Fine Structure Doublet Research Articles

Examining quasar absorption-line analysis methods: the tension between simulations and observational assumptions key to modelling clouds

ABSTRACT A key assumption in quasar absorption-line studies of the circumgalactic medium (CGM) is that each absorption component maps to a spatially isolated ‘cloud’ structure that has single valued properties (e.g. density, temperature, metallicity). We aim to assess and quantify the degree of accuracy underlying this assumption. We used adaptive mesh refinement hydrodynamic cosmological simulations of two z = 1 dwarf galaxies and generated synthetic quasar absorption-line spectra of their CGM. For the Si ii λ1260 transition, and the C iv λλ1548, 1550 and O vi λλ1031, 1037 fine-structure doublets, we objectively determined which gas cells along a line of sight (LOS) contribute to detected absorption. We implemented a fast, efficient, and objective method to define individual absorption components in each absorption profile. For each absorption component, we quantified the spatial distribution of the absorbing gas. We studied a total of 1302 absorption systems containing a total of 7755 absorption components. 48 per cent of Si ii, 68 per cent of C iv, and 72 per cent of O vi absorption components arise from two or more spatially isolated ‘cloud’ structures along the LOS. Spatially isolated ‘cloud’ structures were most likely to have cloud–cloud LOS separations of 0.03Rvir (1.3 kpc), 0.11Rvir (4.8 kpc), and 0.13Rvir (5.6 kpc) for Si ii, C iv, and O vi, respectively. There can be very little overlap between multiphase gas structures giving rise to absorption components. If our results reflect the underlying reality of how absorption lines record CGM gas, they place tension on current observational analysis methods as they suggest that component-by-component absorption-line formation is more complex than is assumed and applied for chemical-ionization modelling.

Time-resolved 3D imaging of ultrafast spin–orbit wave packet dynamics

We combine bichromatic polarization pulse shaping with photoelectron imaging tomography for time-resolved spatial imaging of ultrafast spin–orbit wave packet (SOWP) dynamics in atoms. Polarization-shaped two-color pump-probe sequences are generated by spectral amplitude and phase modulation of a femtosecond input pulse and used to excite SOWPs in the potassium fine-structure doublet. By selecting different spectral bands for pump and probe pulse, we achieve interference-free detection of the spatiotemporal SOWP dynamics. Using tomographic techniques, we reconstruct the three-dimensional photoelectron momentum distribution (3D-ED) created by the probe pulse. Time-resolved measurement of the 3D-ED reveals the orbital realignment dynamics induced by spin–orbit interaction in the neutral atom.

Collinear laser spectroscopy at ion-trap accuracy: Transition frequencies and isotope shifts in the 6s2S1/2→6p2P1/2,3/2 transitions in Ba+

The rest-frame transition frequencies of the $6s{\phantom{\rule{0.16em}{0ex}}}^{2}{S}_{1/2}\ensuremath{\rightarrow}6p{\phantom{\rule{0.16em}{0ex}}}^{2}{P}_{1/2}$ (${D}_{1}$) and $6s{\phantom{\rule{0.16em}{0ex}}}^{2}{S}_{1/2}\ensuremath{\rightarrow}6p{\phantom{\rule{0.16em}{0ex}}}^{2}{P}_{3/2}$ (${D}_{2}$) lines in the stable isotopes of ${\mathrm{Ba}}^{+}$ were measured with an accuracy better than $200\phantom{\rule{4pt}{0ex}}\mathrm{kHz}$ through (quasi)simultaneous collinear and anticollinear laser spectroscopy, leading to an improvement in the accuracy of the isotope shifts by more than an order of magnitude compared to previous data. The ratio of the field-shift constants in the ${D}_{1}$, ${D}_{2}$ fine-structure doublet has been determined to $f=1.0186(9)$ through a King plot analysis. These collinear laser spectroscopy measurements have reached an accuracy comparable to ion-trap measurements on allowed dipole transitions. Our result provides an accurate benchmark for atomic many-body calculations and is compared to pure Dirac-Hartree-Fock calculations and to elaborate multiconfigurational Dirac-Fock calculations. The influence of correlation effects and the Breit interaction on the isotope shift in ${\mathrm{Ba}}^{+}$ are discussed.

