83 Matching Results

Search Results

Advanced search parameters have been applied.

A Hybrid approach to molecular continuum processes combiningGaussian basis functions and the discrete variable representation

Description: Gaussian basis functions, routinely employed in molecular electronic structure calculations, can be combined with numerical grid-based functions in a discrete variable representation to provide an efficient method for computing molecular continuum wave functions. This approach, combined with exterior complex scaling, obviates the need for slowly convergent single-center expansions, and allows one to study a variety of electron-molecule collision problems. The method is illustrated by computation of various bound and continuum properties of H2+.
Date: August 29, 2005
Creator: Rescigno, Thomas N.; Horner, Daniel A.; Yip, Frank L. & McCurdy,C. William
Partner: UNT Libraries Government Documents Department

Electronic states of B$sup +$ produced by electron bombardment of boron trihalides: some ion-molecule reactions of B$sup +$

Description: Beam attenuation studies of B/sup +/ as a function of attenuating gas pressure shows that the beam has a composition of 35% /sup 3/P and 65% /sup 1/S when produced by 70 eV electrons from BF/sub 3/. As one proceeds in the series from BF/sub 3/ to BI/sub 3/, the amount of excited state varies in a regular manner and B/sup +/ ions are completely in the ground state when BI/sub 3/ is used as a target. The composition of the beam could also be varied by varying the electron energy. The results of the attenuation studies were also confirmed by studying the cross section vs. beam energy curves for the B/sup +/ (/sup 1/S) and B/sup +/ (/sup 3/P) reactions with N/sub 2/O. (auth)
Date: January 1, 1972
Creator: Lin, K. C.; Cotter, R. J. & Koski, W. S.
Partner: UNT Libraries Government Documents Department

Theory of Ionization by Electron Collisions

Description: The problem of calculating comprehensive sets of cross sections for ionization of atoms and molecules is reviewed. Emphasis is put on targets that are already ionized and lower-energy collisions for which the incident electron must be treated on a par with target electrons. The physical circumstances are discussed in which perturbation methods should be adequate, and special investigations are recommended to identify the relevant ranges of parameters more precisely and dependably. A new R-matrix approach is outlined which should permit rather simple, approximate, but dependable calculations when the perturbation methods fail.
Date: June 1976
Creator: Fano, Ugo & Inokuti, Mitio
Partner: UNT Libraries Government Documents Department

The linear algebraic method for electron-molecule collisions

Description: In order to find numerical solutions to many problems in physics, chemistry and engineering it is necessary to place the equations of motion (classical or quantal) of the variables of dynamical interest on a discrete mesh. The formulation of scattering theory in quantum mechanics is no exception and leads to partial differential or integral equations which may only be solved on digital computers. Typical approaches introduce a numerical grid or basis set expansion of the scattering wavefunction in order to reduce `the problem to the solution of a set of algebraic equations. Often it is more convenient to deal with the scattering matrix or phase amplitude rather than the wavefunction but the essential features of the numerics are unchanged. In this section we will formulate the Linear Algebraic Method (LAM) for electron-atom/molecule scattering for a simple, one-dimensional radial potential. This will illustrate the basic approach and enable the uninitiated reader to follow the subsequent discussion of the general, multi-channel, electron-molecule formulation without undue difficulty. We begin by writing the Schroedinger equation for the s-wave scattering of a structureless particle by a short-range, local potential.
Date: September 1995
Creator: Collins, L. A. & Schneider, B. I.
Partner: UNT Libraries Government Documents Department

Numerically Solvable Model for Resonant Collisions of Electronswith Diatomic Molecules

Description: We describe a simple model for electron-molecule collisions that has one nuclear and one electronic degree of freedom and that can be solved to arbitrarily high precision, without making the Born-Oppenheimer approximation, by employing a combination of the exterior complex scaling method and a finite-element implementation of the discrete variable representation. We compare exact cross sections for vibrational excitation and dissociative attachment with results obtained using the local complex potential approximation as commonly applied in the ''boomerang'' model, and suggest how this two-dimensional model can be used to test the underpinnings of contemporary nonlocal approximations to resonant collisions.
Date: January 27, 2006
Creator: Houfek, Karel; Rescigno, T.N. & McCurdy, C.W.
Partner: UNT Libraries Government Documents Department

