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Piezoelectric Effects on the Optical Properties of GaN/Al(x)Ga(1-x)N Multiple Quantum Wells

Description: Piezoelectric effects on the optical properties of GaN/AlGaN multiple quantum wells (MQWS) have been investigated by picosecond time-resolved photoluminescence (PL) measurements. For MQWS with well thickness 30 and 40 the excitonic transition peak positions at 10 K in continuous wave (CW) spectra are red-shifted with respect to the GaN epilayer by 17 meV and 57 meV, respectively. The time-resolved PL spectra of the 30 and 40 well MQWS reveal that the excitonic transition is in fact blue-shifted at early delay times due to quantum confinement of carriers. The spectral peak position shifts toward lower energies as the delay time increases and becomes red-shifted at longer delay times. We have demonstrated that the results described above is due to the presence of the piezoelectric field in the GaN wells of GaN/AlGaN MQWS subject to elastic strain together with screening of the photoexcited carriers. By comparing experimental and calculation results, we conclude that the piezoelectric field strength in GaN/Al.15G~.85N MQWS has a lower limit value of about 560 kV/cm: The electron and hole wave function distributions have also been obtained. The implication of our findings on the practical applications of GaN based optoelectronic devices is also discussed.
Date: November 10, 1998
Creator: Botchkarev, A.; Chow, W.W.; Jiang, H.X.; Kim, H.S.; Lin, J.Y. & Morkoc, H.
Partner: UNT Libraries Government Documents Department

Test of {Delta}I = 2 staggering in the superdeformed bands of {sup 194}Hg

Description: Superdeformed (SD) states in {sup 194}Hg were populated in {sup 150}Nd({sup 48}Ca,4n) using a 201 MeV {sup 48}Ca beam from the 88- inch cyclotron. A high statistics experiment was done to test for the previously reported evidence for a {Delta}I = 2 staggering in the three SD bands in {sup 194}Hg. The transition energies were determined with a precision of at least 60 eV for most transitions. From this improvement, we cannot confirm evidence for an extended regular {Delta}I = 2 staggering in any of the three SD bands of {sup 194}Hg. However, we observe deviations from a smooth reference in the SD bands 2 and 3 which differ from previous results. Oscillation patterns of the {gamma}-ray energies that can be induced by a simple band crossing or level shift are discussed. Even though such level shifts would explain the observed effects, other experimental signatures, such as a crossing band, are needed to fully understand the results of the present work; no such band was found.
Date: November 1996
Creator: Kruecken, R.; Deleplanque, M. A. & Hackman, G.
Partner: UNT Libraries Government Documents Department

Magneto-Excitons in (411)A and (100)-Oriented GaAs/AlGaAs Multiple Quantum Well Structures

Description: We report magneto-exciton spectroscopy studies of (411)A and (100)-oriented GaAs/Al{sub 0.3}Ga{sub 0.7}As multiquantum well structures. The samples consisted of seven GaAs quantum wells with widths varying between 0.6 and 12nm, were grown on (411)A and (100)-oriented GaAs substrates. The exciton diamagnetic energy shifts and linewidths were measured between 0 and 14T at 1.4K The dependence of the exciton diamagnetic shifts with magnetic field were calculated using a variational approach and good agreement with experiment for both substrate orientations was found.
Date: January 20, 1999
Creator: Bajaj, K.K.; Hiyamizu, S.; Jones, E.D.; Krivorotov, I.; Shimomura, S. & Shinohara, K.
Partner: UNT Libraries Government Documents Department

Insights on the local density approximation plasma polarization shift as provided by the optimum potential method

Description: The plasma polarization shift computed with a Local Density Functional model of an ion-sphere model is compared with results calculated using an optimum central field effective exchange potential. Indications are that the bulk of the shift is an artifact of the approximate exchange functional describing the interaction between bound and continuum orbitals in the LDA.
Date: January 18, 1995
Creator: Wilson, B. & Liberman, D.A.
Partner: UNT Libraries Government Documents Department

Precision measurements of atomic lifetimes in alkali like systems. Progress report, September 15, 1995--January 15, 1998

