Energy-resolved Spectra Research Articles

Upgrade of the ion beam analysis end-station with the wavelength dispersive X-ray spectrometer for use with the focused MeV ion beams

A novel modular end-station has been developed for use with focused MeV ion beams. It supports analysis of micro-samples using simultaneously several ion beam analysis (IBA) techniques, including wavelength dispersive (WD) and energy dispersive (ED) Particle Induced X-ray Emission (PIXE) spectrometry, Elastic Backscattering Spectrometry (EBS and RBS) and Nuclear Reaction Analysis (NRA). The end-station enables quantitative elemental analysis, including the creation of two-dimensional elemental maps based on the ED-PIXE and EBS/RBS/NRA measurements. The in-house developed wavelength dispersive X-ray (WDX) spectrometer can measure high energy resolution spectra on selected sample micro-regions. In this work the description of the modular end-station is presented, highlighting the features of the WDX spectrometer: i) in the complementary analysis of complex ED X-ray spectra with many overlapping peaks measured simultaneously and ii) in chemical speciation studies. An example is presented to demonstrate the capabilities of the modular system for simultaneous analytical measurements of selected heterogeneous micro-samples.

Identification and Suppression of Point Defects in Bromide Perovskite Single Crystals Enabling Gamma-Ray Spectroscopy.

Methylammonium lead tribromide (MAPbBr3) stands out as the most easily grown wide-band-gap metal halide perovskite. It is a promising semiconductor for room-temperature gamma-ray (γ-ray) spectroscopic detectors, but no operational devices are realized. This can be largely attributed to a lack of understanding of point defects and their influence on detector performance. Here, through a combination of crystal growth design and defect characterization, including positron annihilation and impedance spectroscopy, the presence of specific point defects are identified and correlated to detector performance. Methylammonium (MA) vacancies, MA interstitials, and Pb vacancies are identified as the dominant charge-trapping defects in MAPbBr3 crystals, while Br vacancies caused doping. The addition of excess MABr reduces the MA and Br defects and so enables the detection of energy-resolved γ-ray spectra using a MAPbBr3 single-crystal device. Interestingly, the addition of formamidinium (FA) cations, which converted to methylformamidinium (MFA) cations by reaction with MA+ during crystal growth further reduced MA defects. This enabled an energy resolution of 3.9% for the 662keV 137Cs line using a low bias of 100V. The work provides direction toward enabling further improvements in wide-bandgap perovskite-based device performance by reducing detrimental defects.

Phosphorene nanoribbon field effect transistor with a dual material gate

In this paper, we present a dual-material gate phosphorene nanoribbon field-effect transistor (DMG-PNRFET) that combines the advantages of a PNRFET with a DMG configuration. In this structure, the difference in the work function creates an extra barrier for band-to-band tunneling from the valence band (VB) to the conduction band (CB) inside the channel leading to lower off-currents. An illustration for transmission coefficients with relevant band diagrams is included to demonstrate energy-resolved current spectrum and tunneling emissions within the transport window for both on and off-states. Results also show that DMG-PNRFET possesses a higher ION/IOFF ratio, delay, and power delay product (PDP) compared to a conventional PNRFET. Hence, the DMG-PNRFET is better suited for digital applications. Our simulations rely on combining the density functional-based tight binding method with the non-equilibrium Green’s function.

Speed of sound in methane under conditions of planetary interiors

We present direct observations of acoustic waves in warm dense matter. We analyze wave-number- and energy-resolved x-ray spectra taken from warm dense methane created by laser heating a cryogenic liquid jet. X-ray diffraction and inelastic free-electron scattering yield sample conditions of 0.3±0.1 eV and 0.8±0.1 g/cm−3, corresponding to a pressure of ∼13 GPa. Inelastic x-ray scattering was used to observe the collective oscillations of the ions. With a highly improved energy resolution of ∼50 meV, we could clearly distinguish the Brillouin peaks from the quasielastic Rayleigh feature. Data at different wave numbers were utilized to derive a sound speed of 5.9±0.5 km/s, marking a high-temperature data point for methane and demonstrating consistency with Birch's law in this parameter regime. Published by the American Physical Society 2024

Variable spin texture in Mn3Sn/Pt heterostructures revealed by magnetic circular dichroism