Probing the nonlocal approximation to resonant collisions ofelectrons with diatomic molecules

Description: A numerically solvable two-dimensional model introduced bythe authors [Phys. Rev. A 73, 032721 (2006)]is used to investigate thevalidity of the nonlocal approximation to the dynamics of resonantcollisions of electrons with diatomic molecules. The nonlocalapproximation to this model is derived in detail, all underlyingassumptions are specified and explicit expressions for the resonant andnon-resonant (background) T matrix for the studied processes are given.Different choices of the so-called discrete state, which fully determinesthe nonlocal approximation, are discussedand it is shown that a physicalchoice of this state can in general give poorer results than otherchoices that minimize the non-adiabatic effects and/or the backgroundterms of the T matrix. The background contributions to the crosssections, which are usually not considered in the resonant theory ofelectron-molecule collisions, can be significant not only for elasticscattering but also for the inelastic process of vibrationalexcitation.
Date: September 7, 2007
Creator: Houfek, Karel; Rescigno, Thomas N. & McCurdy, C. William
Partner: UNT Libraries Government Documents Department

Recent theoretical results on electron-polyatomic molecule collisions

Description: Until recently, the principal barrier to the accurate theoretical description of electronic collisions with polyatomic molecules was the computational problem of scattering by a nonlocal, arbitrarily asymmetric potential. Effective numerical techniques capable of solving this variety of potential scattering problem for electronic collisions have now matured, and the first applications of methods for treating many-body aspects of collisions of electrons with polyatomic molecules have begun to appear in the literature. The past two years have seen the appearance of a large collection of calculations on electron-polyatomic collisions which compare favorably with experimental determinations. In addition to the dramatic developments in methods which explicitly exploit the methods of quantum chemistry to treat the effects of electron correlation, polarization, etc., parameter-free model potential methods for electronically elastic collisions have also evolved markedly in recent years. Progress in both electronically elastic and inelastic processes is reviewed briefly.
Date: March 1, 1994
Creator: McCurdy, C. W.
Partner: UNT Libraries Government Documents Department

Linking the gaseous and the condensed phases of matter: The slow electron and its interactions

Description: The interfacing of the gaseous and the condensed phases of matter as effected by interphase and cluster studies on the behavior of key reactions involving slow electrons either as reacting initial particles or as products of the reactions themselves is discussed. Emphasis is placed on the measurement of both the cross sections and the energetics involved, although most of the available information to date is on the latter. The discussion is selectively focussed on electron scattering (especially the role of negative ion states in gases, clusters, and dense matter), ionization, electron attachment and photodetachment. The dominant role of the electric polarization of the medium is emphasized.
Date: December 31, 1993
Creator: Christophorou, L. G.
Partner: UNT Libraries Government Documents Department

Interaction of Slow Electrons With High-Pressure Gases (`Quasi-Liquids`); Synthesis of Our Knowledge on Slow Electron-Molecule Interactions; Final Progress Report, March 15, 1986--August 14, 1987

Description: The rate constant for electron attachment to F{sub 2} was measured at 233 to 373 K in nitrogen at 0.04--0.75 eV and in argon at 298 K at 0.4--2.95 eV. The unfolded cross section was compared with calculations. The ionization threshold of N,N,N{prime},N{prime}-tetramethyl-p-phenylenediamine (TMPD) was measured in ethane over the density range 0.15--13.3 M/l at 295--413 K using multiphoton ionization conductivity.
Date: 1987-23~
Creator: McCorkle, D. L. & Christophorou, L. G.
Partner: UNT Libraries Government Documents Department

Properties of auroral-line induced transitions

Description: The radiation induced near the 0($sup 1$S)--0($sup 1$D) auroral-line of atomic oxygen by collisions with rare gas atoms has been studied. Emission from both bound, quasi-molecular states and from free atoms perturbed in collisions is observed. Experimental measurements of kinetic rates and their temperature dependences will be presented. (auth)
Date: August 1, 1975
Creator: Murray, J. R. & Powell, H. T.
Partner: UNT Libraries Government Documents Department

