1,126 Matching Results

Search Results

Advanced search parameters have been applied.

Computational Materials Science Network Team on ''Excited states and response functions''

Description: This report covers the final one-year period of work done by the Principal Investigator (S. T. Pantelides) and his group in collaboration with other team members. The focus of the work was to pursue understanding of core excitation spectra in doped manganites where experimental data obtained at Oak Ridge National Laboratory by S. J. Pennycook showed inequivalent Mn atoms. Calculations found that doping sets up a Peierls-like instability that drives the observed distortion. Further calculations of electron-energy-loss spectra to account for the observed L23 ratios in the Mn L2,3 spectra will be pursued in the future.
Date: April 22, 2005
Creator: Pantelides, Sokrates T.
Partner: UNT Libraries Government Documents Department


Description: Two new positron active isotopes, B{sup 8} and Na{sup 20}, have been found to decay to excited states of Be{sup 8} and Ne{sup 20}, which in turn decay 'instantaneously' by alpha emission. Their half-lives are 0.65 {+-} 0.1 sec. and 1/4 sec. respectively. N{sup 12} is also found to have a low energy positron group which leads to an {alpha}-unstable excited state in C{sup 12}. The masses of B{sup 8} and Na{sup 20} are 8.027 and 20.015 respectively. B{sup 8} decays by a 13.7 {+-} 0.3 Mev positron, through the same excited state of Be{sup 8} as does Li{sup 8}. Estimates of the energies of the excited state in C{sup 12} and Ne{sup 20} are made.
Date: May 31, 1950
Creator: Alvarez, Luis W.
Partner: UNT Libraries Government Documents Department

Femtosecond isomerization dynamics in the ethylene cation measured in an EUV-pump NIR-probe configuration

Description: Dynamics in the excited ethylene cation C{sub 2}H{sub 4}{sup +} lead to isomerization to the ethylidene configuration (HC-CH{sub 3}){sup +}, which is predicted to be a transient configuration for electronic relaxation. With an intense femtosecond EUV (extreme ultraviolet) pump pulse to populate the excited state, and an NIR (near infrared) probe pulse to produce the fragments CH{sup +} and CH{sub 3}{sup +} (which provides a direct signature of ethylidene), we measure optimum fragment yields at a probe delay of 80 fs. Also, an H{sub 2}-stretch transient configuration, yielding H{sub 2}{sup +} upon probing, is found to succeed the ethylidene configuration. We find that a simple single- or double-decay model does not match the data, and we present a modified model (introduction of an isomerization delay of 50 {+-} 25 fs) that does provide agreement.
Date: March 17, 2009
Creator: van Tilborg, Jeroen; Allison, Tom; Wright, Travis; Hertlein, Marc; Falcone, Roger; Liu, Yanwei et al.
Partner: UNT Libraries Government Documents Department

Spectroscopic and dynamical studies of highly energized small polyatomic molecules. Technical progress report, July 1, 1993--October 31, 1994

Description: This is a progress report on a project to spectroscopically study small polyatomic molecules which are highly excited. The authors describe work on acetylene (HCCH) and HCO. Their work involves dispersed fluorescence spectroscopy, and the development and application of superpolyad models for studying intramolecular vibrational redistribution.
Date: February 1, 1995
Creator: Field, R. W. & Silbey, R. J.
Partner: UNT Libraries Government Documents Department

Nuclear Spin Orientation Dependence of Magnetoconductance: A New Method for Measuring the Spin of Charged Excitations in the QHE

Description: A new method for measuring the spin of the electrically charged ground state excitations m the Q$j~j quantum Hall effect ia proposed and demonstmted for the tirst time in GaAs/AIGaAs nndtiquantum wells. The method is &sed on the nuclear spin orientation dependence of" the 2D dc conductivity y in the quantum Hall regime due to the nuclear hyperfine interaction. As a demonstration of this method the spin of the electrically charged excitations of the ground state is determined at filling factor v = 1.
Date: December 1, 1998
Creator: Bowers, C.R.; Reno, J.L.; Simmons, J.A. & Vitkalov, S.A.
Partner: UNT Libraries Government Documents Department

Room temperature observation of quantum jumps of single molecule into dark states

Description: Fluctuations in the room temperature emission rate from single dye molecules which are excited with the near field scanning optical microscope reveal long (seconds) and short ({approximately} milliseconds) lived dark states.
Date: November 1, 1995
Creator: Ha, T.; Enderle, T.; Ogletree, D.F.; Selvin, P.R.; Weiss, S. & Chemla, D.S.
Partner: UNT Libraries Government Documents Department

