Measurements Of Cross Sections For Excitation Research Articles

Electron collisional excitation cross-section measurements and modeling for select Ni-like to Ge-like gold transitions

We have experimentally determined the electron collisional excitation cross-sections for several 3d→4f and 3d→5f excitations in Ni- to Ge-like Au at energies of ∼ 0.4, 1, 2, and 3 keV above threshold energy, ET, for the 3d→4f excitations ( ET ∼ 2.5 keV) and ∼ 0.2, 1, and 2 keV above threshold energy for the 3d→5f excitations ( ET ∼ 3.3 keV). The cross-section measurements are possible by using the GSFC micro-calorimeter to record emission spectra from beam plasmas created in the Livermore EBIT-I electron beam ion trap. The cross-sections are experimentally determined from the ratio of the measured intensities of the collisionally excited lines to the intensities of the radiative recombination lines in monoenergetic electron distribution EBIT-I plasmas. The effects of polarization and Auger processes in the beam plasmas are accounted for in the cross-section determination. Experimentally determined cross-sections are compared with those from HULLAC, DWS, and FAC calculations. The measurements exhibit significant differences with the calculations of these excitation cross-sections.

Isospin symmetry in B(E2) values: Coulomb excitation study of Mg21

The $T_z$~=~$-\frac{3}{2}$ nucleus ${}^{21}$Mg has been studied by Coulomb excitation on ${}^{196}$Pt and ${}^{110}$Pd targets. A 205.6(1)-keV $\gamma$-ray transition resulting from the Coulomb excitation of the $\frac{5}{2}^+$ ground state to the first excited $\frac{1}{2}^+$ state in ${}^{21}$Mg was observed for the first time. Coulomb excitation cross-section measurements with both targets and a measurement of the half-life of the $\frac{1}{2}^+$ state yield an adopted value of $B(E2;\frac{5}{2}^+\rightarrow\frac{1}{2}^+)$~=~13.3(4)~W.u. A new excited state at 1672(1)~keV with tentative $\frac{9}{2}^+$ assignment was also identified in ${}^{21}$Mg. This work demonstrates large difference of the $B(E2;\frac{5}{2}^+\rightarrow\frac{1}{2}^+)$ values between $T$~=~$\frac{3}{2}$, $A$~=~21 mirror nuclei. The difference is investigated in the shell-model framework employing both isospin conserving and breaking USD interactions and using modern \textsl{ab initio} nuclear structure calculations, which have recently become applicable in the $sd$ shell.

Comment on electron-impact excitation cross section measurements for He-like xenon

We discuss lower-than-predicted collisional–excitation cross sections for helium-like xenon measured at an Electron Beam Ion Trap facility. In a review paper (H. Chen and P. Beiersdorfer. Can. J. Phys. 86, 55 (2008)), the authors find a significant effect due to the Breit interaction between the free and the bound electrons in the excitation process of He-like xenon. The authors state that the agreement between the measured and calculated cross section values can only be found when the generalized Breit interaction is included in the calculations. We have performed new calculations with a multi-configuration Dirac–Fock code, as well as with the Penn State University suite of codes, and our conclusions are that the contribution of the Breit interaction is much lower than found in the calculations presented in the abovementioned article. In fact, our predictions are subsequently almost twice as large as the experimental values. We present these considerations in hopes of motivating new experimental investigations.

Comment on “Energy-Dependent Excitation Cross Section Measurements of the Diagnostic Lines of Fe XVII”

Comment on “Energy-Dependent Excitation Cross Section Measurements of the Diagnostic Lines of Fe XVII”

Electron-impact excitation cross-section measurements at EBITs from 1986 to 2006

This paper reviews the electron-impact excitation (EIE) measurements at electron beam ion trap (EBIT) facilities in the last 20~years. EIE cross sections are important atomic parameters fundamental to understanding the spectroscopic properties of ions. The properties of an EBIT make it an ideal device to measure the EIE cross section of highly charged ions. As a matter of fact, a report of EIE measurement was among the first papers published on the first EBIT ever built, EBIT-I. Since then, a wide range of measurements have been performed for K-shell and L-shell highly charged ions of Ti, V, Cr, Mn, Fe, Xe, and Ba using a combination of crystal spectrometers and solid-state X-ray detectors. In the last few years, the measurements were extended to all strong Fe L-shell lines by using a 6 × 6 pixel array microcalorimeter.PACS Nos.: 32.30.Jc, 32.30.Rj, 34.50.Fa, 32.70.Cs

Direct evidence for channel-coupling effects in molecules: electron impact excitation of the a″ 1Σ+g state of N2

New measurements of differential cross-sections for excitation of the a″ 1Σ+g (v′ = 0, 1) state in molecular nitrogen reveal a cusp-like angular distribution. This feature is distinctly observed for the first time in the present electron energy-loss experiment as a result of finer scattering angle grid and impact energy coverage than previous measurements. This feature is similar to that observed in atomic targets such as He, Hg and Ba. The observed phenomenon suggests an interference effect related to configuration–interaction coupling between lower and excited states that are of the same symmetry. It is hoped that the present work will stimulate theoretical investigations into the physics that governs this cusp-like behaviour.

