
Creator: Karo, A.M.; DeBoni, T.M. & Hiskes, J.R.
Partner: UNT Libraries Government Documents Department

Computer simulations of particle-surface dynamics

Description: Our simulations of particle-surface dynamics use the molecular dynamics codes that we have developed over the past several years. The initial state of a molecule and the parameters defining the incoming trajectory can be specifically described or randomly selected. Statistical analyses of the states of the particles and their trajectories following wall collisions are carried out by the code. We have carried out calculations at high center-of-mass energies and low incidence angles and have examined the survival fraction of molecules and the dependence upon the incoming trajectory. We report also on preliminary efforts that are being made to simulate sputtering and recombinant desorption processes, since the recombinant desorption of hydrogen from typical wall materials may be an important source for vibrationally-excited hydrogen in volume sources; for surface sources the presence of occluded hydrogen may affect the concentration of atomic species.
Date: October 1, 1986
Creator: Karo, A.M.; Hiskes, J.R. & DeBoni, T.M.
Partner: UNT Libraries Government Documents Department

Generation of H/sup -/, D/sup -/ ions on composite surfaces with application to surface/plasma ion source systems

Description: We review some salient features of the experimental and theoretical data pertaining to hydrogen negative ion generation on minimum-work-function composite surfaces consisting of Cs/transition metal substrates. Cesium or hydrogen ion bombardment of a cesium-activated negatively-biased electrode exposed to a cesium-hydrogen discharge results in the release of hydrogen negative ions. These ions originate through desorbtion of hydrogen particles by incident cesium ions, desorbtion by incident hydrogen ions, and by backscattering of incident hydrogen. Each process is characterized by a specific energy and angular distribution. The calculation of ion formation in the crystal selvage region is discussed for different approximations to the surface potential. An ab initio, all-electron, local density functional model for the composite surface electronics is discussed.
Date: August 19, 1983
Creator: Hiskes, J.R.; Karo, A.M.; Wimmer, E.; Freeman, A.J. & Chubb, S.R.
Partner: UNT Libraries Government Documents Department

Generation of H sup minus , H sub 2 (v double prime ), and H atoms by H sub 2 sup + and H sub 3 sup + ions incident upon barium surfaces

Description: The generation of vibrationally excited molecules by electron excitation collisions and the subsequent generation of negative ions by dissociative attachment to these molecules has become a standard model for volume source operation. These processes have been supplemented recently by the demonstration of atom-surface recombination to form vibrationally excited molecules, and enhanced negative ion formation by protons incident upon barium electrodes. In this paper we consider the additional processes of molecular vibrational excitation generated by recombination of molecular ions on the electrode surfaces, and negative ion formation by vibrationally excited molecules rebounding from low work-function electrodes. 10 refs., 4 figs.
Date: January 1, 1989
Creator: Hiskes, J.R. & Karo, A.M.
Partner: UNT Libraries Government Documents Department

Simulation of shock-induced energy flux in molecular solids. Revision 1

Description: Computer molecular dynamics has been used to study the time evolution of the energy of diatomic molecules embedded in a monatomic host lattice when the system is shock loaded. Center-of-mass, rotational, and internal energies were each monitored. For H/sub 2/ and CH groups in an iron host, the results demonstrate rapid and violent internal excitation of a totally athermal nature. The origins of this are discussed as are the reasons for the absence of a similar effect for a CH group in a carbon lattice. From these results for diatomic systems it is argued that large molecules, similarly treated, may easily be excited to the point of rupture. If they are so situated (e.g., at or near a surface) that during, or shortly after, excitation they escape from the lattice, they will rupture rather than de-excite and thus generate molecular fragments (e.g., free radicals) which could, in the case of an explosive system, serve to initiate detonation.
Date: February 27, 1984
Creator: Karo, A.M.; Walker, F.E.; DeBoni, T.M. & Hardy, J.R.
Partner: UNT Libraries Government Documents Department

Study of shock-induced signals and coherent effects in solids by molecular dynamics

Description: Molecular dynamics calculations are presented that address the extent of microscopic detail that can be deduced from macroscopic gauge measurements of shock propagation in condensed systems. We have simulated large asymmetrically shock-loaded lattices, varying the initial temperature of both the loading plates and the lattice. Specifically: we have studied triple shock loading of a thin lattice; double shock loading of the same lattice; and triple shock loading of a thick lattice. In all cases we found strong memory effects in that the spall pattern always mirrors the loading history. This is a direct consequence of our two basic results: (a) the energy in finite width shocks is only weakly coupled to motions transverse to their direction of propagation; and (b) coupling between shock motion and random thermal motion is relatively weak, even in lattices that are near their melting temperatures.
Date: August 31, 1981
Creator: Karo, A.M.; Walker, F.E.; Cunningham, W.G. & Hardy, J.R.
Partner: UNT Libraries Government Documents Department

Theoretical studies of shock dynamics in two-dimensional structures. V. Microscopic constraints on shock-induced signals

Description: Molecular dynamics calculations are presented that address the extent of microscopic detail that can be deduced from macroscopic gauge measurements of shock propagation in condensed systems. Large asymmetrically shock-loaded lattices were simulated, varying the initial temperature and the strength of shock loading. Randomly-placed mass defects were introduced into the lattice, and the degradation of the shock front with the subsequent development of fracture and chunky spall were studied and compared with the coherent microscopic spall found for perfect lattices.
Date: June 18, 1981
Creator: Karo, A.M.; Walker, F.E.; Cunningham, W.G. & Hardy, J.R.
Partner: UNT Libraries Government Documents Department