Continuous Faraday measurement of spin precession without light shifts

We describe a dispersive Faraday optical probe of atomic spin which performs a weak measurement of spin projection of a quantum gas continuously for more than one second. To date focusing bright far-off-resonance probes onto quantum gases has proved invasive, due to strong scalar and vector light shifts exerting dipole and Stern-Gerlach forces. We show that tuning the probe near the magic-zero wavelength at 790 nm between the fine-structure doublet of $^{87}$Rb cancels the scalar light shift, and careful control of polarization eliminates the vector light shift. Faraday rotations due to each fine-structure line reinforce at this wavelength, enhancing the signal-to-noise ratio for a fixed rate of probe-induced decoherence. Using this minimally-invasive spin probe we perform microscale atomic magnetometry at high temporal resolution. Spectrogram analysis of the Larmor precession signal of a single spinor Bose-Einstein condensate measures a time-varying magnetic field strength with 1 {\mu}G accuracy every 5 ms; or equivalently makes > 200 successive measurements each at $10\,\mathrm{pT/\sqrt{Hz}}$ sensitivity.

Unexpectedly large difference of the electron density at the nucleus in the $$ 4p\, ^2{\mathrm {P}}_{{1}/{2},{3}/{2}}$$ 4 p 2 P 1 / 2 , 3 / 2 fine-structure doublet of Ca $$^+$$ +

We measured the isotope shift in the $^2$S$_{1/2}$-$^2$P$_{3/2}$ (D2) transition in singly-ionized calcium ions using photon recoil spectroscopy. The high accuracy of the technique enables us to resolve the difference between the isotope shifts of this transition to the previously measured isotopic shifts of the $^2$S$_{1/2}$-$^2$P$_{1/2}$ (D1) line. This so-called splitting isotope shift is extracted and exhibits a clear signature of field shift contributions. From the data we were able to extract the small difference of the field shift coefficient and mass shifts between the two transitions with high accuracy. This J-dependence is of relativistic origin and can be used to benchmark atomic structure calculations. As a first step, we use several ab initio atomic structure calculation methods to provide more accurate values for the field shift constants and their ratio. Remarkably, the high-accuracy value for the ratio of the field shift constants extracted from the experimental data is larger than all available theoretical predictions.

Precision Test of Many-Body QED in the Be+ 2p Fine Structure Doublet Using Short-Lived Isotopes.

Absolute transition frequencies of the 2s 2S{1/2}→2p2P{1/2,3/2} transitions in Be^{+} were measured for the isotopes ^{7,9-12}Be. The fine structure splitting of the 2p state and its isotope dependence are extracted and compared to results of ab initio calculations using explicitly correlated basis functions, including relativistic and quantum electrodynamics effects at the order of mα(6) and mα(7) ⁢ln α. Accuracy has been improved in both the theory and experiment by 2 orders of magnitude, and good agreement is observed. This represents one of the most accurate tests of quantum electrodynamics for many-electron systems, being insensitive to nuclear uncertainties.

Hyperfine Structure inTh229g3+as a Probe of theTh229g→Th229mNuclear Excitation Energy

We identify a potential means to extract the $^{229g}$Th $\rightarrow$ $^{229m}$Th nuclear excitation energy from precision microwave spectroscopy of the $5F_{5/2,7/2}$ hyperfine manifolds in the ion $^{229g}$Th$^{3+}$. The hyperfine interaction mixes this ground fine structure doublet with states of the nuclear isomer, introducing small but observable shifts to the hyperfine sub-levels. We demonstrate how accurate atomic structure calculations may be combined with measurement of the hyperfine intervals to quantify the effects of this mixing. Further knowledge of the magnetic dipole decay rate of the isomer, as recently reported, allows an indirect determination of the nuclear excitation energy.