Description: Precision measurements of atomic lifetimes are important to the analysis of data from many areas of physics and provide fundamental atomic structure information. Scientists in the fields of astrophysics, geophysics, and plasma fusion all depend on oscillator strengths to determine the relative abundances of elements. Assessing the operation of discharge lamps and atomic resonance line filters also depends on knowing accurately atomic oscillator strengths. Often relative values of oscillator strengths are measured precisely, but accurate atomic lifetimes are needed to obtain absolute values. In addition, the interpretation of parity nonconservation (PNC) experiments requires accurate knowledge of the atomic structure including radial matrix elements. Many of these scientific needs are addressed theoretically with sophisticated many-electron atomic structure calculations. In this program they address these needs experimentally with a precision that surpasses current theoretical accuracy. The lifetime measurements also play the important roles of assessing the accuracy of many-electron atomic structure calculations and of guiding further theoretical development. Alkali like atoms, with a single electron outside of a closed shell, provide the simplest open shell systems for detailed comparisons between experiment and theory. To date, the research has focused on measurements of excited state lifetimes in neutral alkali systems along with the development of the necessary equipment and techniques for studying alkali-like ionic systems. The accomplishments of this program are summarized in Section 2 and are supported by the reprints and preprints that appear in the Appendix.
Date: January 1, 1998
Creator: Tanner, C. E.
Partner: UNT Libraries Government Documents Department

Collisional population transfer in yterbium ions

Description: Long-lived metastable states of Yb+ ions are used for atomic frequency standards, precision measurements, and quantum information research. The effect of population trapping and transfer in these states must be well understood. We report here the transfer of Yb+ ions into the long-lived {sup 2}F{sub 7/2} state by means of collisions between He buffer gas and Yb+ ions held in a linear Paul trap. Transfer rates were measured as functions of buffer-gas pressure and repump-laser power, and the collisional population transfer rates were extracted. The measured transfer rate coefficients are 8.32(75)x10-11 and 8.65(33)x10-11 cm3/s for the collisional processes {sup 2}P{sub 1/2}{yields}{sup 2}D{sub 5/2} and {sup 2}D{sub 3/2}{yields}{sup 2}F{sub 7/2}, respectively.
Date: January 1, 2009
Creator: Schauer, Martin Michael; Torgerson, Justin R; Danielson, Jeremy R; Zhao, Xinxin; Nguyen, Ahn - Tuan & Wang, Li - Bang
Partner: UNT Libraries Government Documents Department

Hyperfine quenching of the 2$sup 3$P$sub 0$ state in heliumlike ions

Description: An estimate is presented of the lifetime of the 2$sup 3$P$sub 0$ state for odd-Z heliumlike ions in the range Z = 9 to 29. An approximation scheme is employed which utilizes the fact that both Z$sup -1$ and (Z$alpha$)$sup 2$ are small parameters for the range of Z under consideration. 1 fig, 2 tables, 14 refs. (GHT)
Date: November 19, 1975
Creator: Mohr, P.J.
Partner: UNT Libraries Government Documents Department

Test of {Delta}I = 2 staggering in the superdeformed bands of {sup 194}Hg

Description: The presence of {Delta}I = 2 staggering in the three known superdeformed bands of {sup 194}Hg has been reexamined in a new experiment with Gammasphere. A relative accuracy of better than 30 eV was achieved for most transition energies. No statistically significant oscillations in the transition energies were found for band 1 while staggering patterns were observed in bands 2 and 3. The statistical significance of the observed effects was analyzed. The patterns display some similarities with expectations based on a band crossing picture, even though such a picture cannot reproduce the observations in a straightforward way. No evidence was found for additional superdeformed bands in {sup 194}Hg which could account for possible band-crossings.
Date: June 5, 1996
Creator: Kruecken, R.; Deleplanque, M.A.; Lee, I.Y. & Asztalos, S.
Partner: UNT Libraries Government Documents Department

Search for Neutron to Anti-Neutron Transitions at HFIR/ORNL

Description: The transition of neutron to anti-neutron might be the first observed signal of the baryon instability long-awaited in Grand Unification models and required for the explanation of baryon asymmetry in the universe. A newly-proposed experiment to search for neutron-antineutron transitions at High Flux Isotope Reactor at Oak Ridge National Laboratory can improve the discovery potential by factor of {approximately} 1,000 relative to the existing limits. Further prospects of n -> n(overbar) search are also discussed in this paper.
Date: October 22, 1998
Creator: Kamyshkov, Y.
Partner: UNT Libraries Government Documents Department

Around the back-bend in an excited {sup 150}Gd superdeformed band

Description: The excited superdeformed band (band 5) in {sup 150}Gd has a discontinuity (back-bend) in the smooth variation of gamma-ray energy with spin at around 1 MeV. It has been established that the band also has a decay branch that is probably a continuation of band 5 below the back-bend. Gamma-rays have been identified which link band 5 with the yrast superdeformed band in {sup 150}Gd.
Date: December 1, 1996
Creator: Twin, P. J.; Ertuerk, S. & Beausang, C. W.
Partner: UNT Libraries Government Documents Department

