Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “PHOTOEMISSION”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Direct observation of spin-resolved full and empty electron states in ferromagnetic surfaces

We present a versatile apparatus for the study of ferromagnetic surfaces, which combines spin-polarized photoemission and inverse photoemission spectroscopies. Samples can be grown by molecular beam epitaxy and analyzed in situ. Spin-resolved photoemission spectroscopy analysis is done with a hemispherical electron analyzer coupled to a 25 kV-Mott detector. Inverse photoemission spectroscopy experiments are performed with GaAs crystals as spin-polarized electron sources and a UV bandpass photon detector. As an example, measurements on the oxygen passivated Fe(100)-p(1×1)O surface are presented.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Origin of electrons emitted into vacuum from InGaN light emitting diodes

The mechanism responsible for efficiency droop in InGaN light-emitting diodes (LEDs) has long been elusive due to indirect measurement techniques used for its identification. Auger recombination is unique among proposed efficiency droop mechanisms, in that it is the only mechanism capable of generating hot carriers. In a previous study [J. Iveland et al., Phys. Rev. Lett. 110, 177406 (2013)], we performed electron energy analysis of electrons emitted into vacuum from a forward biased InGaN LED that had been brought into negative electron affinity by cesiation. Three peaks were observed in the energy spectrum of vacuum emitted electrons. In this Letter, we unambiguously identify the origin of the peaks. The two higher energy peaks correspond to accumulation of electrons transported to the surface in the bulk Γ and side L conduction band valleys. The L-valley peak is a direct signature of a hot Auger electron population. The lower energy peak results from surface photoemission induced by the internal LED light emitted from the InGaN quantum wells. Two control experiments were performed. In the first, a simple GaN pn junction generated only a single Γ peak in electroemission. In the second, selective detection of the photoemission from an LED under modulated light excitation and DC electrical injection confirms that only the low energy peak is photogenerated and that LED light is incapable of generating Γ or L-valley peaks, the latter only occurring due to the Auger effect in the LED active region.

42 ENGINEERING↗
DOE OSTI2025

Photoelectron–residual-ion interaction in angle-resolved streaked shake-up ionization of helium

Streaked photoelectron emission spectra access the correlated dynamics of photoelectrons and residual target electrons with attosecond temporal resolution. Here, we calculated ab initio single-ionization spectra for photoemission from helium atoms by colinearly polarized ultrashort XUV and assisting few-femtosecond IR pulses. Distinguishing direct and shake-up ionization resulting in ground-state and excited (𝑛 = 2, 3) He + residual ions, respectively, we examined the effects of the correlated photoemission dynamics on the photoelectron phase accumulation as a function of the observable photoelectron detection direction and kinetic energy and XUV–IR pulse delay. We tracked the dynamical evolution of the residual ion in relative streaked photo- emission delays and found dominant contributions for shake-up emission from the residual-ion–photoelectron interaction. These are in very good and fair agreement, respectively, for 𝑛 = 2 and 𝑛 = 3 shake-up photoemission along the pulse-polarization directions, with previous experimental and theoretical investigations [Ossiander et al ., Nat. Phys. 13, 280 (2017)] and reveal a strong photoemission-direction dependence for shake-up ionization due to the coupling between the photoelectron and evolving residual-ion charge distribution in the IR-laser field.

Stark effect↗
DOE OSTI2018

Perspectives on Designer Photocathodes for X-ray Free-Electron Lasers: Influencing Emission Properties with Heterostructures and Nanoengineered Electronic States

Here, the development of photoemission electron sources to specifically address the competing andnincreasingly stringent requirements of advanced light sources such as X-ray Free Electron Lasers (XFELs) motivates a comprehensive material-centric approach that integrates predictive compu- tational physics models, advanced nano-synthesis methods, and sophisticated surface science char- acterization with in situ correlated study of photoemission performance and properties. Related efforts in material science are adopting various forms of nanostructure (such as compositionally graded stoichiometry in heterostructured architectures, and quantum features) allowing for tailored electronic structure to control and enhance opto-electronic properties. These methods influence the mechanisms of photoemission (absorption, transport, and emission) but have not, as yet, been systematically considered for use in photocathode applications. Recent results and near-term op- portunities are described to exploit controlled functionality of nanomaterials for photoemission. An overview of the requirements and status is provided.

