Population Of Charge States Research Articles

Charge states of nitrogen-vacancy centers in Fermi level controlled diamond n-i-n junctions

Control of the charge state of the nitrogen-vacancy (NV) center is crucial because of its instability and its transitions between the negative (NV–) and neutral (NV0) NV charge states under laser irradiation In this study, we fabricated an n-i-n junction, with an i-layer sandwiched between two phosphorus-doped n-layers; then, we measured the charge state of NV centers under steady state and laser irradiation in a known band structure where the Fermi energy changes gradually. The steady-state charge state measured by a nondestructive single shot exhibited stable NV– and NV0 signals when the Fermi level was even slightly above and below the transition level, respectively. This result indicates that the charge state can be significantly stabilized through band engineering. Both charge-state populations were observed only when the Fermi level was close to the transition level. Under continuous green laser irradiation, the ratio of NV– measured by the photoluminescence spectra changed gradually with the Fermi level in the depletion layer because of the balance between excitation from the laser and the supply of charge from the band. This outcome agrees reasonably with the calculated bands. Furthermore, we measured the PL spectra of the ensemble NV centers and discovered that their charge state can be well-controlled, as in the single NV center. The charge state of the i-layer at the interface can be stabilized by depositing a thin n-layer on the surface. These results would contribute significantly to improve sensor performance.

Multicolor-illuminated charge-state dynamics of the nitrogen-vacancy center in diamond

Charge-state conversion has been a very useful tool for improving the performance of nitrogen-vacancy (NV) centers in diamond in many application scenarios. Lasers of different colors are employed to manipulate the NV charge, ranging from blue to infrared. Here, we focus on the study of the charge-state dynamics under illumination of multicolors and their combination with infrared (IR) light, and the results are directly presented in terms of charge-state population. It is shown that each color of light serves some kind of superiority over others in charge-state conversion and a greatly improved conversion rate can be induced by the additional IR illumination. We report that the steady-state ${\mathrm{NV}}^{\ensuremath{-}}$ population is increased up to 82% by IR light, and it presents basically the same response trend to infrared power as that of fluorescence counts. We also report a fast-oscillating fluorescence time trace by modulating the IR power. We believe that our work offers some straightforward results that might be of interest to researchers and lead to a clearer understanding of the NV charge-state dynamics.

Beam particle identification and tagging of incompletely stripped heavy beams with HEIST.

A challenge preventing successful inverse kinematics measurements with heavy nuclei that are not fully stripped is identifying and tagging the beam particles. For this purpose, the HEavy ISotope Tagger (HEIST) has been developed. HEIST utilizes two micro-channel plate timing detectors to measure the time-of-flight, a multi-sampling ion chamber to measure energy loss, and a high-purity germanium detector to identify isomer decays and calibrate the isotope identification system. HEIST has successfully identified 198Pb and other nearby nuclei at energies of about 75 MeV/A. In the experiment discussed, a typical cut containing 89% of all 198Pb80+ in the beam had a purity of 86%. We examine the issues of charge state contamination. The observed charge state populations of these ions are presented and, using an adjusted beam energy, are well described by the charge state model GLOBAL.

Attosecond pump-attosecond probe spectroscopy of Auger decay.

Attosecond pump-attosecond probe spectroscopy is becoming possible due the development of sub-femtosecond free electron laser (FEL) pulses as well as intense high-order harmonic generation-based attosecond sources. Here we investigate theoretically whether these developments can provide access to direct time-resolved measurement of Auger decay through detection of the total yield of an ionic decay product, in analogy to the photodissociation product detection in femtochemistry. We show that the ion yield based measurement is generally possible and in the case of the inner-valence hole decay can be background-free. Extensive first principles calculations are used to optimise the probe photon energies for a variety of prototypical systems.