Energy-Dependent Excitation Cross Section Measurements of the Diagnostic Lines of Fe XVII

By implementing a large-area, gain-stabilized microcalorimeter array on an electron beam ion trap, the electron-impact excitation cross sections for the dominant x-ray lines in the Fe XVII spectrum have been measured as a function of electron energy establishing a benchmark for atomic calculations. The results show that the calculations consistently predict the cross section of the resonance line to be significantly larger than measured. The lower cross section accounts for several problems found when modeling solar and astrophysical Fe XVII spectra.

An electronically tunable ultrafast laser source applied to fluorescence imaging and fluorescence lifetime imaging microscopy

Fluorescence imaging is used widely in microscopy and macroscopic imaging applications for fields ranging from biomedicine to materials science. A critical component for any fluorescence imaging system is the excitation source. Traditionally, wide-field systems use filtered thermal or arc-generated white light sources, while point scanning confocal microscope systems require spatially coherent (point-like) laser sources. Unfortunately, the limited range of visible wavelengths available from conventional laser sources constrains the design and usefulness of fluorescent probes in confocal microscopy. A ‘hands-off’ laser-like source, electronically tunable across the visible spectrum, would be invaluable for fluorescence imaging and provide new opportunities, e.g. automated excitation fingerprinting and in situ measurement of excitation cross-sections. Yet more information can be obtained using fluorescence lifetime imaging (FLIM), which requires that the light source be pulsed or rapidly modulated. We show how a white light continuum, generated by injecting femtosecond optical radiation into a micro-structured optical fibre, coupled with a simple prism-based tunable filter arrangement, can fulfil all these roles as a continuously electronically tunable (435–1150 nm) visible ultrafast light source in confocal, wide-field and FLIM systems.

CMOS Compatible Erbium Coupled Si Nanocrystal Thin Films for Microphotonics

ABSTRACTEr doped Si-rich SiO2 films were deposited through reactive RF magnetron co-sputtering and subjected to a single annealing step to simultaneously form silicon nanocrystals (Si-nc's) and activate the Er emission. Reference Er in stoichiometric SiO2 (Er:SiO2) films were deposited for comparison and the Er emission in the presence of Si-nc's was optimized with respect to the annealing temperature. The Er emission from Er in SiO2 containing Si-nc's (Er:SiO2+Si-nc) films is maximized at annealing temperatures between 600 °C and 800 °C, where the 1.54 μm emission is enhanced by more than two orders of magnitude relative to Er:SiO2 samples. Efficient energy coupling between Si-nc's and Er ions was demonstrated through excitation cross section measurements and non-resonant Er excitation experiments for samples annealed at temperatures as low as 600 °C. Since strong emission can be achieved from Er:SiO2+Si-nc films deposited through a standard CMOS process and annealed at temperatures below 700 °C, they can be used to fabricate CMOS compatible light

Use of a fast beam target for the determination and reduction of the cascade contribution to electron excitation cross-section measurements

This paper describes a method for reducing the influence of cascades on the measurement of electron excitation cross sections using the optical method and a fast beam atomic target. By using a fast beam of target atoms one can reduce the influence of cascades on a measurement, and estimate the cascade contribution to the excitation signal.

New experimental initiatives using very highly charged ions from an “electron beam ion trap”

A short review of the experimental program in highly-charged heavy ion physics conducted at the Lawrence Livermore National Laboratory Electron Beam Ion Trap (EBIT) facility is presented. The heavy-ion research, involving ions up to fully stripped U92+, includes precision x-ray spectroscopy and lifetime studies, electron impact ionization and excitation cross section measurements. The investigations of ion-surface interactions following the impact of high-Z highly charged ions on surfaces are aimed to study the neutralization dynamics effecting the ion and the response of the surface as well. The combination of an EBIT% with a cryogenic Penning Trap (“RETRSP”) allows to conduct experiments with ultra cold (e.g. 10 K) very highly charged ions. These studies of charge exchange processes, Coulomb crystals and measurements of hyperfine transitions using laser spectroscopy are in progress.

Resonant vibrational excitation of the c 3Πu state in HD and D2 by electron impact

Metastable excitation cross section measurements are presented for the first time for deuterium hydride and deuterium. Narrow resonance features observed in the 11-13 eV energy range are compared with the previously observed resonance features in molecular hydrogen. A time-of-flight technique is used to isolate and measure the metastable molecules from the other products of the collision and this has allowed the detection of the v=0,1,2,3,4 vibrational levels in the c 3Πu state. The positions of the resonance features are calibrated on an absolute scale and compared with earlier studies.

Electron-impact excitation cross-section measurements of highly charged heliumlike and lithiumlike ions.