47 OTHER INSTRUMENTATION↗
DOE OSTI2011

Photocathode Optimization for a Dynamic Transmission Electron Microscope: Final Report

The Dynamic Transmission Electron Microscope (DTEM) team at Harvey Mudd College has been sponsored by LLNL to design and build a test setup for optimizing the performance of the DTEM's electron source. Unlike a traditional TEM, the DTEM achieves much faster exposure times by using photoemission from a photocathode to produce electrons for imaging. The DTEM team's work is motivated by the need to improve the coherence and current density of the electron cloud produced by the electron gun in order to increase the image resolution and contrast achievable by DTEM. The photoemission test setup is nearly complete and the team will soon complete baseline tests of electron gun performance. The photoemission laser and high voltage power supply have been repaired; the optics path for relaying the laser to the photocathode has been finalized, assembled, and aligned; the internal setup of the vacuum chamber has been finalized and mostly implemented; and system control, synchronization, and data acquisition has been implemented in LabVIEW. Immediate future work includes determining a consistent alignment procedure to place the laser waist on the photocathode, and taking baseline performance measurements of the tantalum photocathode. Future research will examine the performance of the electron gun as a function of the photoemission laser profile, the photocathode material, and the geometry and voltages of the accelerating and focusing components in the electron gun. This report presents the team's progress and outlines the work that remains.

42 ENGINEERING↗
DOE OSTI2010

Electronic band structure and Kondo coupling in YbRh2Si2

The electronic band structure of YbRh2Si2 is calculated in a relativistic framework including correlation corrections and magnetization of the Yb ion and compared to detailed angle-resolved photoemission spectra. The photoemission spectra for LuRh2Si2 are used as reference to identify electronic bands with no f symmetry. The calculated band structure manifests a 4f13 spin-polarized configuration leaving the unoccupied state at 1.4eV above the Fermi energy. At the band theory level, the 4f bands are located far below the Fermi level and the anisotropic Coulomb interaction within the 4f shell spreads the multilevel into broader 4f complexes below -2.5eV . The photoemission spectra obtained on YbRh2Si2 show a clear f -multilevel splitting into j=7/2 and 5/2 excitations. The interaction of the 4f7/2 levels close to the Fermi energy with two conduction bands shows visible hybridization gaps of 45 and 80meV, respectively. We discuss the origin of these excitations and provide an analysis according to Anderson's single-impurity model with parameters suggested by the band-structure calculation and the photoemission spectra. Both experiment and theory indicate nearly identical Fermi surfaces for LuRh2Si2 and YbRh2Si2 . The valency of Yb in YbRh2Si2 is estimated to be close to +3.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Spectral and polarization based imaging in deep-ultraviolet excited photoelectron microscopy

Using photoelectron emission microscopy, nanoscale spectral imaging of atomically thin MoS2 buried between Al2O3 and SiO2 is achieved by monitoring the wavelength and polarization dependence of the photoelectron signal excited by deep-ultraviolet light. Although photons induce the photoemission, images can exhibit resolutions below the photon wavelength as electrons sense the response. To validate this concept, the dependence of photoemission yield on the wavelength and polarization of the exciting light was first measured and then compared to simulations of the optical response quantified with classical optical theory. A close correlation between experiment and theory indicates that photoemission probes the optical interaction of UV-light with the material stack directly. The utility of this probe is then demonstrated when both the spectral and polarization dependence of photoemission observe spatial variation consistent with grains and defects in buried MoS2. Taken together, these new modalities of photoelectron microscopy allow mapping of optical property variation at length scales unobtainable with conventional light-based microscopy.

Beechem, Thomas E. (ORCID:0000000275360719)↗
DOE OSTI2015

Mott localization in a pure stripe antiferromagnet Rb 1 - δ Fe 1.5 - σ S 2

A combination of neutron diffraction and angle-resolved photoemission spectroscopy measurements on a pure antiferromagnetic stripe ${\mathrm{Rb}}_{1{-}{\delta}}{\mathrm{Fe}}_{1.5{-}{\sigma}}{\mathrm{S}}_{2}$ is reported here. A neutron diffraction experiment on a powder sample shows that a $98$% volume fraction of the sample is in the antiferromagnetic stripe phase with rhombic iron vacancy order and a refined composition of ${\mathrm{Rb}}_{0.66}{\mathrm{Fe}}_{1.36}{\mathrm{S}}_{2}$, and that only $2$% of the sample is in the block antiferromagnetic phase with $\sqrt{5}\times{}\sqrt{5}$ iron vacancy order. Furthermore, a neutron diffraction experiment on a single crystal shows that there is only a single phase with the stripe antiferromagnetic order with the refined composition of ${\mathrm{Rb}}_{0.78}{\mathrm{Fe}}_{1.35}{\mathrm{S}}_{2}$, while the phase with block antiferromagnetic order is absent. Angle-resolved photoemission spectroscopy measurements on the same crystal with the pure stripe phase reveal that the electronic structure is gapped at the Fermi level with a gap larger than 0.325 eV. The data collectively demonstrate that the extra $10$% iron vacancies in addition to the rhombic iron vacancy order effectively impede the formation of the block antiferromagnetic phase; the data also suggest that the stripe antiferromagnetic phase with rhombic iron vacancy order is a Mott insulator.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Metal-Insulator Photocathode Heterojunction for Directed Electron Emission