Photophysics of fullerenes: Thermionic emission

Description: Multiphoton ionization of fullerenes using long-pulse length lasers occurs mainly through vibrational autoionization. In many cases the laser ionization can be described as thermionic in analogy to the boiling off of electrons from a filament. Thermionic emission manifests itself as a delayed emission of electrons following pulsed laser excitation. Klots has employed quasiequilibrium theory to calculate rate constants for thermionic emission from fullerenes which seem to quantitatively account for the observed delayed emission times and the measured electron energy distributions. The theory of Klots also accounts for the thermionic emission of C{sub 60} excited by a low power CW Argon Ion laser. Recently Klots and Compton have reviewed the evidence for thermionic emission from small aggregates where mention was also made of experiments designed to determine the effects of externally applied electric fields on thermionic emission rates. The authors have measured the fullerene ion intensity as a function of the applied electric field and normalized this signal to that produced by single photon ionization of an atom in order to correct for all collection efficiency artifacts. The increase in fullerene ion signal relative to that of Cs{sup +} is attributed to field enhanced thermionic emission. From the slope of the Schottky plot they obtain a temperature of approximately 1,000 K. This temperature is comparable to but smaller than that estimated from measurements of the electron kinetic energies. This result for field enhanced thermionic emission is discussed further by Klots and Compton. Thermionic emission from neutral clusters has long been known for autodetachment from highly excited negative ions. Similarly, electron attachment to C{sub 60} in the energy range from 8 to 12 eV results in C{sub 60} anions with lifetimes in the range of microseconds. Quasiequilibrium theory (QET) calculations are in reasonable accord with these measurements.
Date: September 1, 1996
Creator: Compton, R.N.; Tuinman, A.A. & Huang, J.
Partner: UNT Libraries Government Documents Department

Generation of metastable rare-gas beams and studies of electron-metastable collision processes. Final technical report, January 1, 1990--September 30, 1991

Description: The basic goals of this research effort of designing, developing, and testing a high-flux, high-purity metastable rare-gas beam source suitable for measurements of electron-collisions have been achieved. Here the authors briefly describe the status of this effort. This effort will be pursued with support from other sources of funding and results of this subsequent effort will be made available to Los Alamos National Laboratory. On the theoretical side they have extended their procedure for calculating the cross sections for electron impact excitation of molecules to determine these cross sections for collisions with metastable states of molecules. As a relevant application they have studied the differential and integral cross sections for the a {sup 1}{Delta}{sub g} {yields} b {sup 1}{Sigma}{sub g}{sup +} transition in O{sub 2} for which measurements (R.I. Hall and S. Trajmar, J. Phys. B 8, L293 (1975)) were available at a single energy (4.5 eV). The agreement between the calculated and measured values of these cross sections is encouraging and support the large value reported experimentally.
Date: December 31, 1993
Creator: McKoy, V. & Trajmar, S.
Partner: UNT Libraries Government Documents Department

Generalized oscillator strengths. Progress report, July 1, 1973--July 1, 1974

Description: Progress is described on research under Contract AT(11-1)3247 and research still to be completed in the period July 1, 1973 to July 1, 1974. The research objectives defined in the original proposal were closely followed. The principal accomplishments during the period were: The introduction of a computer controlled counting system with provision for automatic control and operation of the electron spectrometer currently in use. Electron scattering with excitation of singlet-triplet transitions for helium was studied during the year both to acquire information on collision cross sections and to test a general theory of the abnormally high cross sections for forward scattering found for certain types of transitions. A theoretically predicted minimum in the cross section (at zero scattering angle) was found in a study over the range 100 to 500 eV of the 1/sup 1/S yields 2/sup 3/S transition of helium. Abnormally high cross sections for singlet-triplet transitions at high kinetic energy are predicted when the orbital term symbol is unchanged on excitation. As a test of the theory the X/sup 1/ SIGMA /sup +/ yields b/sup 3/ SIGMA /su p +/ transition in CO was looked for and found at THETA = 0 deg at both 200 and 300 eV thus confirming the theory. New electron scattering studies on both CO and CO/sub 2/ are described. A new method for the calculation of singlet-triplet energy differences from generalized oscillator strengths is described. (auth)
Date: January 1, 1974
Creator: Lassettre, Edwin N.
Partner: UNT Libraries Government Documents Department