Spectral characterization of an industrial EUV light source for nanolithography

The emission spectra from an industrial, droplet-based, laser-produced plasma, extreme ultraviolet light source for nanolithography are here presented and analyzed. The dependence of spectral features on the CO2-drive-laser intensity is studied by changing the beam spot size at constant pulse energy and duration. We characterize the spectrum by fitting the results of atomic structure calculations to the short-wavelength region (7–11 nm), where the contributions from various charge states can be resolved, and obtain the relative contributions of charge states Sn9+ –Sn15+ . These relative contributions are compared to charge state populations as calculated with the non-equilibrium plasma kinetics code flychk. The calculations are shown to be in good qualitative agreement with the results, showing that the effective plasma temperature, and with it, the shape of the unresolved, main emission feature at 13.5 nm, is a remarkably weak function of laser intensity under this source normal operating conditions.

Charge-state populations for the neon-XFEL system**Project supported by the National Natural Science Foundation of China (Grant No. 11474208).

The interaction between neon and x-ray free-electron lasers with different laser parameters is systematically studied by solving a set of coupled rate equations. As an example, the evolution of 1s12s22p6 configuration is given under different incident photon numbers, pulse widths, and photon energies. We have also determined all of the charge-state populations as a function of three laser pulse parameters by averaging over time. The result shows that the variations of these charge-state populations demonstrate a pattern when the pulse width is shorter than 10 fs: some of the charge-states decrease rapidly, while the others rise but remain relatively constant for pulse width larger than 10 fs. The variation of the average charge with three parameters has also obtained. The average charge decreases for a pulse width shorter than 10 fs but remains basically unchanged for a pulse width longer than 10 fs.

Photo-EPR study of compensated defects in Be-doped GaN substrates

Time-dependent photo-induced electron paramagnetic resonance measurements were performed on GaN crystals grown by the high nitrogen pressure solution method. A resonance was induced in Be-doped samples by illuminating with photon energies greater than 2.7 eV. The EPR was thermally stable at T=4K and was quenched with photon energies between 0.5 and 2.7 eV. Optical cross sections for EPR excitation and quenching processes were extracted from time-dependent measurements using a model for defect charge state populations. A fit to the cross sections was performed using a lineshape for photon absorption by deep centers in GaN. The threshold for photo-excitation of a compensated acceptor (A−+hν→A0+eCB−) was 2.5 eV followed by a lattice relaxation of 0.3 eV. The threshold for the complimentary process (A0+hν→A−+hVB+) was 0.75 eV with a relaxation energy of 0.3 eV. These values are consistent with CN being the EPR-active acceptor. Thermal quenching is attributed to delocalization of carriers from a beryllium-related electron trap, which are then captured by neutral CN.

Engineering of Fermi level by nin diamond junction for control of charge states of NV centers

The charge-state control of nitrogen-vacancy (NV) centers in diamond is very important toward its applications because the NV centers undergo stochastic charge-state transitions between the negative charge state (NV−) and the neutral charge state (NV0) of the NV center upon illumination. In this letter, engineering of the Fermi level by a nin diamond junction was demonstrated for the control of the charge state of the NV centers in the intrinsic (i) layer region. By changing the size (d) of the i-layer region between the phosphorus-doped n-type layer regions (nin) from 2 μm to 10 μm, we realized the gradual change in the NV− charge-state population in the i-layer region from 60% to 80% under 532 nm excitation, which can be attributed to the band bending in the i-layer region. Also, we quantitatively simulated the changes in the Fermi level in the i-layer region depending on d with various concentrations of impurities in the i-layer region.

Optical charge state control of spin defects in 4H-SiC

Defects in silicon carbide (SiC) have emerged as a favorable platform for optically active spin-based quantum technologies. Spin qubits exist in specific charge states of these defects, where the ability to control these states can provide enhanced spin-dependent readout and long-term charge stability. We investigate this charge state control for two major spin qubits in 4H-SiC, the divacancy and silicon vacancy, obtaining bidirectional optical charge conversion between the bright and dark states of these defects. We measure increased photoluminescence from divacancy ensembles by up to three orders of magnitude using near-ultraviolet excitation, depending on the substrate, and without degrading the electron spin coherence time. This charge conversion remains stable for hours at cryogenic temperatures, allowing spatial and persistent patterning of the charge state populations. We develop a comprehensive model of the defects and optical processes involved, offering a strong basis to improve material design and to develop quantum applications in SiC.