Non-collinear antiferromagnets with a D019 hexagonal structure have attracted tremendous attention for their potential applications in topological spintronics. Exploring the relationship between spin texture and electronic band structure is crucial for understanding the physical nature of these chiral antiferromagnets. Here, we systematically investigated the variation of topological spin texture of the non-collinear antiferromagnet Mn3Sn film using magnetic circular dichroism (MCD) spectroscopy. The evolution of Mn spin texture from coplanar inverted triangular structure to swirling spin texture was achieved in Mn3Sn/Pt heterostructures through introducing an interfacial Dzyaloshinskii–Moriya interaction (DMI) at room temperature. Correspondingly, unconventional zero-crossing points in energy-resolved MCD spectra as well as a gradual shift of a zero-crossing point to longer wavelength were observed. Our work provides a spin texture modulation approach via interfacial DMI and an effective non-contact magneto-optical detection method to reveal the spin texture in the non-collinear antiferromagnet/heavy metal system.

Resolution of zigzag magnetic correlations in Na-deficient NaxIrO3 without long-range ordering

The materials search for Kitaev quantum spin liquids led to the discovery of many honeycomb lattice materials. Much attention has been paid to materials without magnetic order down to the lowest temperatures. The newly synthesized Na-deficient ${\mathrm{Na}}_{x}{\mathrm{IrO}}_{3}$ has been found to bear no sign of long-range magnetic order above 1 K from physical property measurements. In this paper, we report momentum- and energy-resolved excitation spectra in Na-deficient ${\mathrm{Na}}_{x}{\mathrm{IrO}}_{3}$ measured using a resonant inelastic x-ray scattering spectrometer. Orbital excitation spectra show that the octahedral and trigonal crystal field splittings are larger in ${\mathrm{Na}}_{x}{\mathrm{IrO}}_{3}$ than in ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$. On the other hand, the low-energy spectrum at low temperature shows a wave-vector dispersion and a spectral weight distribution that are similar to those of ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$, revealing that the two-dimensional zigzag magnetic correlations in ${\mathrm{Na}}_{x}{\mathrm{IrO}}_{3}$ are similar to those in ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ in terms of the ordered magnetic moment direction and three dynamically fluctuating zigzag orders. The azimuth angle dependence of the low-energy spectrum corroborates these results. The two-dimensional zigzag magnetic correlations rapidly weaken until 50 K. At high temperatures, the spectral weight distribution of the low-energy excitation resembles that of the pure Kitaev model, indicating that the Kitaev interaction dominates the dynamic magnetic response at high temperature. We suggest that the larger crystal field and distortion and the weakened longer-range Heisenberg exchange interactions due to the Na deficiency in ${\mathrm{Na}}_{x}{\mathrm{IrO}}_{3}$ contribute to bring ${\mathrm{Na}}_{x}{\mathrm{IrO}}_{3}$ away from the zigzag long-range magnetic order phase.

Above-threshold ionization of argon with ultrashort orbital-angular-momentum beams

Light-matter interaction with laser pulses endowed with orbital angular momentum (OAM) raises a fundamental question about the nature of the transfer of this property of light to matter. In this work, a ``reaction microscope'' is used for precise measurement of the momentum of ionized photoions and photoelectrons from the interaction of Ar atoms with a linearly polarized, ultrashort ($\ensuremath{\sim}25$ fs), moderately intense (${10}^{13}--{10}^{14}\phantom{\rule{0.16em}{0ex}}\phantom{\rule{3.33333pt}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$) OAM carrying laser pulses. The angle and energy-resolved photoelectron spectrum is compared with the photoelectron spectrum obtained from the interaction with the laser beams with no OAM at similar intensities. No evidence of angular momentum transfer to the electrons from light is observed in our experiment.

Performance study of tunneling field effect transistors based on the graphene and phosphorene nanoribbons

The present work deals with the simulation and comparison of two tunneling field effect transistors (TFETs) based on the single layers of armchair graphene nanoribbons (AGNRs) and armchair phosphorene nanoribbons (APNRs). The transmission properties of the transistor have been obtained by solving the Schrödinger-Poisson equations self-consistently using the non-equilibrium Green's function (NEGF) formalism and the real-space approach. In addition, the energy band structure of the two materials have been obtained using tight-binding approximation method. The simulation results in each bias include the energy band structure, energy-resolved current spectrum, transmission function and drain current versus gate voltage diagrams. The results indicate that the structure of phosphorene TFETs meets a better ION to IOFF ratio and sub-threshold swing than the graphene TFETs despite a relative decrease in the amount of ION.

