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At least 271 records · Page 15

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DOE OSTI2025

Electron recombination of rotationally cold D 2 H + ions

Dissociative recombination (DR) of electrons with small molecular ions is a fundamental process for the physics and chemistry of the interstellar medium and planetary atmospheres. In previous DR studies, detailed analysis of the experimental rate coefficients has been hindered by the difficulty of preparing the ions in well-defined quantum states. For polyatomic ions in particular, truly state-selective measurements have been elusive, allowing only qualitative benchmarks of theory. Here, we present DR studies of the deuterated triatomic hydrogen ion D 2 H + , where the molecular ions were stored for up to 1000 seconds inside the Cryogenic Storage Ring (CSR) prior to the DR measurements. Our experiments with rotationally cold D 2 H + ions allow for detailed comparison to state-of-the-art theoretical calculations. We obtain very good agreement between experiment and theory even in the important collision energy range from 1 meV to 0.5 eV, where a multitude of Rydberg resonances reveal their imprint on the rate coefficient.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

NASA NTRS1980

Unimolecular decomposition reactions at low-pressure: A comparison of competitive methods

The lack of a simple rate coefficient expression to describe the pressure and temperature dependence hampers chemical modeling of flame systems. Recently developed simplified models to describe unimolecular processes include the calculation of rate constants for thermal unimolecular reactions and recombinations at the low pressure limit, at the high pressure limit and in the intermediate fall-off region. Comparison between two different applications of Troe's simplified model and a comparison between the simplified model and the classic RRKM theory are described.

Adams, G. F.↗

NASA NTRS1973

Comparison of electron and electronic temperatures in recombining nozzle flow of ionized nitrogen-hydrogen mixture. I, II.

Relaxation of the population distribution of electronic states is studied theoretically for a highly ionized nitrogen-hydrogen mixture expanding through a nozzle wherein the hydrogen content is less than 0.1%. The analysis incorporates quantum-mechanical rate coefficients, and considers the effects of wall cooling and absorption of radiation. Calculations are carried out for a condition produced experimentally. Visible and infrared line radiations from nitrogen and hydrogen were measured with a spectrograph. The geometry and stagnation conditions were those calculated theoretically. The experiment confirms quantitatively the predictions that the electronic excitation temperatures of hydrogen and (3P) core states of nitrogen are higher than the electron temperature, and that (3P) excitation temperatures exhibit maxima within the nozzle.

Park, C.↗

DOE OSTI2022

Prompt D 0 , D + , and D* + production in Pb–Pb collisions at $ \sqrt{s_{\mathrm{NN}}} $ = 5.02 TeV

The production of prompt D 0 , D + , and D* + mesons was measured at midrapidity (|y| < 0.5) in Pb–Pb collisions at the centre-of-mass energy per nucleon–nucleon pair $\sqrt{s_{NN}}$ = 5.02 TeV with the ALICE detector at the LHC. The D mesons were reconstructed via their hadronic decay channels and their production yields were measured in central (0–10%) and semicentral (30–50%) collisions. The measurement was performed up to a transverse momentum (p T ) of 36 or 50 GeV/c depending on the D meson species and the centrality interval. For the first time in Pb–Pb collisions at the LHC, the yield of D 0 mesons was measured down to p T = 0, which allowed a model-independent determination of the p T -integrated yield per unit of rapidity (dN/dy). A maximum suppression by a factor 5 and 2.5 was observed with the nuclear modification factor (R AA ) of prompt D mesons at p T = 6–8 GeV/c for the 0–10% and 30–50% centrality classes, respectively. The D-meson RAA is compared with that of charged pions, charged hadrons, and J/ψ mesons as well as with theoretical predictions. The analysis of the agreement between the measured R AA , elliptic (v 2 ) and triangular (v 3 ) flow, and the model predictions allowed us to constrain the charm spatial diffusion coefficient D s . Furthermore the comparison of R AA and v 2 with different implementations of the same models provides an important insight into the role of radiative energy loss as well as charm quark recombination in the hadronisation mechanisms.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

NASA NTRS1987

Vertical O(sub 3) distribution as a diagnostic for eddy diffusion profile

A model study of the distribution of ozone on Mars is presented. It is showed that knowledge of the vertical ozone distribution, such as could be obtained from ultraviolet measurements from an orbiter, could be used to infer vertical transport rates at various levels in the atmosphere. The dependence of the vertical distribution of O(sub 3) on the height variation of eddy diffusion coefficient is illustrated. Ozone abundance is a valuable diagnostic for other climatological parameters. In addition, the sensitivity of O(sub 3) distribution to eddy diffusion may aid in determining the role of surface oxidation and recombination processes and lead to a better understanding of the volatiles released or adsorbed cyclically in the Martian regolith.

