skip to main content

Title: Linear polarization of anisotropically excited x-ray lines from the n=2 complex in He-like Ar16+
High-resolution x-ray spectra were recorded at the National Institute of Standards and Technology electron beam ion trap (EBIT) using two Johann-type crystal spectrometers, with their dispersion planes oriented parallel and perpendicular to the beam direction. The linear polarizations of the 1s2−1s2l transitions in He-like argon ions were determined from the measured spectra at electron beam energies of 3.87 and 7.91 keV. The theoretical analysis was performed using detailed collisional-radiative modeling of the non-Maxwellian EBIT plasma with the NOMAD code modified to account for magnetic sublevel atomic kinetics. Effects influencing the polarizations of the observed 1s2−1s2l lines were investigated, including radiative cascades, the 1s2 1S0−1s2s 1S0 two-photon transition, and the charge exchange recombination of H-like argon ions. With these included, the measured polarizations of the resonance (1s2 1S0−1s2p 1P1), intercombination (1s2 1S0−1s2p 3P1), and forbidden lines (1s2 1S0−1s2s 3S1, 1s2 1S0−1s2p 3P2 ) were found to be in good agreement with the calculations.
Authors:
; ; ; ; ; ; ;
Award ID(s):
1806494
Publication Date:
NSF-PAR ID:
10180566
Journal Name:
Journal of physics
Volume:
53
Page Range or eLocation-ID:
115701
ISSN:
0953-4075
Sponsoring Org:
National Science Foundation
More Like this
  1. Radiative double-electron capture (RDEC), in which two-electron capture is accompanied by simultaneousemission of a single photon, was investigated for fully stripped and one-electron projectiles colliding withgaseous and thin-foil targets. RDEC can be considered the inverse of double photoionization by a single photon.For the gaseous targets, measurements were done for 2.11 MeV/uF9+and F8+ions interacting with N2and Ne,while for the thin-foil target the measurements were done for 2.11 MeV/uF9+and F8+and 2.19 MeV/uO8+andO7+ions striking thin C targets. Reports on this work were already published separately in shorter accounts by LaMantiaet al.[Phys. Rev. Lett.124, 133401 (2020)for the gas targets andPhys.Rev.A102, 060801(R) (2020)forthe thin-foil targets].more »The gas targets were studied under single-collision conditions, while the foil targets sufferedunavoidable multiple collisions. The measurements were carried out by detecting x-ray emission from the targetat 90◦to the beam direction in coincidence with outgoing ions undergoing double, single, and, in the caseof the foil targets, no charge change inside the target. Striking differences between the gaseous and foil targetswere found from these measurements, with RDEC for the gaseous targets occurring only in coincidence with q-2outgoing projectiles as expected, while RDEC for the foil targets was seen in each of the outgoing q-2, q-1, and nocharge-change states. The no charge-change result was totally unexpected. The cross sections for RDEC for thefully stripped ions on gas targets were found to be about six times larger than those for the one-electron projec-tiles. For the foil targets, the RDEC cross sections for the fully stripped and one-electron projectiles differ some-what from one another but not to the the extent they did for the gas targets. In this work the cross sections for allof the projectiles for the foil targets were adjusted due to the target contaminant background from potassium andcalcium atoms that existed in the spectra. Also, the cross sections for the incident one-electron projectiles weremodified due to a correction for the fraction of these ions that becomes fully stripped in passage through the foil.These differences are attributed to the effects of the multiple collisions that occur for the foil targets. The differ-ential cross sections at 90◦determined for each of the projectiles interacting with each of the targets are comparedwith each other and with the previous measurements. To the extent that the cross sections follow a sin2θdepen-dence, the total cross sections are compared with theoretical calculations [E. A. Mistonova and O. Yu. Andreev,Phys. Rev. A87, 034702 (2013)], for which the agreement is poor, with the measured cross section exceedingthe predicted ones by about an order of magnitude. Possible reasons for this discrepancy will be discussed.« less
  2. We present spectroscopic measurements and detailed theoretical analysis of inner-shell LMn and LNn (n  4) dielectronic resonances in highly charged M-shell ions of tungsten. The x-ray emission from W49+ through W64+ was recorded at the electron-beam ion trap (EBIT) facility at the National Institute of Standards and Technology with a high-purity Ge detector for electron-beam energies between 6.8 and 10.8 keV. The measured spectra clearly show the presence of strong resonance features as well as direct excitation spectral lines. The analysis of the recorded spectra with large-scale collisional-radiative modeling of the EBIT plasma allowed us to unambiguously identify numerousmore »dielectronic resonances associated with excitations of the inner-shell 2s1/2, 2p1/2, and 2p3/2 electrons.« less
  3. Non-thermal electron distributions, such as beams of electrons, are found in many laboratory and astrophysical plasma sources and can produce anisotropic and polarized emission. Theories used to model the emission require sublevel specific analysis, which can be difficult to verify experimentally. Using two polarization-sensitive Johann-type crystal spectrometers at the National Institute of Standards and Technology (NIST) electron beam ion trap facility, we measured the linear polarization of well-known dielectronic recombination satellite transitions from Li-like Ar ions and two blended features from Be-like ions. The spectrometers observed the plasma at 90◦ relative to the electron beam propagation direction, and the crystalmore »dispersion planes were oriented perpendicular relative to each other to allow for differing polarization sensitivities. Measurements were taken near the resonance energies of each line and compared with theoretical predictions based on relativistic magnetic sublevel atomic kinetics using the density-matrix theory. Most of the predictions are in excellent agreement with measured values.« less
