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The effect of chalcogen and metal on the electronic properties and stability of metal–chalcogenides clusters, TM6Xn(PH3)6 (TM = Mo, Cr, Re, Co, Ni; X = Se, Te; n = 8,5)
摘要： We have performed a comparative study of the electronic structure, stability, and magnetic properties of a series of metal–chalcogen clusters stabilized by PH3 ligands. Clusters studied include TM6X8(PH3)6, TM = Cr, Mo, Re, Co, X = Se, Te, and Ni6X(PH3)6, X = Se, and Te. We find that the phosphine ligands act as charge donors, transferring charge to the metal sites, creating an electrostatic effect that lowers the ionization energy. The electronic structure of the cluster also has a significant effect on its charge donor properties, as the Re cluster has a closed electronic shell with a charge state of +2, making it an alkaline earth superatom. The chromium clusters are found to have a series of close lying magnetic isomers. Selenium is a better charge acceptor than tellurium and this causes the telluride clusters to have lower ionization potentials, while the enhanced charge transfer to selenium increases the binding energy of the phosphine ligand.
关键词： metal–chalcogen clusters,PH3 ligands,ionization energy,superatom,stability,charge transfer,magnetic properties,electronic structure
更新于2025-09-10 09:29:36
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UVIT <i>–HST</i> – <i>GAIA</i> view of NGC 288: a census of the hot stellar population and its properties from UV
摘要： A complete census of the blue horizontal branch (BHB) and blue straggler star (BSS) population within a 10(cid:2) radius from the centre of the globular cluster NGC 288 is presented, based on images from the Ultraviolet Imaging Telescope (UVIT). The UV and UV?optical colour–magnitude diagrams (CMDs) are constructed by combining the UVIT, HST-ACS and ground data and are compared with the Bag of Stellar Tracks and Isochrones (BaSTI) isochrones generated for UVIT ?lters. We used stellar proper motion data from GAIA DR2 to select the cluster members. Our estimations of the temperature distribution of 110 BHB stars reveal two peaks, with the main peak at Teff ～ 10 300 K, and with the distribution extending up to Teff ～ 18 000 K. We identify the well-known photometric gaps, including the Grundahl jump (G-jump) in the BHB distribution, which are located between the peaks. We detect a plateau in the far-ultraviolet (FUV) magnitude for stars hotter than Teff ～ 11 500 K (G-jump), which could be caused by atomic diffusion. We detect two extreme horizontal branch (EHB) candidates, with temperatures ranging from 29 000 to 32 000 K. The radial distribution of 68 BSSs suggests that the bright BSSs are more centrally concentrated than the faint BSSs and the BHB stars. We ?nd that the BSSs have a mass range of 0.86–1.25 M(cid:4) and an age range of 2–10 Gyr, with peaks at 1 M(cid:4) and 4 Gyr respectively. This study showcases the importance of combining UVIT with HST, ground and GAIA data in deriving HB and BSS properties.
关键词： stars: blue stragglers,ultraviolet: stars,catalogues,stars: imaging,globular clusters: individual: NGC 288,stars: Hertzsprung–Russell and colour–magnitude diagrams,stars: horizontal branch
更新于2025-09-10 09:29:36
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Observing multiple stellar populations with VLT/FORS2
摘要： Aims. We present the photometric analysis of the external regions of three Galactic globular clusters: NGC 6121, NGC 6397, and NGC 6752. The main goal is the characterization of multiple stellar populations along the main sequence (MS) and the study of the radial trend of di?erent populations hosted by the target clusters. Methods. The data have been collected using the FOcal Reducer and low dispersion Spectrograph 2 (FORS2) mounted at the Very Large Telescope (VLT) UT1 of the European Southern Observatory (ESO) in U BV I ?lters. From these data sets we extracted high-accuracy photometry and constructed color–magnitude diagrams (CMDs). We exploit appropriate combinations of colors and magnitudes, which are powerful tools for identifying multiple stellar populations, such as B versus U ? B and V versus cU,B,I = (U ? B) ? (B ? I) CMDs. Results. We con?rm previous ?ndings of a split MS in NGC 6752 and NGC 6121. Apart from the extreme case of ω Centauri, this is the ?rst detection of multiple MS from ground-based photometry. For NGC 6752 and NGC 6121, we compare the number ratio of the blue MS to the red MS in the cluster outskirts with the fraction of ?rst and second generation stars measured in the central regions. There is no evidence for signi?cant radial trend. The MS of NGC 6397 is consistent with a simple stellar population. We propose that the lack of multiple sequences is due both to observational errors and to the limited sensitivity of U, B, V, I photometry to multiple stellar populations in metal-poor GCs. Finally, we compute the helium abundance for the stellar populations hosted by NGC 6121 and NGC 6752, ?nding a mild (ΔY ～ 0.02) di?erence between stars in the two sequences.