Measuring the Density of a Molecular Cluster Injector via Visible Emission from an Electron Beam

Description: A method to measure the density distribution of a dense hydrogen gas jet is pre- sented. A Mach 5.5 nozzle is cooled to 80K to form a flow capable of molecular cluster formation. A 250V, 10mA electron beam collides with the jet and produces Hα emission that is viewed by a fast camera. The high density of the jet, several 10<sup>16</sup>cm<sup>-3</sup>, results in substantial electron depletion, which attenuates the H<sub>α</sub> emission. The attenuated emission measurement, combined with a simplified electron-molecule collision model, allows us to determine the molecular density profile via a simple iterative calculation.
Date: June 28, 2010
Creator: Lundberg, D. P.; Kaita, R.; Majeski, R. M. & Stotler, D. P.
Partner: UNT Libraries Government Documents Department

High accuracy electron-impact photoemission cross section measurements for the EUV radiometric standard. Final report

Description: This project was concerned with high-accuracy measurements of electron-impact photoemission cross sections for transitions of atoms and molecules in the EUV spectral region (30 nm to 200 nm). The spectrometer-detector system that was employed in the measurements was calibrated using well-parameterized synchrotron radiation. The 58.4 resonance line of He was chosen to establish the overall accuracy of the apparatus and data collection procedures. The measurement of this line can be compared to accurate theoretical calculations and to one other experimental cross section measurement in which a different excitation channel was observed.
Date: December 31, 1989
Creator: Risley, J. S.
Partner: UNT Libraries Government Documents Department

Enhanced negative ion formation via electron attachment to electronically-excited states

Description: Recent basic studies on electron attachment to laser-excited molecules show that electron attachment to electronically-excited states can have orders of magnitude larger cross sections compared to the respective ground electronic states. Even though systematic studies have not been conducted, there are indications that electronically-excited states may play a significant role in negative ion formation in gas discharges. The high-lying Rydberg states could be of particular significance since, (i) their production efficiencies are high, and (ii) they have comparatively long lifetimes. Such states could be populated in discharge sources via direct electron impact or via excitation transfer from metastable states of inert gases.
Date: December 31, 1995
Creator: Pinnaduwage, L.A.
Partner: UNT Libraries Government Documents Department

Ab initio calculations on collisions of low energy electrons with polyatomic molecules

Description: The Kohn variational method is one of simplest, and oldest, techniques for performing scattering calculations. Nevertheless, a number of formal problems, as well as practical difficulties associated with the computation of certain required matrix elements, delayed its application to electron--molecule scattering problems for many years. This paper will describe the recent theoretical and computational developments that have made the complex'' Kohn variational method a practical tool for carrying out calculations of low energy electron--molecule scattering. Recent calculations on a number of target molecules will also be summarized. 41 refs., 7 figs.
Date: August 1, 1991
Creator: Rescigno, T. N.
Partner: UNT Libraries Government Documents Department

Electron-excited molecule interactions

Description: In this paper the limited but significant knowledge to date on electron scattering from vibrationally/rotationally excited molecules and electron scattering from and electron impact ionization of electronically excited molecules is briefly summarized and discussed. The profound effects of the internal energy content of a molecule on its electron attachment properties are highlighted focusing in particular on electron attachment to vibrationally/rotationally and to electronically excited molecules. The limited knowledge to date on electron-excited molecule interactions clearly shows that the cross sections for certain electron-molecule collision processes can be very different from those involving ground state molecules. For example, optically enhanced electron attachment studies have shown that electron attachment to electronically excited molecules can occur with cross sections 10{sup 6} to 10{sup 7} times larger compared to ground state molecules. The study of electron-excited molecule interactions offers many experimental and theoretical challenges and opportunities and is both of fundamental and technological significance. 54 refs., 15 figs.
Date: January 1, 1991
Creator: Christophorou, L.G. (Oak Ridge National Lab., TN (USA) Tennessee Univ., Knoxville, TN (USA). Dept. of Physics)
Partner: UNT Libraries Government Documents Department