Two-phonon excitations in <sup>170</sup>Er

Description: Recent experiments at the GEANIE/WNR facility and the University of Kentucky accelerator have yielded strong evidence for a two-gamma excitation in <sup>170</sup>Er. This new case can be added to a handful of previously identified examples of two-gamma vibrations, all of them discovered in this decade. In this paper the experimental evidence for a two-phonon excitation <sup>170</sup>Er is presented and the current state of understanding of these structures is reviewed in the context of this and other recent findings.
Date: September 29, 1998
Creator: Archer, D E; Becker, J A; Bernstein, L A; Garrett, P E; Johns, G D; Kadi, M et al.
Partner: UNT Libraries Government Documents Department

Theoretical studies of collisional relaxation of highly excited SO{sub 2} in an Ar bath

Description: This paper describes molecular dynamics studies of collisional relaxation of highly excited SO{sub 2} in an Ar bath. Most of the calculations use a newly developed global ab initio potential surface for SO{sub 2} that correctly describes the superoxide (SOO) and ring isomers of SO{sub 2} that occur as secondary minima on the ground state potential surface at high energies (about 75% of the dissociation energy) above the C{sub 2v} minimum. Rate constants for the S + O{sub 2} and O + SO reactions are calculated to test this surface, and to examine the importance of electronically excited states in the O + SO recombination. The Ar + SO{sub 2} collisions are described by summing the ab initio potential with empirical intermolecular potentials. The resulting average vibrational energy transfer <{Delta}E> per collision is in good agreement with direct measurements (done at energies where the secondary minima are not populated) at 1000K, but the agreement is poorer at 300K. The agreement is significantly better than was obtained in a previous theoretical study, and our results indicate that the use of improved intramolecular and intermolecular potentials is crucial to obtaining the better results. The energy dependence of <{Delta}E> is found to be much stronger at energies where the secondary minima on the potential surface are accessible, however much of this effect is reproduced using a potential that has the same dissociation energy but not the secondary minima.
Date: December 31, 1995
Creator: Lendvay, G.; Schatz, G.C. & Harding, L.B.
Partner: UNT Libraries Government Documents Department

Characterization of liquid phase epitaxial GaAs forblocked-impurity-band far-infrared detectors

Description: GaAs Blocked-Impurity-Band (BIB) photoconductor detectors have the potential to become the most sensitive, low noise detectors in the far-infrared below 45.5 cm{sup -1} (220 {micro}m). We have studied the characteristics of liquid phase epitaxial GaAs films relevant to BIB production, including impurity band formation and the infrared absorption of the active section of the device. Knowledge of the far-infrared absorption spectrum as a function of donor concentration combined with variable temperature Hall effect and resistivity studies leads us to conclude that the optimal concentration for the absorbing layer of a GaAs BIB detector lies between 1 x 10{sup 15} and 6.7 x 10{sup 15} cm{sup -3}. At these concentrations there is significant wavefunction overlap which in turn leads to absorption beyond the 1s ground to 2p bound excited state transition of 35.5 cm{sup -1} (282 {micro}m). There still remains a gap between the upper edge of the donor band and the bottom of the conduction band, a necessity for proper BIB detector operation.
Date: April 7, 2004
Creator: Cardozo, B.L.; Reichertz, L.A.; Beeman, J.W. & Haller, E.E.
Partner: UNT Libraries Government Documents Department

Enhanced Electron Attachment to Highly-Excited Molecules and Its Applications in Pulsed Plasmas

Description: Studies conducted over the past several years have shown that electron attachment to highly-excited states of molecules have extremely large cross sections. We will discuss the implications of this for pulsed discharges used for H<sup>-</sup> generation, material processing, and plasma remediation.
Date: June 27, 1999
Creator: Ding, W. X.; Ma, C. Y.; McCorkle, D. L. & Pinnaduwage, L. A.
Partner: UNT Libraries Government Documents Department

Nuclear Structure of the Closed Subshell Nucleus 90Zr Studied with the (n,n'(gamma)) Reaction