关键词： globular clusters: individual: M 4,globular clusters: individual: NGC 6752,globular clusters: individual: NGC 6397,globular clusters: general
更新于2025-09-09 09:28:46
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The <i>Swift</i> X-ray Telescope Cluster Survey
摘要： Aims. We present a spectral analysis of a new, ?ux-limited sample of 72 X-ray selected clusters of galaxies identi?ed with the X-ray Telescope (XRT) on board the Swift satellite down to a ?ux limit of ～10?14 erg s?1 cm?2 (SWXCS). We carry out a detailed X-ray spectral analysis with the twofold aim of measuring redshifts and characterizing the properties of the intracluster medium (ICM) for the majority of the SWXCS sources. Methods. Optical counterparts and spectroscopic or photometric redshifts for some of the sources are obtained with a cross-correlation with the NASA/IPAC Extragalactic Database. Additional photometric redshifts are computed with a dedicated follow-up program with the Telescopio Nazionale Galileo and a cross-correlation with the SDSS. In addition, we also blindly search for the Hydrogen-like and He-like iron Kα emission line complex in the X-ray spectrum. We detect the iron emission lines in 35% of the sample, and hence obtain a robust measure of the X-ray redshift zX with typical rms error 1–5%. We use zX whenever the optical redshift is not available. Finally, for all the sources with measured redshift, background-subtracted spectra are ?tted with a single-temperature mekal model to measure global temperature, X-ray luminosity and iron abundance of the ICM. We perform extensive spectral simulations to accounts for ?tting bias, and to assess the robustness of our results. We derive a criterion to select reliable best-?t models and an empirical formula to account for ?tting bias. The bias-corrected values are then used to investigate the scaling properties of the X-ray observables. Results. Overall, we are able to characterize the ICM of 46 sources with redshifts (64% of the sample). The sample is mostly constituted by clusters with temperatures between 3 and 10 keV, plus 14 low-mass clusters and groups with temperatures below 3 keV. The redshift distribution peaks around z ～ 0.25 and extends up to z ～ 1, with 60% of the sample at 0.1 < z < 0.4. We derive the luminosity?temperature relation for these 46 sources, ?nding good agreement with previous studies. Conclusions. Thanks to the good X-ray spectral quality and the low background of Swift/XRT, we are able to measure ICM temperatures and X-ray luminosities for the 46 sources with redshifts. Once redshifts are available for the remaining 26 sources, this sample will constitute a well-characterized, ?ux-limited catalog of clusters distributed over a broad redshift range (0.1 ≤ z ≤ 1.0) providing a statistically complete view of the cluster population with a selection function that allows a proper treatment of any measurement bias. The quality of the SWXCS sample is comparable to other samples available in the literature and obtained with much larger X-ray telescopes. Our results have interesting implications for the design of future X-ray survey telescopes, characterized by good-quality PSF over the entire ?eld of view and low background.