The 6.7 keV thermal emission lines in the stellar flare spectra of two chromospherically active Binaries: Algol and GT Mus

Chromospherically active binaries’ (CABs) are late-type evolved stars with strong chromosphere/coronae and high flare activity levels. Starspot evolution in these CABs and binaries reveals strong stellar and chromospheric activities. In binaries, the tidal interaction via the internal dynamo motion, during rapid rotation of the component stars generates magnetic fields. The magnetic activity heats the tenuous coronal plasma, and the reconnection of energetic particles leads to stellar flare formation. Cyclic-variation signatures in brightness and emission lines are indicators of coronal/chromospheric activities. The spatial population densities of CABs are large, and their hard energy spectral characteristics are prominent in our galaxy. Photo-ionization/collisional excitation processes in the coronae of CABs generate a strong thermal emission line at 6.7 keV. In the present work, we re-analyzed stellar flare data of two CABs (Algol & GT Muscae) observed with Suzaku. We resolved the 6.7 keV emission spectrum and equivalent width (EW) in each source’s stellar flare data. The resolved energy spectra are remarkably similar to that of the hard energy spectra at 6.7 keV of X-ray point sources observed in different Galactic regions. The EW of the 6.7 keV emission line obtained in this work compares favorably with the EW range (260 eV – 980 eV) of the 6.7 keV energy spectra of Galactic point sources. Hence, cumulative X-ray emission lines of coronal plasma (and their EWs) emitted by numerous point sources in different Galactic plane positions give clues on the Galactic Ridge X-ray Emission (GRXE) mechanism. We are of the view that CABs could account for a large number of fast-transient X-ray sources that emit thermal emission line at 6.7 keV during their quiescent and flared phases, and these underlying population sources could contribute significantly to the total luminosity of 6.7 keV GRXE.

Spectra of Saturn’s proton belts revealed

Saturn is permanently surrounded by 6 discrete proton radiation belts that are rigidly separated by the orbits of its inner moons and dense rings. These radiation belts are ideal environments to study the details of radial diffusion and the CRAND source process, yet progress has been hindered by the fact that the energy spectra are not known with certainty: Reanalysis of the response functions of the LEMMS instrument on-board the Cassini orbiter has shown that measurements of ≲10MeV protons may be easily contaminated by ≳10MeV protons and that many available measurements characterize a very broad energy range, so that the calculation of an energy-resolved spectrum is not as straightforward as previously assumed. Here we use forward modeling of the measurements based on the instrument response and combine this technique where useful with numerical modeling of the proton belt physics in order to determine Saturn’s spectra with higher certainty. We find significant proton intensities up to ≈1 GeV. While earlier studies reported on proton spectra roughly following a power law with exponent ≈−2, our more advanced analysis shows harder spectra with exponent ≈−1. The observed spectra provide independent confirmation that Saturn’s proton belts are sourced by CRAND and are consistent with the provided protons being subsequently cooled in the tenuous gas originating from Saturn or Enceladus. The intensities at Saturn are found to be lower than at Jupiter and Earth, which is also consistent with the source of Saturn being exclusively CRAND, while the other planets can draw from additional processes. Our new spectra can be used in the future to further our understanding of Saturn’s proton belts and the respective physical processes that occur at other magnetized planets in general. Also, the spectra have applications for several topics of planetary science, such as space weathering of Saturn’s moons and rings, and can be useful to constrain properties of the main rings through their production of secondary particles.

The influence of molecular alignment and orientation in the ground state and excited state on the resonance-enhanced multi-photon ionization dynamics

We explore the influence of molecular alignment and orientation in the ground and excited states on the ionization probability, photoelectron angular distribution (PAD), energy-resolved photoelectron energy spectrum (PES) and two-dimensional momentum spectrum in the resonance-enhanced multiphoton ionization (REMPI) process. The calculated results for the LiH molecule show that molecular pre-alignment and -orientation have different effects on molecular photoionization. The ionization probability and energy-resolved photoelectron spectrum are associated with molecular pre-alignment. The angular distribution of photoelectrons and angular distribution of the momentum spectra are closely dependent on molecular pre-orientation. The ionization probability is also related to the center time and overlap area of the pump and probe pulses.