Tyler, A. L.↗

NASA NTRS1976

Post sunset behavior of the 6300 A atomic oxygen airglow emission

A theoretical model of the 6300 A OI airglow emission was developed based on the assumptions that both the charged and neutral portions of the Earth's upper atmosphere are in steady state conditions of diffusive equilibrium. Intensities of 6300 A OI emission line were calculated using electron density true height profiles from a standard C-4 ionosonde and exospheric temperatures derived from Fabry-Perot interferometer measurements of the Doppler broadened 6300 A emission line shape as inputs to the model. Reaction rate coefficient values, production mechanism efficiencies, solar radiation fluxes, absorption cross sections, and models of the neutral atmosphere were varied parametrically to establish a set of acceptable inputs which will consistently predict 6300 A emission intensities that closely agree with intensities observed during the post-sunset twilight period by an airglow observatory consisting of a Fabry-Perot interferometer and a turret photometer. Emission intensities that can only result from the dissociative recombination of molecular oxygen ions were observed during the latter portion of the observational period. Theoretical calculations indicate that contamination of the 6300 A OI emission should be on the order of or less than 3 percent; however, these results are very sensitive to the wavelengths of the individual lines and their intensities relative to the 6300 A OI intensity. This combination of a model atmosphere, production mechanism efficiencies, and quenching coefficient values was used when the dissociative photoexcitation and direct impact excitation processes were contributing to the intensity to establish best estimates of solar radiation fluxes in the Schumann--Runge continuum and associated absorption cross sections. Results show that the Jacchia 1971 model of the upper atmosphere combined with the Ackerman recommended solar radiation fluxes and associated absorption cross sections produces theoretically calculated intensities that more closely agree with the observed intensities than all the other combinations.

Smith, R. E.↗

DOE OSTI2020

Radiative capture rates at deep defects from electronic structure calculations

We present a methodology to calculate radiative carrier capture coefficients at deep defects in semiconductors and insulators from first principles. Electronic structure and lattice relaxations are accurately described with hybrid density functional theory. Calculations of capture coefficients provide an additional validation of the accuracy of these functionals in dealing with localized defect states. We also discuss the validity of the Condon approximation, showing that even in the event of large lattice relaxations the approximation is accurate. We test the method on GaAs:VGa-TeAs and GaN:CN, for which reliable experiments are available, and demonstrate very good agreement with measured capture coefficients.

36 MATERIALS SCIENCE↗

NASA NTRS1978

Global circulation and distribution of hydrogen in thermosphere of Venus

The global density distribution of atomic H in the upper atmosphere of Venus is determined in terms of a global circulation model. It is shown that H produced on the dayside is efficiently advected to the nightside by the major gas CO2 where it is then convected to lower altitudes and recombines. A maximum night/day H density ratio of about 5 is derived, in which case exospheric return flow from night to day strongly limits the nightside density enhancement. A previously postulated constraint connecting the eddy diffusion coefficient with the escape flux of H in one dimensional models is no longer required when global circulation of H is considered.

Hartle, R. E.↗

DOE OSTI2025

Efficiency droop contributors in InGaN green light emitting diodes

Here, efficiency droop contributors (i.e., inherent Auger–Meitner recombination, polarization-induced effects, thermal effects, and light extraction) in InGaN green light emitting diodes (LEDs) are decoupled and quantified. First, a modified ABC model is developed, and external quantum efficiency measurements are taken under constant and pulsed currents (⁠ EQE Constant and EQE Pulsed ⁠, respectively). The LED internal quantum efficiency with and without thermal effects (⁠ IQE$^{ABC}_{Constant}$ and IQE$^{ABC}_{Pulsed}$⁠, respectively) is extracted using the modified model. Then, using Raman spectroscopy, the LED junction temperature is extracted. Finally, using the optical-electrical model (OEM), the polarization- and temperature-independent LED internal quantum efficiency (⁠ IQE OEM ) is calculated from the modified ABC model and the extracted junction temperature. By comparing external (⁠ EQE Constant ⁠) and the three internal quantum efficiencies (⁠ IQE$^{ABC}_{Constant}$, IQE$^{ABC}_{Pulsed}$⁠, and IQE OEM ), the impacts of inherent Auger–Meitner recombination, polarization-induced effects, thermal effects, and light extraction on the efficiency droop are decoupled and quantified. It is found that inherent Auger–Meitner recombination-induced droop is approximately 49% of the total efficiency droop in commercial green LEDs, while polarization-induced effects contribute about 35%, and thermal droop accounts for nearly 16%. Lastly, these findings suggest, to quash the green gap, it is critical to search for materials and device designs with low inherent Auger–Meitner coefficients and polarization fields, respectively.