Ionization competition effects on population distribution and radiative opacity of mixture plasmas

Ionization competition arising from the electronic shell structures of various atomic species in the mixture plasmas was investigated, taking SiO2 as an example. Using a detailed-level-accounting approximation, we studied the competition effects on the charge state population distribution and spectrally resolved and Planck and Rosseland mean radiative opacities of mixture plasmas. A set of coupled equations for ionization equilibria that include all components of the mixture plasmas are solved to determine the population distributions. For a given plasma density, competition effects are found at three distinct temperature ranges, corresponding to the ionization of M-, L-, and K-shell electrons of Si. Taking the effects into account, the spectrally resolved and Planck and Rosseland mean opacities are systematically investigated over a wide range of plasma densities and temperatures. For a given mass density, the Rosseland mean decreases monotonically with plasma temperature, whereas Planck mean does not. Although the overall trend is a decrease, the Planck mean increases over a finite intermediate temperature regime. A comparison with the available experimental and theoretical results is made.

Effect of He/Ne/Ar on EUV emission and Xe plasma pumped by capillary discharge

The influence of the He, Ne and Ar on Xe plasma pumped by capillary discharge process is studied theoretically and experimentally. The charge-state populations and average ionization of Xe as a function of electron temperature for pure Xe and mixed gases are calculated with a collision-radiation model. The emission of Xe10+ 4d 8-4d 75p 13.5 nm in 2% bandwidth is obtained by a EUV emission monitor (E-Mon). Additional He/Ne/Ar gas can influence on the time scale and the intensity of the appearance for the E-Mon signals by effecting the electron density and temperature.

NMR Detection of pH-Dependent Histidine–Water Proton Exchange Reveals the Conduction Mechanism of a Transmembrane Proton Channel

The acid-activated proton channel formed by the influenza M2 protein is important for the life cycle of the virus. A single histidine, His37, in the M2 transmembrane domain (M2TM) is responsible for pH activation and proton selectivity of the channel. Recent studies suggested three models for how His37 mediates proton transport: a shuttle mechanism involving His37 protonation and deprotonation, a H-bonded imidazole-imidazolium dimer model, and a transporter model involving large protein conformational changes in synchrony with proton conduction. Using magic-angle-spinning (MAS) solid-state NMR spectroscopy, we examined the proton exchange and backbone conformational dynamics of M2TM in a virus-envelope-mimetic membrane. At physiological temperature and pH, (15)N NMR spectra show fast exchange of the imidazole (15)N between protonated and unprotonated states. To quantify the proton exchange rates, we measured the (15)N T(2) relaxation times and simulated them for chemical-shift exchange and fluctuating N-H dipolar fields under (1)H decoupling and MAS. The exchange rate is 4.5 × 10(5) s(-1) for Nδ1 and 1.0 × 10(5) s(-1) for Nε2, which are approximately synchronized with the recently reported imidazole reorientation. Binding of the antiviral drug amantadine suppressed both proton exchange and ring motion, thus interfering with the proton transfer mechanism. By measuring the relative concentrations of neutral and cationic His as a function of pH, we determined the four pK(a) values of the His37 tetrad in the viral membrane. Fitting the proton current curve using the charge-state populations from these pK(a)'s, we obtained the relative conductance of the five charge states, which showed that the +3 channel has the highest time-averaged unitary conductance. At physiologically relevant pH, 2D correlation spectra indicated that the neutral and cationic histidines do not have close contacts, ruling out the H-bonded dimer model. Moreover, a narrowly distributed nonideal helical structure coexists with a broadly distributed ideal helical conformation without interchange on the sub-10 ms time scale, thus excluding the transporter model in the viral membrane. These data support the shuttle mechanism of proton conduction, whose essential steps involve His-water proton exchange facilitated by imidazole ring reorientations.