High gain x-ray lasers pumped by transient collisional excitation

Description: We present recent results of x-ray laser amplification of spontaneous emission in Ne-like and Ni-like transient collisional excitation schemes. The plasma formation, ionization and collisional excitation can be optimized using two laser pulses of 1 ns and 1 ps duration at table-top energies of 5 J in each beam. High gain of 35 cm{sup -1} has been measured on the 147 {Angstrom} 4d{r_arrow}4p J=0{r_arrow}1 transition of Ni-like Pd and is a direct consequence of the nonstationary population inversion produced by the high intensity picosecond pulse. We report the dependence of the x-ray laser line intensity on the laser plasma conditions and compare the experimental measurements with hydrodynamic and atomic kinetics simulations for Ne-like and Ni-like lasing.
Date: June 16, 1998
Creator: Dunn, J., LLNL
Partner: UNT Libraries Government Documents Department

Pressure dependence of Se absorption lines in AlSb

Description: Using far infrared absorption spectroscopy, the authors have investigated electronic transition spectra of Se donors in AlSb as a function of hydrostatic pressure. At least two distinct ground to bound excited state transition lines, which depend quadratically on the pressure, can be seen. At pressures between 30 and 50 kbar, evidence of an anti-crossing between one of the electronic transitions and a peak which they attribute to the 2 zone center LO phonon mode can be seen.
Date: September 1, 1996
Creator: Hsu, L.; Haller, E.E. & Ramdas, A.K.
Partner: UNT Libraries Government Documents Department

Molecular iodine absolute frequencies. Final report

Description: Fifty specified lines of {sup 127}I{sub 2} were studied by Doppler-free frequency modulation spectroscopy. For each line the classification of the molecular transition was determined, hyperfine components were identified, and one well-resolved component was selected for precise determination of its absolute frequency. In 3 cases, a nearby alternate line was selected for measurement because no well-resolved component was found for the specified line. Absolute frequency determinations were made with an estimated uncertainty of 1.1 MHz by locking a dye laser to the selected hyperfine component and measuring its wave number with a high-precision Fabry-Perot wavemeter. For each line results of the absolute measurement, the line classification, and a Doppler-free spectrum are given.
Date: June 25, 1990
Creator: Sansonetti, C. J.
Partner: UNT Libraries Government Documents Department

A theoretical analysis of the reaction between ethyl and molecular oxygen

Description: Using a combination of electronic-structure theory, variational transition-state theory, and solutions to the time-dependent master equation, the authors have studied the kinetics of the title reaction theoretically over wide ranges of temperature and pressure. The agreement between theory and experiment is quite good. By comparing the theoretical and experimental results describing the kinetic behavior, they have been able to deduce a value for the C{sub 2}H{sub 5}-O{sub 2} bond energy of {approximately}34 kcal/mole and a value for the exit-channel transition-state energy of {minus}4.3 kcal/mole (measured from reactants). These numbers compare favorably with the electronic-structure theory predictions of 33.9 kcal/mole and {minus}3.0 kcal/mole, respectively. The master-equation solutions show three distinct temperature regimes for the reaction, discussed extensively in the paper. Above T {approx} 700 K, the reaction can be written as an elementary step, C{sub 2}H{sub 5} + O{sub 2} {leftrightarrow} C{sub 2}H{sub 4} + HO{sub 2}, with the rate coefficient, k(T) = 3.19 x 10{sup {minus}17} T{sup 1.02} exp(2035/RT) cm{sup 3}/molec.-sec., independent of pressure even though the intermediate collision complex may suffer a large number of collisions.
Date: December 13, 2000
Creator: Miller, James A.; Klippenstein, Stephen J. & Robertson, Stuart H.
Partner: UNT Libraries Government Documents Department

L-shell emission from high-Z solid targets by intense 10{sup 19}W/cm{sup 2} irradiation with a 248nm laser