The microwave and millimeter spectrum of ZnCCH ($\tilde {X}$X̃2Σ+): A new zinc-containing free radical

The pure rotational spectrum of the ZnCCH (X̃(2)Σ(+)) radical has been measured using Fourier transform microwave (FTMW) and millimeter direct-absorption methods in the frequency range of 7-260 GHz. This work is the first study of ZnCCH by any type of spectroscopic technique. In the FTMW system, the radical was synthesized in a mixture of zinc vapor and 0.05% acetylene in argon, using a discharge assisted laser ablation source. In the millimeter-wave spectrometer, the molecule was created from the reaction of zinc vapor, produced in a Broida-type oven, with pure acetylene in a dc discharge. Thirteen rotational transitions were recorded for the main species, (64)ZnCCH, and between 4 and 10 for the (66)ZnCCH, (68)ZnCCH, (64)ZnCCD, and (64)Zn(13)C(13)CH isotopologues. The fine structure doublets were observed in all the data, and in the FTMW spectra, hydrogen, deuterium, and carbon-13 hyperfine splittings were resolved. The data have been analyzed with a (2)Σ Hamiltonian, and rotational, spin-rotation, and H, D, and (13)C hyperfine parameters have been established for this radical. From the rotational constants, an r(m) ((1)) structure was determined with r(Zn-C) = 1.9083 Å, r(C-C) = 1.2313 Å, and r(C-H) = 1.0508 Å. The geometry suggests that ZnCCH is primarily a covalent species with the zinc atom singly bonded to the C≡C-H moiety. This result is consistent with the hyperfine parameters, which suggest that the unpaired electron is localized on the zinc nucleus. The spin-rotation constant indicates that an excited (2)Π state may exist ∼19,000 cm(-1) in energy above the ground state.

Effects of pulse shape on strongly driven two-level systems

We present an experimental study of the dynamics of a two-level system driven by strong nonresonant electromagnetic pulses as a function of pulse intensity and detuning. We have explored the qualitative and quantitative behavior of the transition probability as a function of pulse area for five different temporal profiles: Lorentzian, Lorentzian squared, hyperbolic secant, hyperbolic secant squared, and Gaussian. The two-level system consists of a fine-structure doublet in sodium Rydberg states coupled by Raman transitions driven through far-off-resonance intermediate states. The pulses are in the microwave regime and have high fidelity and uniform intensity. Experiments show that, despite the similarity in the pulse shapes, the behavior of the population transfer versus intensity depends dramatically on the temporal shape and that the spectral properties and area of the pulse do not adequately describe the response.

Polarization spectroscopy of positive and negative trions in an InAs quantum dot

Using polarization-sensitive photoluminescence and photoluminescence excitation spectroscopy, we study single InAs/GaAs self-assembled quantum dots. The dots were embedded in an n-type, Schottky diode structure allowing for control of the charge state. We present here the exciton, singly charged exciton (positive and negative trions), and the twice negatively charged exciton. For non-resonant excitation below the wetting layer, we observed a large degree of polarization memory from the radiative recombination of both the positive and negative trions. In excitation spectra, through the p-shell, we have found several sharp resonances in the emission from the s-shell recombination of the dot in all charged states. Some of these excitation resonances exhibit strong coulomb shifts upon addition of charges into the quantum dot. One particular resonance of the negatively charged trion was found to exhibit a fine structure doublet under circular polarization. This observation is explained in terms of resonant absorption into the triplet states of the negative trion.

Microwave spectroscopy of Al I atoms in Rydberg states: D and G terms

We present results of microwave and millimetre-wave resonance measurements in D, F and G Rydbergstates of neutral aluminium in the frequency range 4–423 GHz for principal quantum numbersn = 22–43. In all, 12, 27 and 19 resonances of or , or and or series, respectively, have been recorded. D-state fine-structure doublet splittings have been obtained withaccuracy ± 1.4 MHz. For D and G states, quantum defect Ritz-expansion parameters have been determined,from which the Rydberg series can be accurately reconstructed to an accuracy of order ± 1 MHz(3 × 10−5 cm−1) forall n.