New photocathode materials capable of producing intense and directed electron pulses are needed for development of next generation light sources and dynamic transmission electron microscopy. Ideal photocathodes should have high photoemission quantum efficiency (QE) and be capable of delivering collimated and well-shaped pulses of consistent charge under high-field operating conditions. High-brightness and low-intrinsic emittance electron pulses have been predicted for hybrid metal-insulator photocathode designs constructed from three to four monolayer MgO films on atomically flat silver. Here we use angle-resolved photoelectron spectroscopy to confirm directional photoemission and a large increase in QE under ultraviolet laser excitation of an ultrathin MgO film on Ag(001). We observe new low-binding energy photoemission, not seen for Ag(001), and greater electron emission in the normal direction. Under 4.66 eV laser excitation, the photoemission quantum efficiency of the MgO/Ag(001) hybrid photocathode is a factor of seven greater than that for clean Ag(001).

Environmental Molecular Sciences Laboratory↗
DOE OSTI2019

Hole-induced electronic and optical transitions in L a 1 - x S r x Fe O 3 epitaxial thin films

We have investigated the electronic and optical properties of epitaxial La 1-x Sr x FeO 3 for 0 ≤ x ≤ 1 prepared by molecular-beam epitaxy. Core-level and valence-band x-ray photoemission features monotonically shift to lower binding energy with increasing x, indicating downward movement of the Fermi level toward the valence-band maximum. Combining valence-band photoemission and O K -edge x-ray absorption data, we map the evolution of the occupied and unoccupied bands and observe a narrowing of the gap, along with a transfer of state density from just below to just above the Fermi level as a result of hole doping. In-plane transport measurements confirm that the material becomes a p -type semiconductor at lower doping levels and exhibits a conversion from semiconducting to metallic behavior at x = 1. As a result, low-energy optical transitions revealed by spectroscopic ellipsometry are explained based on insight from theoretical densities of states and first-principles calculations of optical absorption spectra.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Bulk electronic structure of lanthanum hexaboride (LaB 6 ) by hard x-ray angle-resolved photoelectron spectroscopy

In the last decade rare-earth hexaborides have been investigated for their fundamental importance in condensed matter, and for their applications in advanced technological fields. Among these compounds, LaB 6 has a special place, being a traditional d-band metal without additional f- bands. In order to understand the bulk electronic structure of the more complex rare-earth hexaborides, in this paper we investigate the bulk electronic structure of LaB 6 using tender/hard x-ray photoemission spectroscopy, measuring both core-level and angle-resolved valence-band spectra. Furthermore, we compare the La 3d core level spectrum to cluster model calculations in order to understand the bulk-like core-hole screening effects. The results show that the La 3d well-screened peak is at a lower binding energy compared to the main poorly-screened peak; the relative intensity between these peaks depends on how strong the hybridization is between La and B atoms. Here, we show that the recoil effect, negligible in the soft x-ray regime, becomes prominent at higher kinetic energies for lighter elements, such as boron, but is still negligible for heavy elements, such as lanthanum. In addition, we report the bulk-like band structure of LaB6 determined by tender/hard x-ray angle-resolved photoemission spectroscopy (HARPES). We compare HARPES experimental results to the free-electron final-state calculations and to the more precise one-step photoemission theory including matrix element and phonon excitation effects. The agreement between the features present in the experimental ARPES data and the theoretical calculations is very good. In addition, we consider the nature and the magnitude of phonon excitations in order to interpret HARPES experimental data measured at different temperatures and excitation energies. We demonstrate that one step theory of photoemission and HARPES experiments provide, at present, the only approach capable of probing, both experimentally and theoretically, true “bulk-like” electronic band structure of rare-earth hexaborides and strongly correlated materials.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Ultrafast Dynamics of Molecules on Surfaces Studied with Time-Resolved XUV Photoelectron Spectroscopy (Final Report)