42 ENGINEERING↗

NASA NTRS1983

The problem of cooling the cold Io torus

Two models are developed for the transport of ions inward from Io's orbit, the first using the assumption that radial diffusion is the dominant transport mechanism while the second uses a combination of diffusive and convective transport. The models include thermal and number density transport, radiation, ionization and pickup of local neutrals, recombination, charge exchange, and Coulomb interactions. It is found that pure diffusive transport can account for the dramatic inward depletion of the torus only by invoking recombination or by postulating a massive increase in the production rate of torus ions sometime prior to the Voyager mission. Recombination is determined to be far too slow to be the cause of the observed density decrease inside of Io. The model combining convection and diffusion is shown to reasonably match the data, but only if a diffusion coefficient 100 times less than that derived from Pioneer observations is used. In addition, it is shown that the Pioneer derived diffusion rate combined with Voyager temperature and density measurements indicate a large nonradiative sink of energy in the inner torus.

Richardson, J. D.↗

DOE OSTI2022

Efficiency Improvement Progress in Green, Yellow and Red III Nitride LEDs

Large improvements in the efficiencies of visible light emitting diodes (LEDs) have been realized over the last decade. Extending the range of wavelengths over which efficient emission can be realized continues to be a goal of active research. Efficiency droop refers to a non-thermal decrease in internal quantum efficiency with increasing current density that affects III-nitride LEDs in general, but more severely those with longer emission wavelengths. The so-called green gap is mainly related to the wavelength dependence of efficiency droop. The peak EQE in commercial green LEDs exceeds 60% but corresponds to a current density that is an order of magnitude lower than required for applications. In this presentation, we will review some of the underlying physics that contribute to the wavelength dependence of droop. We will discuss some challenges in the development of long wavelength III-nitride LEDs and possible strategies to mitigate efficiency droop based on the understanding that nonradiative Auger recombination is the root cause. One general strategy is to engineer the multi-quantum well active region to minimize the average carrier density. Another approach is to design wells with a more favorable balance between the radiative and Auger coefficients. Finally, we will present the current status of the performance of green, yellow and red III-Nitride LEDs.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗

NASA NTRS1978

Non-thermal hydrogen in the Venus exosphere - The ionospheric source and the hydrogen budget

A nonthermal, or 'hot', Venus corona of H atoms has been observed by Mariners 5 and 10 and Venera 9. Of the sources investigated, reaction of H2 with ionospheric O(+) is still the strongest. It can explain the smaller densities but falls somewhat short of the largest (from Mariner 5). The subsequent recombination of OH(+), supplemented by solar-wind processes, may give an escape flux of 10 million atoms per sq cm/s. The low density of thermal H atoms on the day side has previously been attributed to either a large eddy diffusion coefficient or an escape flux tenfold greater than this. An alternative mechanism, suggested by Hartle and Mayr (1977) is supported: the hydrogen is swept to the night side by strong thermospheric winds. This process is analogous to the 'Johnson pump' for the terrestrial winter helium bulge. Large nightside bulges of H and H2 are predicted; the night/day density ratio is estimated to be as large as 100 for each.

Kumar, S.↗

NASA NTRS1989

Photochemical stability of CO2 in the Martian atmosphere - Reevaluation of the eddy diffusion coefficient and the role of water vapor

Theoretical models of CO2 photodissociation in the atmosphere of Mars are developed and analyzed, with a focus on the conditions prevailing during the Mariner 6/7 experiments. The aeronomy of Martian CO2 is reviewed; the atomic O distribution in the upper atmosphere is characterized; the construction of the basic model is described; and simulation results are presented in extensive tables and graphs and discussed in detail. The vertical eddy-diffusion coefficient corresponding to the estimated upper-atmosphere O density (1-2 percent at 135 km) is found to be about 5 x 10 to the 7th sq cm/sec, and HO(x) is found to play a significant role in catalyzing the recombination of CO and O to form CO2. The reasons why the model overpredicts the production of HO(x) and hence of CO2 are explored.