Description: States in {sup 90}Zr have been observed with the (n,n{prime}{gamma}) reaction using both spallation and monoenergetic accelerator-produced neutrons. A scheme comprised of 81 levels and 157 transitions was constructed concentrating on levels below 5.6 MeV in excitation energy. Spins have been determined by considering data from all experimental studies performed for {sup 90}Zr. Lifetimes have been deduced using the Doppler-shift attenuation method for many of the states and transition rates have been obtained. A spherical shell-model interpretation in terms of particle-hole excitations assuming a {sup 88}Sr closed core is given. In some cases, enhancements in B(M1) and B(E2) values are observed that cannot be explained by assuming simple particle-hole excitations. Shell-model calculations using an extended f pg-shell model space reproduce the spectrum of excited states very well, and the gross features of the B(M1) and B(E2) transition rates. Transition rates for individual levels show discrepancies between calculations and experimental values.
Date: June 12, 2003
Creator: Garrett, P E; Younes, Y; Becker, J A; Bernstein, L A; Baum, E M; DiPrete, D P et al.
Partner: UNT Libraries Government Documents Department

Direct observation of photoinduced bent nitrosyl excited-state complexes

Description: Ground state structures with side-on nitrosyl ({eta}{sup 2}-NO) and isonitrosyl (ON) ligands have been observed in a variety of transition-metal complexes. In contrast, excited state structures with bent-NO ligands have been proposed for years but never directly observed. Here we use picosecond time-resolved infrared spectroscopy and density functional theory (DFT) modeling to study the photochemistry of Co(CO){sub 3}(NO), a model transition-metal-NO compound. Surprisingly, we have observed no evidence for ON and {eta}{sup 2}-NO structural isomers, but have observed two bent-NO complexes. DFT modeling of the ground and excited state potentials indicates that the bent-NO complexes correspond to triplet excited states. Photolysis of Co(CO){sub 3}(NO) with a 400-nm pump pulse leads to population of a manifold of excited states which decay to form an excited state triplet bent-NO complex within 1 ps. This structure relaxes to the ground triplet state in ca. 350 ps to form a second bent-NO structure.
Date: June 28, 2008
Creator: Sawyer, Karma R.; Steele, Ryan P.; Glascoe, Elizabeth A.; Cahoon, James F.; Schlegel, Jacob P.; Head-Gordon, Martin et al.
Partner: UNT Libraries Government Documents Department

E2 Transition Probabilities in 114Te: a Conundrum

Description: Lifetimes in {sup 114}Te were determined using the recoil distance Doppler-shift technique with a plunger device coupled to five HP Ge detectors enhanced by one Euroball Cluster detector. The experiment was carried out at the Cologne FN Tandem facility using the {sup 93}Nb({sup 24}Mg,p2n) reaction at 90 MeV. The differential decay curve method in coincidence mode was employed to derive lifetimes for seven excited states, while the lifetime of an isomeric state was obtained in singles mode. The resulting E2 transition probabilities are shown to be very anomalous in comparison with the vibrational energy spacings of the ground state band.
Date: May 13, 2005
Creator: Moller, O; Warr, N; Jolie, J; Dewald, A; Fitzler, A; Linnemann, A et al.
Partner: UNT Libraries Government Documents Department

Relativistic Multireference Many-body Perturbation Theory for Open-shell Ions with Multiple Valence Shell Electrons: the Transition Rates and Lifetimes of the Excited Levels in Chlorinelike Fe X

Description: A recently developed relatistic multireference many-body perturbation theory based on multireference configuration-interaction wavefunctions as zeroth order wavefunctions is outlined. The perturbation theory employs a general class of configuration-interaction wve functions as reference functions, and thus is applciable to multiple open valence shell systems with near degeneracy of a manifold of strongly interacting configurations. Multireference many-body perturbation calculations are reported for the ground and excited states of chlorine-like Fe X in which the near degeneracy of a manifold of strongly interacting configurations mandates a multireference treatment. Term energies of a total of 83 excited levels arising from the 3s{sup 2}3p{sup 5}, 3s3p{sup 6}, 3s{sup 2}3p{sup 4}3d, 3s3p{sup 5}3d, and 3s{sup 2}3p{sup 3}3d{sup 2} configurations of the ion are evaluated to high accuracy. Transition rates associated with E1/M1/E2/M2/E3 radiative decays and lifetimes of a number of excited levels are calculated and compared with laboratory measurements to critically evaluate recent experiments.
Date: September 30, 2009
Creator: Ishikawa, Y; Santana, J A & Trabert, E
Partner: UNT Libraries Government Documents Department

Testing the Physics of Nuclear Isomers

Description: For much of the past century, physicists have searched for methods to control the release of energy stored in an atom's nucleus. Nuclear fission reactors have been one successful approach, but finding other methods to capitalize on this potential energy source have been elusive. One possible source being explored is nuclear isomers. An isomer is a long-lived excited state of an atom's nucleus--a state in which decay back to the nuclear ground state is inhibited. The nucleus of an isomer thus holds an enormous amount of energy. If scientists could develop a method to release that energy instantaneously in a gamma-ray burst, rather than slowly over time, they could use it in a nuclear battery. Research in the late 1990s indicated that scientists were closer to developing such a method--using x rays to trigger the release of energy from the nuclear isomer hafnium-178m ({sup 178m}Hf). To further investigate these claims, the Department of Energy (DOE) funded a collaborative project involving Lawrence Livermore, Los Alamos, and Argonne national laboratories that was designed to reproduce those earlier results.
Date: January 25, 2006
Creator: Hazi, A
Partner: UNT Libraries Government Documents Department