In this project we set out to make technical advances in time-resolved photoemission from surfaces and study molecule-surface charge transfer processes. Combining a novel light source developed at Stony Brook with a new type of time-of-flight photoemission electron microscope (TOF k-mic) developed in Mainz, Germany, we achieved the former goal. Our light source, based on cavity-enhanced high harmonic generation at 61 MHz repetition rate, enables ultrafast time-resolved photoemission experiments to be conducted with orders of magnitude higher photocurrents than otherwise possible. Critically, this provides the high dynamic range necessary to study perturbatively excited samples, such that one can study the intrinsic dynamics of electrons and holes as they would occur in a device. Combining our light source with the TOF k-mic, we’ve achieved an overall improvement in data rate of time- and angle-resolved photoemission (tr-ARPES) of approximately four orders of magnitude over the previous state-of-the-art using kHz-repetition rate sources and conventional hemispherical electron analyzers. This new instrument is now being used in experiments on 2D materials and organic semiconductors.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

Correlating Tomographic Chemical Inhomogeneity and Low Energy Electronic Structure in Layered Quantum Materials

Photoemission spectroscopy (PES) is a suite of experimental tools to learn about the electronic and chemical structure of materials and surfaces. Normally implemented in a surface-sensitive manner, the research performed under this grant focused on pushing PES into less explored regimes, to reveal bulk electronic structure, to reveal tomographic (layer-resolved) chemistry and electronic structure of layered materials and heterostructures, and to reveal interface phenomena at the junction of two different materials. Standing wave (SW) spectroscopies have also been applied to PdCoO2, a material of interest due to its high conductivity and electron-hydrodynamic tendencies. This material can be modeled as an alternating layered structure consisting of metallic Pd layers and insulating CoO2 layers. Using SW XPS, the total electronic structure has been decomposed into contributions from the two layers, and computations highlighted the different many-body interactions in the two layers (Comm. Phys. 4, 143 (2021)). We have also used hard-x-ray angle-resolved photoemission spectroscopy (ARPES), to investigate LaB6, a technologically important material with widespread application as a cathode material for electron microscopes. We measured the bulk electronic structure of this material and found that the one-step model of photoemission better captured the electronic structure and correlations. This model treats the three steps of the photoemission process—excitation, transport of the photoelectron to the crystal surface, and escape into the vacuum—as a single quantum mechanically coherent process (Phys. Rev. Mater. 5, 055002 (2021)). We also applied x-ray photoelectron spectroscopy, implemented in a near total reflection grazing incidence geometry to elucidate technologically relevant interfaces, such as those between a substrate and photoresist (J. Phys. D: Appl. Phys. 54 464002 (2021)).

36 MATERIALS SCIENCE↗
NASA NTRS1980

OH/A 2 Sigma + - X 2 Pi sub i/ yield from H2O photodissociation in 1050-1370 A

The cross section and quantum yield for the production of the OH(A 2 Sigma + - X 2 Pi sub i) emission from H2O photodissociation by synchrotron radiation in the region 1050-1370 A are determined. Measurements of the photoemission and photoabsorption cross sections obtained for incident radiation bandwidths of 2 A reveal different vibrational structures in the range 1250-1370 A, and the photoemission is attributed to the 3a1-4a1 valence and 3a1-3sa1 Rydberg transitions. Photoemission quantum yields determined from the ratio of photoemission to photoabsorption cross sections are found to increase sharply at a threshold of 1362-5 A to a peak of 11% at 1300 A, then to decrease slowly with decreasing wavelength. Implications of the results for the dissociation processes of H2O are considered.