Shimazaki, Tatsuo↗

NASA NTRS2006

Mestastable State Population in Laser Induced Plasmas

Laser induced plasma has been used as a source of neutrals and ions in the study of astrophysical plasmas. The purity of state of this source is essential in the determination of collision parameters such as the charge transfer rate coefficients between ions and neutrals. We will show that the temperature of the laser induced plasma is a rapidly decreasing function of time. The temperature is initially high but cools off rapidly through collisions with the expanding plasma electrons as the plasma recombines and streams into the vacuum. This rapid expansion of the plasma, similar to a supersonic jet, drastically lowers the internal energy of the neutrals and ions.

Kwong, V. H. S.↗

DOE OSTI2025

Dissociative electron recombination and rotational cooling of the deuterated triatomic hydrogen ions H 2 ⁡D + and D 2 ⁢H +

We have measured the dissociative recombination (DR) of the deuterated triatomic hydrogen ions H 2 ⁡D + and D 2 ⁢H + as a function of storage time at the Cryogenic Storage Ring (CSR). Both molecular ions were stored for up to 1000 s inside the cryogenic vacuum of the CSR prior to the electron recombination measurements, allowing them to cool to their lowest rotational states. We implement a comprehensive model for all relevant processes to predict the internal state evolution of the ions during storage inside the CSR, employing calculated radiative transition strengths and state-selective rate coefficients for electron collisions. Our DR rate coefficient measurements with deuterated triatomic hydrogen ions in defined quantum states allow for meaningful comparisons with state-of-the-art theoretical calculations, paving the way for a better understanding of the complex DR process for polyatomic molecular ions.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

NASA NTRS1997

Mechanism for the Green Glow of the Upper Ionosphere

The generation of the green line of atomic oxygen by dissociative recombination of 02 plus occurs by the capture of an electron into a repulsive state of 02 followed by dissociation along another state of a different electronic symmetry. The two states are coupled together by mixed symmetry Rydberg states. Quantum chemical calculations give a rate coefficient at room temperature of (0.39 (+ 0.31 or -0.19)) x 10 exp -8 cubic centimeters per second. The quantum yield of excited oxygen is within the range deduced from ground, rocket, and satellite observations. The rate coefficients and yields are needed in models of the optical emission, chemistry, and energy balance of planetary ionospheres.

Guberman, Steven L.↗

DOE OSTI2020

Impact of Alloy-Disorder-Induced Localization on Hole Diffusion in Highly Excited c -Plane and m -Plane (In, Ga) N Quantum Wells

The diffusion coefficient of holes can provide knowledge about carrier localization in (In, Ga)N, where the carrier dynamics are altered by randomly fluctuating potential landscape. In group-III nitrides, the diffusivity of holes is difficult to measure by electrical methods but it can be studied using optical techniques. Here, we investigate the dependence of the hole diffusion coefficient on direction and carrier density in c-plane and m-plane (In, Ga)N structures by employing the light-induced transient-grating technique. We show that the hole diffusion coefficient is anisotropic in the m-plane structure, where it is several times larger along the a crystallographic direction than along the c direction. Such anisotropy is observed within the broad range of carrier densities from 10 18 to 10 20 cm –3 . The diffusivity changes nonmonotonously with increasing photoexcitation, this dependence being different in thick and thin layers. We argue that an unexpectedly high diffusion coefficient at low carrier densities in thick quantum wells can be a signature of efficient hole transport via percolative paths occurring due to compositional disorder. In turn, a decrease of diffusivity with the excitation can reflect the effect of Coulomb blockade of these paths. Lastly, we demonstrate that disorder impacts carrier diffusivity even at carrier densities above 10 19 cm –3 , where the overflow of localized states must be included to explain the observed increase of the diffusion coefficient with the carrier density.

36 MATERIALS SCIENCE↗

NASA NTRS1987

Energy transfer of O(D-1) atoms in collision with O(P-3) atoms

Calculations are carried out of the elastic scattering and excitation exchange cross-sections in collisions of O(D-1) and O(P-3) atoms, which determine the degree of thermalization of the O(D-1) atoms produced by dissociative recombination in the thermosphere. The effective elastic scattering and excitation exchange cross-sections are calculated to be 1.55 x 10 to the -15th and 6.25 x 10 to the -16th sq cm, respectively, at a relative collision energy of 1.0 eV. The mutual diffusion coefficient between O(D-1) and O(P-3) atoms is also presented.

Yee, Jeng-Hwa↗