Spatially Resolved Temperature Determination of an Air/Acetylene Flame Using the Two-Step Laser-Enhanced Ionization Technique

We have demonstrated a two-step laser-enhanced ionization (LEI) technique to be potentially useful in measuring a spatially resolved temperature in an atmospheric air/acetylene flame. As a complement to the existing optical methods, LEI detection shares the advantages of extremely high sensitivity and the capability of achieving a high spatial resolution, confined to the dual-beam intersection area of 1 mm × 1 mm. A theoretical model was developed to precisely extract the temperature information from the obtained two-step LEI signal. The physical parameters required in the experiment include simply the relative values of the LEI signals, associated with the fine-structure doublets of a selected thermometric species, and their corresponding laser energies. The subsequent treatment is also convenient without the need to know the absolute energy of the incident laser beam and to accurately calibrate the detection system. Al and Ga were selected as the thermometric species. The resultant radial and axial temperature mappings are consistent with each other and are also comparable with those reported by other optical methods.

Third harmonic generation and destructive interference with femtosecond laser pulses

With an unfocused femtosecond laser tuned to overlap three-photon resonance with a fine structure doublet, a third harmonic field is predicted which has a bandwidth much narrower than that of the laser. Consequently, this third harmonic field decays slowly with time, lasting as long as several nanoseconds. The population left in the excited states is suppressed by a destructive interference between different excitation pathways.

Quantum defects of the sodium atom in f, g and h states

Quantum-defect coefficients have been found to describe the energy levels of neutral Na for based on fitting all recent data for . Separate coefficients are found for each level of the fine-structure doublets. The results may be used to calculate all unperturbed Rydberg levels of Na, with an accuracy no worse than 0.2 MHz for n > 20. The quantum-defect predictions and an expansion in inverse odd powers of the effective quantum number match experimental d- and f-state doublet splittings to an even better accuracy.

Spin Effects in the (e, 2e) Cross Section of Xenon

In a series of experiments investigating the spin-dependent aspects of electron impact induced ionisation of atoms with a spin-resolved incident electron beam we have measured spin-resolved (e, 2e) cross sections for xenon. By experimentally resolving the fine structure levels of the ground state residual ion the existence of an effect analogous to the fine structure effect in excitation has been established, whereby strong and opposite polarisation effects are observed in the ionisation of a spinless closed shell target leading to a fine structure doublet.

Resonantly enhanced two-photon excitation of sodium atoms in a heat pipe

We report the results of a study of two-color laser excitation, ω1 + ω2, in dense sodium vapor where ω1 was tuned in the vicinity of the 3P fine structure doublet and ω2 was tuned to highernl states. Resonant excitation by ω2 of higherns andnd states from one member of the 3P doublet when ω1 is tuned near the other is attributed to collisional excitation transfer. Anomalous dipole forbiddenp-p transitions are observed when ω2 is tuned to highnP states (≥20) and are believed to be DC electric field induced. Sample spectra for the processes considered are calculated and compared with experiment.