关键词： intergalactic medium,galaxies: high-redshift,X-rays: galaxies: clusters,galaxies: clusters: general,cosmology: observations
更新于2025-09-09 09:28:46
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Quantum reflection of rare-gas atoms and clusters from a grating
摘要： Quantum re?ection is a universal property of atoms and molecules when scattered from surfaces in ultracold collisions. Recent experimental work has documented the quantum re?ection and diffraction of He atoms, dimers, trimers, and neon atoms when re?ected from a grating. Conditions for the observation of emerging beam resonances have been discussed and measured. In this paper, we provide a theoretical simulation of the quantum re?ection from a grating for those systems. We con?rm the universal dependence on the incident de Broglie wavelength with the threshold angles where the emerging beam resonances are observed. However, the angular dependence of the re?ection ef?ciencies, that is, the ratio of scattered intensity into speci?c diffraction channels relative to the total intensity is found to be dependent on the details of the particle surface interaction.
关键词： clusters,diffraction,rare-gas atoms,quantum re?ection,grating
更新于2025-09-09 09:28:46
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Impact of pitch angle fluctuations on airborne lidar forward sensing along the flight direction
摘要： Airborne lidar forward sensing along the ?ight direction can serve for noti?cation of clear air turbulence (CAT) and help to prevent injuries or fatal air accidents. The validation of this concept was presented in the framework of the DELICAT (DEmonstration of LIdar-based CAT detec- tion) project. However, the strong variations in signal level, which were observed during the DELICAT measurements but not explained, sometimes indicated the need of a bet- ter understanding the observational errors due to geometrical factors. In this paper, we discuss possible error sources perti- nent to this technique, related to ?uctuations of the ?ight pa- rameters, which may lead to strong signal variations caused by the random deviations of the sensing beam from the for- ward ?ight trajectory. We analyze the variations in backscat- tered lidar signal caused by ?uctuations of the most impor- tant forward-sensing ?ight parameter, the pitch angle. The ?uctuation values considered in the paper correspond to the error limits of the compensational gyro platform used in civil aviation. The part of the pitch angle ?uctuations not com- pensated for by the beam-steering device in the presence of aerosol concentration variations can lead to noticeable signal variations that can be mistakenly attributed to wind shear, turbulence, or fast evolution of the aerosol layer. We formu- late the criteria that allow the recognition of signal variations caused by pitch angle ?uctuations. In?uence of these ?uc- tuations is shown to be stronger for aerosol variations on smaller vertical scales. An example of DELICAT observa- tions indicating a noticeable pitch angle ?uctuation impact is presented.
关键词： clear air turbulence,signal variations,pitch angle fluctuations,aerosol clusters,airborne lidar
更新于2025-09-09 09:28:46
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Nanometer-Sized Crystalline Clusters of IGZO Films Determined from the Grazing Incidence X-ray Scattering and Anomalous X-ray Scattering Data Combined with Reverse Monte Carlo Simulations
摘要： Grazing incidence X-ray scattering measurements have been carried out on c-axis aligned crystalline-indium gallium zinc oxide (CAAC-IGZO) ?lm and nanocrystalline category-indium gallium zinc oxide (NC-IGZO) ?lm and the following results were obtained: (1) the characteristic layered structure of the IGZO crystal did not hold its shape and the X-ray scattering pro?le showed only a relatively sharp ?rst peak at the wave vector (Q) = 21.8 for CAAC ?lm and 23.1 nm11 for NC ?lm, respectively, and additional weak broad peaks were observed at a higher angle. (2) In the case of the CAAC ?lm, tiny peaks were observed at Q = 7 and 14 nm11, corresponding to the positions of the 003 and 006 re?ections, respectively, of the IGZO crystal. Such tiny peaks were not detected in the case of NC ?lm but the asymmetry of the ?rst peak at the low angle side was clearly observed. (3) These structural features implied that more than three polyhedral units, such as InOx (x = 4-6), GaOy (y = 4-6), and ZnOz (z = 4-6), were likely to coexist. It is appropriate to call this structural feature as cluster-1. (4) A composite-type structure formed by combining these polyhedral units is also likely to exist and leads to middle-range ordering. This structure is called cluster-2. The size of such cluster-2 has been estimated to be 2.2 nm for CAAC ?lm and 1.8 nm for NC ?lm using the measured pair distribution function. To gain insights into the structural features of IGZO ?lms, realistic atomic-scale models were obtained to ?t not only the ordinary interference function of grazing incidence X-ray scattering but also the environmental interference function of the anomalous X-ray scattering (AXS) with Zn-absorption edge using reverse Monte Carlo (RMC) simulation. (5) The resultant models indicated the complex and irregular atomic arrangements of two types of IGZO ?lms, which are well characterized by nanometer-sized crystalline clusters. This characteristic feature may be referred to as crystalline-cluster-composite (triple C) structure.