Measurements at laser materials processing machines: spectrum deconvolution including uncertainties and model selection

Abstract. Laser materials processing of workpieces using ultra-short pulsed lasers can lead to unwanted X-ray emission. Their dose rate and spectral distribution have been precisely determined. The measurements were carried out using a thermoluminescence detector (TLD)-based spectrometer in which 30 TLD planes are arranged one behind the other, the first 10 layers made of polymethyl methacrylate, while the remaining 20 layers are interspaced by absorbers with, from the front to the back, increasing atomic charge and thickness. The penetration depth of the radiation into the spectrometer depends on its energy, so that the energy-resolved spectrum of the radiation can be calculated from the TLD dose values by means of mathematical methods (Bayesian deconvolution). The evaluation process also takes into account both the uncertainties of all input quantities and the possibility of adopting different models for the spectrum form. This allowed the resulting spectra to be associated with their realistic uncertainty. The measurements are traceable to the Système international d'unités (SI), i.e. the International System of Units. The results not only provide manufacturers and users of ultra-short pulsed lasers with important information on the design of the machines with regard to radiation protection, but were also included in the recently concluded legislative procedure in the field of radiation protection in Germany.

Tunable Tunneling Magnetoresistance in van der Waals Magnetic Tunnel Junctions with 1T-CrTe2 Electrodes.

Two-dimensional (2D) van der Waals (vdW) heterostructures have opened new avenues for spintronic applications with novel properties. Here, by density functional theory calculations, we investigated the spin-dependent transport in vdW magnetic tunnel junctions (MTJs) composed of 1T-CrTe2 ferromagnetic electrodes. Meanwhile, graphene and h-BN are employed as tunnel barriers. It has been found that the tunneling magnetoresistance (TMR) effects of two types of vdW MTJs present analogous trends: thicknesses of barriers have a great influence on the TMR ratios, which reach up to the maximum when barriers increase to five monolayers. However, despite the similarity, the graphene-barrier junction is more promising for optimization. Through observing the energy-resolved transmission spectra of vdW MTJs, we noticed that TMR ratios of graphene-barrier junctions are tunable and could be enhanced through tuning the position of Fermi energy. Therefore, we successfully realized the TMR optimization by substitutional doping. When substituting one carbon atom with one boron atom in the graphene barrier, TMR ratios are drastically improved, and a TMR ratio as high as 6962% could be obtained in the doped seven-monolayer-barrier junction. Our results pave the way for vdW MTJ applications in spintronics.

Momentum-resolved spectroscopy for the saddle-point excitons in InSb

We use time- and angle-resolved photoemission spectroscopy to reveal momentum-resolved characteristics of interband transitions in InSb. The transitions along the \ensuremath{\Gamma}-K line are correctly described in terms of the independent-quasiparticle band picture up to 1.9 eV. However, the transitions along the \ensuremath{\Gamma}-L line turn out to be strongly excitonic above 1.7 eV, associated with the saddle-point excitons at the ${E}_{1}$ critical point. The momentum- and energy-resolved spectra of photoionization of the saddle-point excitons show that the wave function is composed of the electron-hole pair states with the wave vector of $0.453\ifmmode\pm\else\textpm\fi{}0.055\phantom{\rule{0.16em}{0ex}}{\AA{}}^{\ensuremath{-}1}$ along the \ensuremath{\Gamma}-L direction of the Brillouin zone. Time-resolved characteristics of the exciton photoionization show that the coherently generated excitonic polarization decays with a time constant of 22 \ifmmode\pm\else\textpm\fi{} 1 fs due to the autoionization from the excitonic state to uncorrelated electron-hole pair states.

Mass spectra and incident energy resolved spectra after collisions of hydrocarbon ions with fusion-plasma tungsten thin films

We have performed surface-induced dissociation studies of small deuterated hydrocarbon cations i.e. CDx+ with x=2-4, colliding with two types of tungsten-coated surfaces, in the incident energy range between Ein = 0 eV approximately, up to Ein = 100 eV. A 34 nm thick W layer deposited on stainless steel using the Thermionic Vacuum Arc (TVA) method and a small sample of a tile cut from ASDEX-Upgrade tiles, consisting of PlasmaSprayed (PS) tungsten on carbon, are exposed to ion flux in these experiments. A double-focussing reverse geometry BE mass spectrometer, is used under ultra high vacuum conditions. Mass spectra of secondary charged particles are recorded via time-of-flight mass analyser. For comparison, we have performed equivalent study on polished stainless steel under experimental status explained in Section 2 below. At very low energies i.e. below 10 eV, only surface reflected projectiles are seen, whereas most of them are neutralized by surface micro charges. We observed that the fragmentation pattern of the small molecular ions at a given energy is strongly dependent on the surface. The roughness of both of the said thin films and their reflectivity for the projectile ion beam is studied by analysis of the ion yields of the reaction products