New developments in the ab initio treatment of low energy electron collisions with molecules

Description: Although there has been substantial progress in the ab initio treatment of low energy electron scattering from small diatomic and polyatomic molecules in the last few years a number of problems still remain. Most current research has focused on the calculation of fixed nuclei scattering amplitudes in the static-exchange (SE) approximation. A few calculations have gone beyond this approximation to include electron correlation and/or vibrational and rotational effects, the latter often within the framework of model or parameterized potentials. In this article we review a number of developments which have occurred since the last electron molecule satellite meeting at Daresbury Laboratory in July 1987. Our primary objective shall be to point out the strengths and weaknesses of current computational capabilities and to describe a new approach to electron polyatomic collisions using the complex Kohn method. 30 refs., 1 fig.
Date: January 1, 1989
Creator: Schneider, B.I.; Rescigno, T.N.; McCurdy, C.W. & Lengsfield, B.H. III
Partner: UNT Libraries Government Documents Department

Theoretical aspects of electron correlations in electron collisions

Description: Electron-correlation effects manifest themselves in diverse facets of electron collisions with an atom or molecule. A fast incident electron acts largely as an external agent, and its inelastic collisions probe the structure, including correlations within the target (in both the initial state and the final state), nearly in the same way as photoionization processes do. In inelastic collisions of a slow electron, the central object of study is the correlated motion of the incident electron and an electron excited out of the target core. This elementary observation is illustrated in the lecture by many examples and is elaborated by remarks on some current theoretical methods. (auth)
Date: January 1, 1975
Creator: Inokuti, M.
Partner: UNT Libraries Government Documents Department

Energy-related atomic and molecular structure and scattering studies. Annual progress report, July 1, 1979-June 30, 1980

Description: The basic goals of this program concern the experimental determination of properties of atoms and simple molecules that are important in a wide range of energy-related processes. In particular we have initiated measurements of polarizabilities of highly polar molecules and dimers, and of a number of atoms distributed through the periodic table, and, of the scattering of low energy electrons by highly polar molecules. For this latter series of measurements an entirely new apparatus has been designed and its external components have been completed. A new electron optics assembly has also been designed and is currently under construction. Details of all these projects are presented; analysis and data of the polarizability measurements are also given.
Date: March 1, 1980
Creator: Bederson, B.
Partner: UNT Libraries Government Documents Department

Connections between molecular photoionization and electron-molecule scattering with emphasis on shape resonances

Description: Most of our detailed information on the spectroscopy and dynamics of the electronic continuum of molecules is based on the complementary probes - photoionization and electron scattering. Though usually studied separately, it is most useful to appreciate the connections between these two processes since our understanding of one is often the key to interpreting or even generating new results in the other. We approach this subject in two steps. First, we very briefly outline the well-established connections, e.g., the Bethe-Born theory and comparisons of isoelectronic systems. Then we focus on a point of contact - the role of shape resonances in molecular photoionization and electron-molecule scattering - for which a substantial amount of new information has become available. Specific topics include mapping of resonances from the neutral (h..nu.. + molecule) to the negative ion (e + molecule) system, angular distributions, and interaction with vibration.
Date: January 1, 1979
Creator: Dehmer, J.L. & Dill, D.
Partner: UNT Libraries Government Documents Department

Electron attachment fo halocarbons of interest in gaseous dielectrics

Description: Total electron attachment rate constants and cross sections for halomethane and haloethane (freon) compounds are summarized and new results are reported for haloethylenes. The relationship between the electron attachment cross section and the breakdown strength of dielectric gases reported earlier is further discussed.
Date: January 1, 1984
Creator: McCorkle, D. L.; Christodoulides, A. A. & Christophorou, L. G.
Partner: UNT Libraries Government Documents Department