The cross sections for electron-impact excitation of the heliumlike forbidden line [ital z], intercombination lines [ital y] and [ital x], and the resonance line [ital w], together with the associated lithiumlike collisional satellite lines [ital q], [ital r], [ital t], [ital u], and [ital v] have been measured for Ti, V, Cr, Mn, and Fe ([ital Z]=22--26) on the Livermore electron beam ion trap. The measurements were made near the threshold for excitation of the heliumlike transitions, where no other processes can contribute to the excitation of the lines, and are normalized to the cross sections for radiative electron capture. The uncertainties in the measurements are less than 8% for the heliumlike Fe transitions and between 11% and 15% for the lithiumlike Fe transitions together with a 3% systematic uncertainty from the normalization to theoretical radiative recombination cross sections. We find good agreement with results from a distorted-wave calculation.

Recent results obtained at the LLNL electron beam ion trap (EBIT)

A brief description of the LLNL Electron Beam Ion Trap (EBIT) facility and a summary of the experimental program in highly-charged heavy ion physics conducted at EBIT are presented. The heavy-ion research, involving ions up to fully stripped U+92, includes precision x-ray spectroscopy and lifetime studies, electron impact ionization, excitation cross section measurements, and investigation of ion-surface interactions and experiments with cold high-Z ions.

Excitation cross section measurements of doubly excited states produced in low energy collisions between multicharged ions and neutrals

5–60% N 5+ (1s2s 3S) metastable beam fraction has been produced. Single-electron capture from this state will give doubly excited 1s2s nl 2,4L states. The quartets are metastable with respect to Coulomb autoionization and transitions between these doubly excited quartet states have been observed by photon spectroscopy. 1s2p nl states are also observed (TE process). Absolute emission and excitation cross sections have been measured.

Beam-gas collision system for excitation cross-section measurements

A beam-gas collision experiment to measure excitation cross sections for translation-to-vibration (T-V) energy-transfer collision processes that produce short-wavelength infrared radiation (SWIR) has been assembled and used to obtain the (0,0°,0 → 0,0°,1) excitation cross section for CO2 by collision with Ar. A unique modulation technique used in this system involves pulse-modulating one of the reactants, injecting the pulses into a flowing plasma carrier, supersonically expanding the pulse and carrier, and extracting a beam from the expansion while preserving the modulation of the reactant. Enhanced radiation due to thermalization of excited CO2 at sphere pressures greater than single collision conditions, has been measured and a correction factor for this effect obtained. Measurements of the argon velocity and flux in a helium beam, CO2 number density in an integrating sphere, and infrared radiation at 4.25 μm were made to determine the excitation cross section at a collision velocity of 2.3 km s−1. The average of three sets of measurements yields a value of σ*=3.7(±3)×19−19 cm2.

Measurement of excitation cross sections of the silver ion by electron impact

Measurement of excitation cross sections of the silver ion by electron impact

Measurement of excitation cross section of the 6 ^3P_1 state of Hg using self-calibrating apparatus

The total cross section for excitation of ground state mercury atoms to the 63P1 state by 50 eV electrons was experimentally determined to be 0.434 ± 0.04 × 10−16 cm2. The apparatus used a pair of single-photon detectors to observe correlated optical photons in the 7 1S0 → 6 3P1 → 6 1S0 and 7 3S1 → 6 3P1 → 6 1S0 cascades. The product of the number of singles counts in the two detectors divided by the number of time-correlated cascade events was used along with angular distribution and correlation factors to determine N, the total number of atoms excited to the 6 3P1 state in the interaction region. The correct number N was arrived at in this way without making a separate determination of the overall photon detection efficiency. No standard lamp was needed for calibration nor was reference made to another standard cross section. The apparatus was self-calibrating. The absolute excitation cross section was determined from N and other parameters of the bombardment apparatus.

The radiation chemistry of nitrogen: primary neutral dissociative excitation yield and the isotopic exchange mechanism

Measurements of the yield of NO2 in N2–NO mixtures give a neutral dissociative excitation yield of [Formula: see text] in good agreement with those derived from excitation cross-section measurements and from product yield measurement in the radiolysis of N2–O2 mixtures.The dependence of exchange yields in the radiolysis of 28N2–30N2 mixtures on pressure, O2 concentration, and dose-rate has been measured. The results are consistent with a significant contribution to these yields from the isotopic exchange reaction of N+ with N2. They also indicate that dose-rates of the order of >1022 eV g−1 s−1 are required for the N2+ ions to contribute to the exchange yield.

Effective cross sections for excitation of atomic oxygen by electron impact

Effective cross sections for excitation of atomic oxygen by 600–3000 eV electrons were obtained for the OII 4351.3 Å spectral line corresponding to the 3p′2D5/20 →3s′2D5/2 transition and the OII 4414.9 Å and OII 4417.0 Å lines of the 3p2D0→3s2P multiplet. Oxygen atoms were produced in the absence of O2 molecules by the N+NO→N2+O titration reaction. To minimize O atom recombination and elastic scattering of the electron beam, the N2–O gas mixture was expanded into a vacuum chamber resulting in a free jet flow. This provides a unique technique for excitation cross section measurements of free radicals and reactive gases.