Performance metrics of current transport in pristine graphene nanoribbon field-effect transistors using recursive non-equilibrium Green's function approach

Abstract Graphene nanoribbons (GNRs) are an emerging material for future nanoelectronic applications. Because GNR fabrication technology is still in an early stage, modelling of GNR field-effect transistors (GNRFETs) is significant for evaluating the performance metrics of these devices. In this study, the charge transport properties of double-gate monolayer GNRFETs with various channel widths and lengths and doped contacts are investigated. The Hamiltonian matrix of the device is derived using the nearest-neighbour tight-binding method. The self-consistent solutions of the Poisson and Schrodinger equations are obtained within a recursive non-equilibrium Green's function formalism using the successive over-relaxation method to reduce the time required for the simulation. The effects of channel length and width of the device on the electronic transport properties such as the total density of states, transmission coefficient, energy-resolved current spectrum, and current–voltage characteristics are investigated. The performance metrics of the device, including the subthreshold swing, drain-induced barrier lowering (DIBL), threshold voltage, and on/off current ratio, are computed. It is found that for narrower and longer devices, the subthreshold swing and DIBL decrease, whereas the on/off current ratio increases. In addition, when the width index is in the 3p + 1 family, the device exhibits better switching performance. 7-armchair GNRFETs at 7 nm exhibits an outstanding subthreshold swing of ~67 mV/dec and a DIBL of ~54 mV/V. Thus, the narrower and longer device is less affected by short-channel effects, and the lower leakage current during the off state enables better switching performance, making it a potential candidate for future nanoelectronic applications in low-power design.

Carrier transport of rough-edged doped GNRFETs with metal contacts at various channel widths

Abstract Graphene nanoribbons (GNRs) have attracted much attention owing to their exotic electronic properties. However, it is impossible to fabricate GNRs with perfect edges. The edge roughness effect will degrade the performance of the field effect transistor (FET). Therefore, modelling GNR FETs (GNRFETs) with the line-edge roughness effect and doping is crucial for evaluating the performance metrics of these devices. In this research study, the carrier transport properties of double-gate, monolayer, doped, rough-edged armchair GNRFETs (AGNRFETs) with various channel widths, and metallic zigzag GNR (ZGNR) contacts are studied. The investigated width index originates from the 3p+1 family, and step and edge doping with nitrogen atoms (n-doping) and boron atoms (p-doping) are applied. In addition, the nearest-neighbour tight-binding method is employed to build the Hamiltonian matrix of the device. The self-consistent solutions of the Poisson and Schrodinger equations are computed with the recursive non-equilibrium Green's function (NEGF) method and successive over-relaxation (SOR) method to minimise the time consumption for convergence. The carrier transport properties of the pristine and non-pristine devices (e.g. the total density of states (DOS), transmission coefficient, energy-resolved current spectrum, and various current–voltage characteristics) are simulated. Based on the output, the performance metrics of the device, including the subthreshold swing, drain-induced barrier lowering (DIBL), threshold voltage, and on/off current ratio, are calculated. The results of different widths are compared separately and for the pristine and doped rough-edged channel devices. For narrower devices, the subthreshold swing and DIBL decrease, whereas the threshold voltage and on/off current ratio increase. The subthreshold swing and DIBL of the doped rough-edged channel device are superior to those of the pristine channel device. Moreover, the on-current, threshold voltage, and on/off current ratio of the rough-edged channel improve with dopants. N-doping achieves a much higher current in the on-state owing to the lighter electron mass and faster electron velocity compared with those of holes. Furthermore, the n-type rough-edged seven-armchair GNRFET with a length of 5 nm exhibits an outstanding subthreshold swing of approximately 85.2 mV/dec and a DIBL of 63.03 mV/V. Thereby, the narrower doped, rough-edged device is less affected by short-channel effects and exhibits lower leakage currents in the off-state. This results in a better switching performance, which makes it a potential candidate for future nano-electronic applications with low-power design.