(Invited) Lifetime Degradation in Boron Doped Czochralski Silicon

The electron lifetime in boron-doped and oxygen-containing silicon decreases in the presence of excess electrons, and tends to some low saturated value. At lower intensity, the time scale of this process is strongly increased. The observed degradation kinetics, in a wide range of intensity, from 1 sun to 0.001 sun, is reproduced quantitatively within a simple model based on the presence of latent boron-oxygen centre (LC). Under illumination, LC captures an electron and then reconstructs into a recombination-active configuration (SRC). The forward reaction LC → SRC as well as the backward one depend on the population of charge states of LC and SRC. To account for the saturated lifetime in dependence of the light intensity, three charge states of SRC should be invoked, implying that SRC has two energy levels: a donor and an acceptor. The latter level accounts for the recombination activity of SRC.

Carrier density saturation in a [formula omitted] heterostructure

We observe a strong saturation of the carrier density in the quantum well of a Ga 0.25 In 0.75 As / InP MISFET at positive gate voltages. Using a self-consistent Schrödinger/Poisson solver, we model the band structure and find that the saturation is caused by the population of charge states between the gate and the quantum well. We discuss the impact of these charge states on the transport properties, and present a fabrication method that avoids parallel conduction in this heterostructure.

Charge-state distributions after radiative capture of helium nuclei by a carbon beam

We present new measurements of the charge-state distribution (CSD) of a 1.068MeV/u C beam in He and of the 6+:5+ charge-state population ratio in the recoils of the 12C(α,γ)16O reaction, both measured at the DRAGON recoil mass spectrometer. A computer simulation to model the CSD of both beam and recoil particles in inverse-kinematics experiments is compared to data from this work and from previous work at ERNA. The simulation provides good agreement with both data sets. The results suggest that, for this fusion reaction on the Jπ=4+ resonance at Ebeam=1.064MeV/u, immediately after fusion, the recoil ions contain only the nucleons and not the electrons of the target He atom.

Ionization, stopping, and thermalization of hydrogen and boron beams injected in fusion plasmas

The ionization, stopping, and thermalization of hydrogen and boron beams, injected, respectively, in boron and hydrogen plasmas, is studied. The evolution of the charge state populations of the neutral beams is described considering the various ionization, excitation, and charge exchange channels. The interaction of the beam with the plasma is described in terms of the Fokker–Planck equation, which is numerically solved to show in detail the evolution of the beam until final thermalization is reached. Beam energies of 640 keV/u (maximum of the cross section for the p-B11 fusion reaction) and 200 keV/u, and various plasma temperatures are considered. It is seen that, due to an important perpendicular-diffusion effect, high energy beams reach effective peak temperatures which are much higher than the plasma temperature, before equilibrium is established. The fraction of fusioned particles is also calculated. Some implications of interest for recently proposed p-B11 fusion reactor systems are drawn out.

Enhancing the performances of traditional electron cyclotron resonance ion sources with multiple-discrete-frequency microwave radiation

The performances of electron cyclotron resonance (ECR) ion sources, in terms of high-charge-state yields and intensities within a particular charge state, can be enhanced by increasing the physical sizes of the ECR zones in relation to the sizes of their plasma volumes. The creation of a large ECR plasma “volume” permits coupling of more power into the plasma, resulting in the heating of a much larger electron population to higher energies, the effect of which is to produce higher charge-state distributions and higher intensities within a particular charge state than possible in present forms of the ECR source. The ECR plasma “volumes” of traditional B-minimum ECR sources can be increased by injecting broadband microwave radiation (multiple-discrete-frequency, variable frequency, or broad-band-width frequency microwave radiation) derived from standard klystron, gyrotron, or traveling-wave-tube (TWT) technologies (frequency domain). To demonstrate that the frequency domain technique can be used to enhance the performance of a traditional B-minimum ECR ion source, comparative studies were made to assess the relative performances of the Oak Ridge National Laboratory Caprice ECR ion source, in terms of multiply charged ion-beam generation capabilities, when excited with high-power, single-frequency, or multiple-discrete-frequency microwave radiation, derived from standard klystron and/or TWT technologies. These studies demonstrate that the charge-state populations for Ar q+ and Xeq+ move toward higher values when excited with two and three discrete-frequency, microwave power compared to those observed when single-frequency microwave power is used. For example, the most probable charge state for Xe is increased by one charge-state unit while the beam intensities for charge states higher than the most probable are increased by factors of ∼3 compared to those observed for single-frequency plasma excitation. The results of these measurements along with details on the modifications to the injection system required to couple the microwave radiation into the plasma volume of the Caprice source will be presented in this report.