Dynamic light-matter coupling across multiple spatial dimensions in a quantum dots-in-a-well heterostructure

Description: Semiconductor heterostructures incorporating multiple degrees of spatial confinement have recently attracted substantial interest for photonic applications. One example is the quantum dots-in-a-well (DWELL) heterostructure, consisting of zero-dimensional quantum dots embedded in a two-dimensional quantum well and surrounded by three-dimensional bulk material. This structure offers several advantages over conventional photonic devices while providing a model system for the study of light-matter interactions across multiple spatial dimensions. Here, we use ultrafast differential transmission spectroscopy2 to temporally and spectrally resolve density-dependent carrier dynamics in a DWELL heterostructure. We observe excitation-dependent shifts of the quantum dot energy levels at low densities, while at high densities we observe an anomalous induced absorption at the quantum dot excited state that is correlated to quantum well population dynamics. These studies of density-dependent light-matter interactions across multiple coupled spatial dimensions provide clues to the underlying physics governing quantum dot properties, with important implications for DWELL-based photonic devices.
Date: January 1, 2009
Creator: Prasankumar, Rohit P; Taylor, Antoinette J; Chow, W W; Attaluri, R S & Shenoi, R
Partner: UNT Libraries Government Documents Department

Predicting Efficient Antenna Ligands for Tb(III) Emission

Description: A series of highly luminescent Tb(III) complexes of para-substituted 2-hydroxyisophthalamide ligands (5LI-IAM-X) has been prepared (X = H, CH{sub 3}, (C=O)NHCH{sub 3}, SO{sub 3}{sup -}, NO{sub 2}, OCH{sub 3}, F, Cl, Br) to probe the effect of substituting the isophthalamide ring on ligand and Tb(III) emission in order to establish a method for predicting the effects of chromophore modification on Tb(III) luminescence. The energies of the ligand singlet and triplet excited states are found to increase linearly with the {pi}-withdrawing ability of the substituent. The experimental results are supported by time-dependent density functional theory (TD-DFT) calculations performed on model systems, which predict ligand singlet and triplet energies within {approx}5% of the experimental values. The quantum yield ({Phi}) values of the Tb(III) complex increases with the triplet energy of the ligand, which is in part due to the decreased non-radiative deactivation caused by thermal repopulation of the triplet. Together, the experimental and theoretical results serve as a predictive tool that can be used to guide the synthesis of ligands used to sensitize lanthanide luminescence.
Date: October 6, 2008
Creator: Samuel, Amanda P.S.; Xu, Jide & Raymond, Kenneth
Partner: UNT Libraries Government Documents Department

Probing Reaction Dynamics of Transition-Metal Complexes in Solution via Time-Resolved X-ray Spectroscopy

Description: We report measurements of the photo-induced Fe(II) spin crossover reaction dynamics in solution via time-resolved x-ray absorption spectroscopy. EXAFS measurements reveal that the iron?nitrogen bond lengthens by 0.21+-0.03 Angstrom in the high-spin transient excited state relative to the ground state. XANES measurements at the Fe L-edge show directly the influence of the structural change on the ligand-field splitting of the Fe(II) 3d orbitals associated with the spin transition.
Date: May 24, 2009
Creator: Huse, Nils; Khalil, Munira; Kim, Tae Kyu; Smeigh, Amanda L.; Jamula, Lindsey; McCusker, James K. et al.
Partner: UNT Libraries Government Documents Department

Nuclear quantum effects in the structure and lineshapes of the N2 NEXAFS spectrum

Description: We study the relative ability of several models of the X-ray absorption spectrum to capture the Franck-Condon structure apparent from an experiment on gaseous nitrogen. In doing so, we adopt the Born-Oppenheimer approximation and a constrained density functional theory method for computing the energies of the X-ray-excited molecule. Starting from an otherwise classical model for the spectrum, we systematically introduce more realistic physics, first by substituting the quantum mechanical nuclear radial density in the bond separation R for the classical radial density, then by adding the effect of zero-point energy and other level shifts, and finally by including explicit rovibrational quantization of both the ground and excited states. The quantization is determined exactly, using a discrete variable representation. We show that the NEXAFS spectrum can be predicted semiquantiatively within this framework. We also address the possibility of non-trivial temperature dependence in the spectrum. Finally, we show that it is possible to improve the predicted spectrum by using constrained DFT in combination with more accurate potentials.
Date: December 4, 2009
Creator: Fatehi, Shervin; Schwartz, Craig P.; Saykally, Richard J. & Prendergast, David
Partner: UNT Libraries Government Documents Department