  4. A combined experimental and theoretical study is presented on the collision-induced dissociation (CID) of 9-methylguanine–1-methylcytosine base-pair radical cation (abbreviated as [9MG·1MC]˙ + ) and its monohydrate ([9MG·1MC]˙ + ·H 2 O) with Xe and Ar gases. Product ion mass spectra were measured as a function of collision energy using guided-ion beam tandem mass spectrometry, from which cross sections and threshold energies for various dissociation pathways were determined. Electronic structure calculations were performed at the DFT, RI-MP2 and DLPNO-CCSD(T) levels of theory to identify product structures and map out reaction potential energy surfaces. [9MG·1MC]˙ + has two structures: a conventional structuremore »9MG˙ + ·1MC (population 87%) consisting of hydrogen-bonded 9-methylguanine radical cation and neutral 1-methylcytosine, and a proton-transferred structure [9MG − H]˙·[1MC + H] + (less stable, population 13%) formed by intra-base-pair proton transfer from the N1 of 9MG˙ + to the N3 of 1MC within 9MG˙ + ·1MC. The two structures have similar dissociation energies but can be distinguished in that 9MG˙ + ·1MC dissociates into 9MG˙ + and 1MC whereas [9MG – H]˙·[1MC + H] + dissociates into neutral [9MG – H]˙ radical and protonated [1MC + H] + . An intriguing finding is that, in both Xe- and Ar-induced CID of [9MG·1MC]˙ + , product ions were overwhelmingly dominated by [1MC + H] + , which is contrary to product distributions predicted using a statistical reaction model. Monohydration of [9MG·1MC]˙ + reversed the populations of the conventional structure (43%) vs. the proton-transferred structure (57%) and induced new reactions upon collisional activation, of which intra-base-pair hydrogen transfer produced [9MG + H] + and the reaction of the water ligand with a methyl group in [9MG·1MC]˙ + led to methanol elimination from [9MG·1MC]˙ + ·H 2 O.« less
  5. Abstract The Electron Loss and Fields Investigation with a Spatio-Temporal Ambiguity-Resolving option (ELFIN-STAR, or heretoforth simply: ELFIN) mission comprises two identical 3-Unit (3U) CubeSats on a polar (∼93 ∘ inclination), nearly circular, low-Earth (∼450 km altitude) orbit. Launched on September 15, 2018, ELFIN is expected to have a >2.5 year lifetime. Its primary science objective is to resolve the mechanism of storm-time relativistic electron precipitation, for which electromagnetic ion cyclotron (EMIC) waves are a prime candidate. From its ionospheric vantage point, ELFIN uses its unique pitch-angle-resolving capability to determine whether measured relativistic electron pitch-angle and energy spectra within the loss conemore »bear the characteristic signatures of scattering by EMIC waves or whether such scattering may be due to other processes. Pairing identical ELFIN satellites with slowly-variable along-track separation allows disambiguation of spatial and temporal evolution of the precipitation over minutes-to-tens-of-minutes timescales, faster than the orbit period of a single low-altitude satellite (T orbit ∼ 90 min). Each satellite carries an energetic particle detector for electrons (EPDE) that measures 50 keV to 5 MeV electrons with $\Delta $ Δ E/E < 40% and a fluxgate magnetometer (FGM) on a ∼72 cm boom that measures magnetic field waves (e.g., EMIC waves) in the range from DC to 5 Hz Nyquist (nominally) with <0.3 nT/sqrt(Hz) noise at 1 Hz. The spinning satellites (T spin $\,\sim $ ∼ 3 s) are equipped with magnetorquers (air coils) that permit spin-up or -down and reorientation maneuvers. Using those, the spin axis is placed normal to the orbit plane (nominally), allowing full pitch-angle resolution twice per spin. An energetic particle detector for ions (EPDI) measures 250 keV – 5 MeV ions, addressing secondary science. Funded initially by CalSpace and the University Nanosat Program, ELFIN was selected for flight with joint support from NSF and NASA between 2014 and 2018 and launched by the ELaNa XVIII program on a Delta II rocket (with IceSatII as the primary). Mission operations are currently funded by NASA. Working under experienced UCLA mentors, with advice from The Aerospace Corporation and NASA personnel, more than 250 undergraduates have matured the ELFIN implementation strategy; developed the instruments, satellite, and ground systems and operate the two satellites. ELFIN’s already high potential for cutting-edge science return is compounded by concurrent equatorial Heliophysics missions (THEMIS, Arase, Van Allen Probes, MMS) and ground stations. ELFIN’s integrated data analysis approach, rapid dissemination strategies via the SPace Environment Data Analysis System (SPEDAS), and data coordination with the Heliophysics/Geospace System Observatory (H/GSO) optimize science yield, enabling the widest community benefits. Several storm-time events have already been captured and are presented herein to demonstrate ELFIN’s data analysis methods and potential. These form the basis of on-going studies to resolve the primary mission science objective. Broad energy precipitation events, precipitation bands, and microbursts, clearly seen both at dawn and dusk, extend from tens of keV to >1 MeV. This broad energy range of precipitation indicates that multiple waves are providing scattering concurrently. Many observed events show significant backscattered fluxes, which in the past were hard to resolve by equatorial spacecraft or non-pitch-angle-resolving ionospheric missions. These observations suggest that the ionosphere plays a significant role in modifying magnetospheric electron fluxes and wave-particle interactions. Routine data captures starting in February 2020 and lasting for at least another year, approximately the remainder of the mission lifetime, are expected to provide a very rich dataset to address questions even beyond the primary mission science objective.« less