Methylammonium Lead Tribromide Single Crystal Detectors towards Robust Gamma‐Ray Photon Sensing

AbstractIn recent years, hybrid perovskite single crystalline solid‐state detectors have shown promise in γ‐ray spectroscopy. Here, the γ‐ray photon induced electrical pulses are investigated, which are produced by perovskite solid‐state detectors made with the commonly used methylammonium lead tribromide crystals with chlorine incorporation. Under low electric field detector operation, slow pulses generated by γ‐rays with average rise times of 65 µs are observed, which decreases to 20 µs when a higher electrical field of 500 V cm−1 is applied. However, the baseline becomes noisy quickly, which prevents collection of clean pulses for spectra construction. Further, by systematically measuring the temperature dependence and current–voltage characteristics, such instability is attributed to the local ion migration under electrical field creating a fluctuating dark noise, which presents a major challenge in perovskite γ‐ray detector technologies. It is demonstrated that cycling the bias between positive and negative polarity can stabilize the detector, allowing for longer periods of pulse accumulation for generating energy resolved spectra with resolutions of ≈35% at 59.6 keV and ≈25% at 662 keV at room temperature. The study indicates that the main limiting factors of perovskite‐based γ‐ray detectors are slow rise times and bias instability. These challenges must be properly addressed to achieve reproducible, high‐resolution γ‐ray detection.

Ion trap MS using high trapping gas pressure enables unequivocal structural analysis of three isobaric compounds in a mixture by using energy-resolved mass spectrometry and the survival yield technique.

Recently, it has been shown that energy-resolved mass spectrometry (MS) can provide quantitative information from two isomeric or isobaric compounds in mixtures by using the survival yield (SY) technique together with "gas-phase collisional purification" (GPCP) strategy (Anal. Chem., 2016, 88, p.10821). Herein, we present an improvement and an extension of this concept to the structural analysis of a model mixture of three isobaric compounds (two peptides and a polyether). By using default collision-induced dissociation (CID) tandem MS parameters on an ion trap instrument, the previous approach did not show any signs of isobaric contamination. However, by modifying CID conditions and using a threefold increase of the He trapping gas pressure (to reach 3.00·10-5 mbar), the SY curve was unexpectedly and strongly shifted to higher excitation voltages with two plateaus appearing. Those plateaus, indicating clearly the presence of three isobaric compounds, were taken as reliable indicators to perform GPCP at carefully selected excitation voltages in order to selectively fragment one compound after the other. In this way, CID mass spectra of each compound were correctly recovered, both in terms of fragment ion peaks and in terms of relative intensities, from energy-resolved MSn spectra of the three compounds mixture. This feature enables their unequivocal structural analysis as if samples were free from isobaric interferences. In this paper, we also discuss the possibility for recovering SY curves for pure compounds directly from the mixture. Clearly, in this case, the higher He trapping gas pressure made it possible to use the SY technique, for the first time, for the structural analysis in the case of mixtures of three isobaric compounds. This observation, quite unexpected, demonstrates that the trapping gas pressure is of paramount importance although it is usually not considered in energy-resolved MS for structural and/or quantitative analysis.

A diagnostic of the orbital spectrum of LS 5039 with Fermi-LAT

LS 5039 is a well-known γ-ray binary system which consists of an unknown compact object and a massive companion O star. It shows rather stable emissions at high energies over years and hence serves as an ideal laboratory to investigate the emission mechanism for such peculiar systems which emit prominent γ-rays. To this end, we take the orbital phase resolved energy spectrum as observed by Fermi-LAT over 10 years. We divide the orbit into four orbital phases, each with an orbital phase range of 0.25, centered at 0.00, 0.25, 0.50 and 0.75 respectively, where the phase 0.0 is the periastron and phase 0.5 is the apastron. The phases around 0.25 and 0.75 are symmetric and hence are supposed to have identical local acceleration environment. The spectral analysis shows that the Fermi-LAT spectra are largely different from these two symmetric orbital phases: the emission from orbital phase 0.25 turns out to be significantly stronger than that from 0.75. This result does not fit a scenario that γ-rays are Doppler boosted emission from bow shock tails if LS 5039 has a shock configuration similar to PSR B1259-63, and indicates that the inverse Compton scatterings between the shock accelerated plasma and the stellar particle environment is the underline procedure. We also find that the previous report for a disappearance of the orbital modulation at 3–20 GeV is due to the similar spectral turn-over energies of the different orbital phases. The spectral properties of periastron and apastron regions are addressed in the context of the measurements in phase regions around 0.25 and 0.75.