Electronic Spectroscopy & Dynamics

Description: The Gordon Research Conference (GRC) on Electronic Spectroscopy and Dynamics was held at Colby College, Waterville, NH from 07/19/2009 thru 07/24/2009. The Conference was well-attended with participants (attendees list attached). The attendees represented the spectrum of endeavor in this field coming from academia, industry, and government laboratories, both U.S. and foreign scientists, senior researchers, young investigators, and students. The GRC on Electronic Spectroscopy & Dynamics showcases some of the most recent experimental and theoretical developments in electronic spectroscopy that probes the structure and dynamics of isolated molecules, molecules embedded in clusters and condensed phases, and bulk materials. Electronic spectroscopy is an important tool in many fields of research, and this GRC brings together experts having diverse backgrounds in physics, chemistry, biophysics, and materials science, making the meeting an excellent opportunity for the interdisciplinary exchange of ideas and techniques. Topics covered in this GRC include high-resolution spectroscopy, biological molecules in the gas phase, electronic structure theory for excited states, multi-chromophore and single-molecule spectroscopies, and excited state dynamics in chemical and biological systems.
Date: June 8, 2010
Creator: Mark Maroncelli, Nancy Ryan Gray
Partner: UNT Libraries Government Documents Department

Ab initio calculation of electron excitation energies in solids

Description: Progress in the first-principles calculation of electron excitation energies in solids is discussed. Quasiparticle energies are computed by expanding the electron self energy to first order in the screened Coulomb interaction in the so-called GW approximation. The method was applied to explain and predict spectroscopic properties of a variety of systems. Several illustrative applications to semiconductors, materials under pressure, chemisorption, and point defects in solids are presented. A recent reformulation of the method employing mixed- space functions and imaginary time techniques is also discussed.
Date: February 1, 1996
Creator: Louie, S. G.
Partner: UNT Libraries Government Documents Department

First excited states in doubly-odd {sup 110}Sb: Smooth band termination in the A {approx} 110 region

Description: Excited states have been identified for the first time in {sup 110}Sb in a comprehensive series of {gamma}-spectroscopy experiments, including recoil-mass and neutron-field measurements. Three high-spin decoupled bands with configurations based on 2p-2h excitations across the Z = 50 shell gap, are observed to show the features of smooth band termination, the first such observation in an odd-odd nucleus. The yrast intruder band has been connected to the low spin levels and is tentatively identified up to its predicred termination at I{sup {pi}} = (45{sup +}). Detailed configuration assignments are made through comparison with configuration-dependent cranked Nilsson-Strutinsky calculations; excellent agreement with experiment is obtained. The systematic occurrence of smoothly terminating bands in the neighboring isotopes is discussed.
Date: November 1, 1996
Creator: Lane, G.J.; Fossan, D.B. & Thorslund, I.
Partner: UNT Libraries Government Documents Department

On the unexpected oscillation of the total cross section for excitation in He{sup 2+} + H collisions

Description: Recent calculations and measurements have revealed unexpected oscillations of the total cross section for excitation in low- to intermediate-energy He{sup 2+} + H collisions. A physical explanation of this behavior is given here stemming from analysis of classical trajectory Monte Carlo simulations, molecular orbital close coupling calculations, and solution of the time-dependent Schroedinger equation on a numerical lattice. These results indicate that the observed behavior should be characteristic of a wide range of reactions in ion-atom collisions.
Date: October 1, 1996
Creator: Schultz, D.R.; Reinhold, C.O. & Krstic, P.S.
Partner: UNT Libraries Government Documents Department

Statistical properties of the S-wave resonances of {sup 235}U

Description: The resonance parameters of {sup 235}U in the energy range 0 eV to 2.25 keV were obtained from a generalized least squares analysis of a large set of experimental data using the Reich-Moore formalism in the fitting code SAMMY. The aim of the present paper is to present the statistical properties of the s-wave resonance parameters generated from this study.
Date: June 1, 1997
Creator: Leal, L.C.; Derrien, H. & Larson, N.M.
Partner: UNT Libraries Government Documents Department