Lee, L. C.↗
DOE OSTI2012

Photocathode Research for Electron Accelerators

This dissertation presents studies of two different types of photocathodes used at electron accelerators: GaAs and K2CsSb. The spin polarization of photoelectrons extracted from GaAs photocathodes was evaluated using traditional one-photon absorption and two-photon absorption, to determine the validity of speculation that two-photon absorption would provide significantly higher polarization. To accomplish this study, a novel compact, retarding-field Mott polarimeter was designed, built and commissioned. The second type of photocathode, K2CsSb, was manufactured at Brookhaven National Laboratory and transported to Jefferson Lab in an ultrahigh vacuum "suitcase" and installed within a DC high voltage gun. Charge lifetime measurements were made using a K2CsSb photocathode and compared to those of GaAs under identical operating conditions. The surface morphology of the used K2CsSb was also studied. Many important atomic, nuclear and high energy physics experiments rely on using highly spin-polarized electron beams obtained via photoemission from GaAs photocathodes. Remarkably, even after 30 years of GaAs photogun operation, there are still some lingering questions associated with the beam polarization values being lower than expected. Bulk GaAs can theoretically provide 50% polarization but typically provides ~ 35%. Reduced polarization is attributed to a number of proposed depolarization mechanisms. Recently, it was proposed that two-photon absorption could be exploited to selectively populate the conduction band with electrons of just one spin state providing 100% polarization. These claims were refuted by others on theoretical grounds. In this thesis, for the first time, two-photon photoemission was demonstrated with beam delivered to a novel polarimeter constructed for this experiment. Polarization was found to be comparable to that obtained with conventional one-photon absorption, and to converge to the same value with a reduction in thickness of the active layer, thereby providing additional experimental insight to the polarized photoemission process an depolarization mechanisms in GaAs. Many proposed accelerators (light sources, energy recovery linacs and electron cooling machines) require extremely high average beam current (~ 100mA). The highest average current accelerator today, the FEL at JLab, can operate at up to 10mA and relies on photoemission from GaAs, even though spin polarization is not required. GaAs is an extremely delicate material, prone to failure and requiring ultrahigh vacuum conditions. In the early 1990s, the multi-alkali photocathode K2CsSb was used to demonstrate 32mA average current inside an RF gun with poor vacuum characteristics. This experiment lasted just minutes and was terminated shortly after the high current milestone was met. Based on this early work, many people believe K2CsSb represents a much better photocathode choice for unpolarized accelerator applications, providing high current for long periods of time and under less stringent operating conditions, but a detailed comparative study using the same apparatus has not been done. To address the high current reliability issue, two K2CsSb photocathodes were manufactured at Brookhaven National Lab and transported to Jefferson Lab via an ultrahigh vacuum transport chamber. They were installed within a 200kV DC high voltage photogun. Systematic charge lifetime measurements were performed using both photocathode materials under identical conditions. The performance of K2CsSb was shown to be highly dependent on extracted current. By combining this decay behavior with surface science measurements of the used photocathodes, it was shown that heat quickly disrupts the capability of the K2CsSb photocathode.

McCarter, James↗
DOE OSTI2010

Effect of ultraviolet and x-ray radiation on the work function of TiO{sub 2} surfaces

The work functions of nanocrystalline anatase (TiO{sub 2}) thin films and a rutile single crystal were measured using photoemission spectroscopy (PES). The nanocrystalline titanium dioxide films were deposited in-vacuum using electrospray thin film deposition. A comparison between ultraviolet photoemission spectroscopy (UPS) and low intensity x-ray photoemission spectroscopy (LIXPS) work function measurements on these samples revealed a strong, immediate, and permanent work function reduction (>0.5 eV) caused by the UPS measurements. Furthermore, it was found that regular XPS measurements also reduce the work function after exposure times ranging from seconds to minutes. These effects are similar in magnitude to artifacts seen previously on indium tin oxide (ITO) substrates characterized with XPS and UPS, and are likely related to the formation of a surface dipole through the photochemical hydroxylation of oxygen vacancies present on the TiO{sub 2} surface.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Full Hemisphere Fermi Surface Mapping Using A Novel Toroidal Electron Spectrometer

An overview is provided of a next-generation toroidal angle-resolving photoemission spectrometer, currently in operation at BESSY II. The instrument features exceptional multi-detection in energy and angle, facilitating rapid full-hemisphere photoemission measurements. The instrument is ideal for band structure and Fermi Surface (FS) mapping using Angle Resolved Photoemission Spectroscopy (ARPES). Recent full-hemisphere FS measurements of a prototypical material, copper, reveal previously unseen fine structure due to lifetime broadening, demonstrating the high angular resolution and dense {kappa}-space sampling capabilities of the instrument.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Properties of the SiO{sub 2}- and SiN{sub x}-capped GaAs(100) surfaces of GaInAsN/GaAs quantum-well heterostructures studied by photoelectron spectroscopy and photoluminescence

SiO{sub 2} and SiN{sub x} layers are routinely deposited onto III-V(100) surfaces at different device processing steps. We elucidate these insulator-interface properties with photoemission and photoluminescence (PL) of SiO{sub 2}- and SiN{sub x}-capped GaAs(100) surfaces of GaInAsN/GaAs quantum wells (QWs). Post-growth annealing led to an increase of the QW-PL intensity, of which origin can be clearly linked to the SiO{sub 2} and SiN{sub x} interfaces. Concomitantly, Ga{sub 2}O-related photoemission increased, indicating useful formation of Ga{sub 2}O at both insulator interfaces. Furthermore, higher Ga-oxidation-state emission, identified with Ga diffused into SiO{sub 2} and SiN{sub x}, correlates with the blue-shift of the QW-PL wavelength. Also, interfacial As-As related photoemission was identified.

36 MATERIALS SCIENCE↗