Use of multiple-discrete-frequency microwave radiation to enhance the performances of traditional B-minimum electron cyclotron resonance ion sources

The performances of electron cyclotron resonance (ECR) ion sources, in terms of high-charge-state yields and intensities within a particular charge-state, can be enhanced by increasing the physical sizes of the ECR zones in relation to the sizes of their plasma volumes. The ECR plasma “volumes” of traditional B-minimum ECR sources can be increased by injecting broadband microwave radiation (multiple-discrete frequency, variable frequency, or broad-bandwidth frequency microwave radiation) derived from standard klystron, gyrotron, or traveling-wave-tube (TWT) technologies (frequency domain). In this work, comparisons were made of the charge-state distributions of Arq+ and Xeq+ extracted from the ORNL Caprice ECR ion source, when excited with single frequency and multiple-discrete-frequency microwave radiation, derived from standard klystron and/or TWT technologies. The charge-state populations for Arq+ and Xeq+ move toward higher values when excited with modest power from two and three discrete frequency, microwave radiation compared to those generated with power from single-frequency radiation. For three-frequency plasma excitation, the most probable charge state for Xe is increased by one charge-state unit while the beam intensities for charge-states higher than the most probable are increased by factors of ∼3 over those for the single frequency plasma case. The results of these measurements along with details on the modifications to the injection system required to couple the microwave radiation into the plasma volume of the Caprice source will be presented in this article.

An EBIS/T with integrated Penning trap

A special problem in atomic physics research with highly charged ions is to prepare ions with a unique charge state inside of EBIS or EBIT devices. On the other hand, there are great losses resulting from the transport of the ions from the source to an external trap. Therefore we are setting up an EBIS/T with internal Penning trap. This new set-up will be able to study electron–ion interaction with well-defined initial and final charge states, distinguishing between single step successive ionisation and multiple step ionisation of charge states similar to the crossed beams method but for much higher charge states. Another feature of this system is to determine with high precision the ion charge state distribution in the EBIS/T by application of Fourier Transform Ion Cyclotron Resonance (FT-ICR). This method allows the on-line monitoring of the ion distribution and the evolution of the charge state population together with its dependence on the degree of space charge compensation of the electron beam in the EBIS/T. It will be possible to study ion dynamics in compensated space charge potentials. In case of high homogeneity of the magnetic field in the trap region, experiments may be considered to measure directly binding energies of highly-charged ions and other topics of high resolution mass spectroscopy.

Effects of non-Maxwellian electron distributions on charge-state populations in laser-produced plasmas

The effects of a non-Maxwellian electron distribution on the charge-state populations in a plasma with the distribution characterized by the function f(v)=Fm exp[−(v/vm)m] with 2≤m≤5 are investigated. In the underdense region of a laser produced plasma, the parameter m would depend on the electron temperature, electron density, and the average ionization state of the plasma in addition to the optical laser intensity and wavelength. The ion populations are obtained by solving the rate equations in which the coefficients are evaluated by integrating the cross sections over the non-Maxwellian electron distributions. The scaling of m with column density and optical laser intensity in laser exploding foils is obtained. The effects of a non-Maxwellian electron distribution on the charge-state populations in both selenium and molybdenum foils, similar to those used to model recent x-ray laser experiments, are calculated. The effects on the dominant populations are found to be small, less than a dozen of percents.