关键词： anomalous X-ray scattering,nanometer-sized crystalline clusters,IGZO ?lm,grazing incidence X-ray scattering,reverse Monte Carlo (RMC) simulation
更新于2025-09-09 09:28:46
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Extreme Ultraviolet to Visible Dispersed Single Photon Detection for Highly Sensitive Sensing of Fundamental Processes in Diverse Samples
摘要： The detection of a single photon is the most sensitive method for sensing of photon emission. A common technique for single photon detection uses microchannel plate arrays combined with photocathodes and position sensitive anodes. Here, we report on the combination of such detectors with grating diffraction spectrometers, constituting a low-noise wavelength resolving photon spectroscopy apparatus with versatile applicability. We recapitulate the operation principle of such detectors and present the details of the experimental set-up, which we use to investigate fundamental mechanisms in atomic and molecular systems after excitation with tuneable synchrotron radiation. Extensions for time and polarization resolved measurements are described and examples of recent applications in current research are given.
关键词： fluorescence,synchrotron radiation,single photon detection,photon spectroscopy,clusters,atomic and molecular physics
更新于2025-09-09 09:28:46
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Deriving physical parameters of unresolved star clusters
摘要： Context. This study is the third of a series that investigates the degeneracy and stochasticity problems present in the determination of physical parameters such as age, mass, extinction, and metallicity of partially resolved or unresolved star cluster populations situated in external galaxies when using broad-band photometry. Aims. This work tests the derivation of parameters of artificial star clusters using models with fixed and free metallicity for the WFC3+ACS photometric system. Then the method is applied to derive parameters of a sample of 203 star clusters in the Andromeda galaxy observed with the HST. Methods. Following Papers I and II, the star cluster parameters are derived using a large grid of stochastic models that are compared to the observed cluster broad-band integrated WFC3+ACS magnitudes. Results. We derive the age, mass, and extinction of the sample of M 31 star clusters with one fixed metallicity in agreement with previous studies. Using artificial tests we demonstrate the ability of the WFC3+ACS photometric system to derive the metallicity of star clusters. We show that the metallicity derived using photometry of 36 massive M 31 star clusters is in a good agreement with the metallicity previously derived using spectroscopy taken from literature.
关键词： galaxies: star clusters: general,galaxies: individual: M 31
更新于2025-09-09 09:28:46
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Influence of the Quantum Well Structure and Growth Temperature on a Five-Layer InGaMnAs Quantum Well with an InGaAs Buffer Layer
摘要： The in?uence of quantum well structure and growth temperature on a synthesized multilayer system composed of a ?ve-layer InMnGaAs quantum well with an InGaAs buffer layer grown on semi-insulating (100)-oriented substrates prepared by low temperature molecular beam epitaxy was studied. The magnetization measurements using a superconducting quantum interference device indicated the existence of ferromagnetism with a Curie temperature above room temperature in the ?ve-layer InGaMnAs quantum well structure with an InGaAs buffer layer in a GaAs matrix. X-ray diffraction and secondary ion mass spectroscopy measurements con?rmed the second phase formation of ferromagnetic GaMn clusters. The ferromagnetism that exists in the ?ve-layer of the InMnGaAs quantum well with the InGaAs buffer layer results from a superposition of the ferromagnetism of the low temperature region from the substitutional Mn ions into Ga sites or interstitial Mn ions as well as the presence of manganese ions dopant clusters such as GaMn clusters.
关键词： Molecular Beam Epitaxy,Clusters,Ferromagnetism,Quantum Wells
更新于2025-09-09 09:28:46