Description: Efficient (1.2% yield) multikilovolt x-ray emission from Ba(L) (2.4--2.8{angstrom}) and Gd(L) (1.7--2.1{angstrom}) is produced by ultraviolet (248nm) laser-excited BaF{sub 2} and Gd solids. The high efficiency is attributed to an inner shell-selective collisional electron ejection. Much effort has been expended recently in attempts to develop an efficient coherent x-ray source suitable for high-resolution biological imaging. To this end, many experiments have been performed studying the x-ray emissions from high-Z materials under intense (>10{sup 18}W/cm{sup 2}) irradiation, with the most promising results coming from the irradiation of Xe clusters with a UV (248nm) laser at intensities of 10{sup 18}--10{sup 19}W/cm{sup 2}. In this paper the authors report the production of prompt x-rays with energies in excess of 5keV with efficiencies on the order of 1% as a result of intense irradiation of BaF{sub 2} and Gd targets with a terawatt 248nm laser. The efficiency is attributed to an inner shell-selective collisional electron ejection mechanism in which the previously photoionized electrons are ponderomotively driven into an ion while retaining a portion of their atomic phase and symmetry. This partial coherence of the laser-driven electrons has a pronounced effect on the collisional cross-section for the electron ion interaction.
Date: January 5, 2000
Creator: Nelson, T.R.; Borisov, A.B. & Boyer, K.
Partner: UNT Libraries Government Documents Department

Energy states and energy flow near the transition states of unimolecular reactions

Description: The use of lasers with jet-cooled samples has improved energy and angular momentum resolution for the reactant and time resolution for the rate constant by orders of magnitude. The resolution of product quantum states has added a new dimension to unimolecular dynamics. In the past, the geometry, barrier height and vibrational frequencies of the transition state in RRKM theory were adjusted to fit thermal unimolecular reaction rate data. There have been successful quantitative tests of the ability of ab initio theory to calculate transition state geometries accurately and barrier heights to a few kJ/mol for simple molecules. Predicted frequencies tend to be somewhat too high for the softest modes which are of most importance in determining rates; however, the basic normal modes and sequence of frequencies seem to be correctly predicted. RRKM theory can be used with ab initio results to predict rate constants to within a factor of two or three and may be used for quantitative extrapolation to conditions not accessible in the laboratory but important in practical situations. Experiments on single molecular eigenstates have revealed quantum statistical fluctuations in rates which are predicted quantitatively in the appropriate extension of RRKM theory. Many experiments seeking to demonstrate non-statistical or non-RRKM dynamics have demonstrated the very wide range of applicability of the RRKM model. A few such experiments have demonstrated a lack of complete vibrational energy randomization in a reactant molecule. Dynamical theory has provided an exact quantum analog to RRKM theory which will combine with future experiments to define the extent to which quantized motion along the reaction coordinate and coupling between the reaction coordinate and vibrational degrees of freedom at the transition state are important. 42 refs., 11 figs.
Date: October 1, 1994
Creator: Moore, C.B.
Partner: UNT Libraries Government Documents Department

Characteristics of the 2.65 {mu}m atomic xenon laser

Description: The laser characteristics of the 2.65 {mu}m xenon laser transition are reviewed. Measured and extrapolated laser efficiency in nuclear pumped and electron beam pumped system is reported. Previous research has indicated that the reported power efficiency is between 0.1 and 2 percent.
Date: October 1, 1995
Creator: Hebner, G.A.
Partner: UNT Libraries Government Documents Department

Single Quark Transition Model Analysis of Electromagnetic Nucleon Resonance Excitations

Description: We apply the single quark transition model to resonance transition amplitudes extracted from photo- and electroproduction data. We use experimental data on the S{sub 11}(1535), and D{sub 13}(1520) nucleon resonances to extract the amplitudes for the electromagnetic transition from the nucleon ground state [56,0+] to the [70,1-] supermultiplet, and make predictions for the transition amplitudes of all other states associated with the [70,1-]. We compare the predictions with data and find surprisingly good agreement. The comparison is hampered by the poor data quality for many of the states especially in the electroproduction sector.
Date: August 1, 2002
Creator: Burkert, V.D.; Vita, R. De; Battaglieri, M.; Ripani, M. & Mokeev, V.
Partner: UNT Libraries Government Documents Department

Electron transfer and physical and chemical processes at low temperatures

Description: We summarize some phenomena that occur at temperatures of the order of 15K, and are dominated by quantum mechanical tunneling. Although electron tunneling dominates many conduction processes at low temperatures, we discuss evidence that phenomena like oxidation, as well as the solution of alkali metals in ammonia, can also be dominated by electron tunneling. Both phenomena demonstrate that the chemical potential of a metastable system can equilibrate at low temperatures by electron tunneling. The case of alkali metal clusters covered with ammonia is contrasted to covering the clusters with Xe. In this case changes in the activated conduction are observed which are consistent with the dielectric constant of the rare gas.
Date: October 1, 1995
Creator: Strongin, M.; Xia, B. & Jacobsen, F.M.
Partner: UNT Libraries Government Documents Department