Dipolar interaction between

A method is outlined to vary reproducibly the density of [111] ${\mathit{P}}_{\mathit{b}}$ centers (\ensuremath{\cdot}Si\ensuremath{\equiv}${\mathrm{Si}}_{3}$ defects with an unpaired ${\mathit{sp}}^{3}$ orbital perpendicular to the interface) at the thermal (111)Si/${\mathrm{SiO}}_{2}$ interface (grown at \ensuremath{\approxeq}920 \ifmmode^\circ\else\textdegree\fi{}C; 1.1 atm ${\mathrm{O}}_{2}$) using alternate non--Iin situR H passivation (hydrogenation in pure ${\mathrm{H}}_{2}$ at temperatures T=253 \ifmmode^\circ\else\textdegree\fi{}C--353 \ifmmode^\circ\else\textdegree\fi{}C) and degassing (high vacuum of p${10}^{\mathrm{\ensuremath{-}}6}$ Torr at T=752 \ifmmode^\circ\else\textdegree\fi{}C--835 \ifmmode^\circ\else\textdegree\fi{}C). These soft thermal treatments may be randomly sequenced and do not affect the interface structure. Only the spin state (electron spin resonance activity) of ${\mathit{P}}_{\mathit{b}}$ centers is modified by bonding or releasing H. The total number of \ensuremath{\cdot}Si\ensuremath{\equiv}${\mathrm{Si}}_{3}$ defects---either passivated or not---remains unaltered and seems to be set by the initial oxidation step. The maximum ${\mathit{P}}_{\mathit{b}}$ density is about 1.5% of the Si atom sites in a (111) plane, which appears as a natural constant for the (111)Si/${\mathrm{SiO}}_{2}$ interface thermally grown at \ensuremath{\approxeq}920 \ifmmode^\circ\else\textdegree\fi{}C. This [${\mathit{P}}_{\mathit{b}}$] monitoring is used as a tool to unveil the dipole-dipole (DD) influence on the K-band electron-spin-resonance spectrum of ${\mathit{P}}_{\mathit{b}}$ centers. The main effects are an overall broadening of the Zeeman resonance and the appearance of fine-structure doublets that grow with increasing concentration.Measurements at low ${\mathit{P}}_{\mathit{b}}$ concentrations reveal the residual dangling-bond resonance (void of dipolar interactions). Simulation of this signal using theoretical hyperfine parameters predicted from a relaxed ${\mathrm{Si}}_{22}$${\mathrm{H}}_{27}$ ${\mathit{P}}_{\mathit{b}}$ model cluster shows these to be somewhat underestimated. For higher ${\mathit{P}}_{\mathit{b}}$ concentrations, dipolar spectra were calculated, starting from a supposed array of possible ${\mathit{P}}_{\mathit{b}}$ sites in a (111)Si plane over which the ${\mathit{P}}_{\mathit{b}}$ centers are randomly distributed, by exact diagonalization of the spin Hamiltonian for a large number of possible configurations over the array. These are compared with experiments to derive the spatial distribution of the ${\mathit{P}}_{\mathit{b}}$ centers. Several site arrays were tested, leading to strong evidence that the ${\mathit{P}}_{\mathit{b}}$ centers are randomly distributed over IallR Si atom sites at the interfacial terraces. Models incorporating a strong clustering of ${\mathit{P}}_{\mathit{b}}$ centers or correlating them with interfacial steps are excluded. Also, the ditrigonal ring ${\mathit{P}}_{\mathit{b}}$ silica-cap model appears inappropriate. Fine structure due to first-, second-, third-, and fourth-neighbor interactions are not observed. This is ascribed to superexchange interaction between neighboring spins; the magnitude of exchange is shown to be compatible with previous observations on other Si dangling-bond-like defects in bulk Si. The superhyperfine structure (of splitting ${\mathit{A}}_{\mathrm{---}}^{\mathrm{shf}}$=14.8\ifmmode\pm\else\textpm\fi{}0.2 G) due to ${\mathit{P}}_{\mathit{b}}$-electron interactions with $^{29}\mathrm{Si}$ nuclei at second-nearest-neighbor positions in the Si substrate, has conclusively been distinguished from the DD fine structure. Previous investigations of the DD interactions are consistently explained resulting in a consistent picture of the two-dimensional ${\mathit{P}}_{\mathit{b}}$ system as regards dipolar and hyperfine interactions, saturation behavior, and moment calculations.

Raman spectra of GaAs/AlAs superlattices with fluctuation of period under off- and near-resonant conditions

Raman spectra of GaAs/AlAs superlattices (SL's), in which three types of building blocks, (GaAs) n (AlAs) n ( n = 8, 10, 12), are stacked randomly, have been measured under off- and near-resonant conditions. Under the off-resonant condition ( λ i = 5145 A ̊ ), an n = 1 doublet of the folded mode of an LA phonon branch is observed like that of a periodic (GaAs) 10(AlAs) 10 superlattice. This doublet has a fine structure and higher order doublets ( n ≥ 2) disappear. These Raman spectral features are reproduced satisfactorily by the bond polarizability model and the elastic continuum model for the folded LA modes, in which the penetration depth of light is taken into account. Under the near-resonant condition ( λ i = 7525 A ̊ ), the n = 1 folded modes are anomalously broadened. This spectral profile can be reproduced well by the model calculation assuming that the bond polarizability depends on the energy gap of each GaAs well under the near-resonant condition. This result shows that the breakdown of the wavevector-conservation rule for nonperiodic superlattices is enhanced under near-resonant conditions.

Reversible H2 passivation of [formula omitted] interface defects in (1 1 1)Si/SiO2

K-band electron spin resonance (ESR) at 4.3 K has revealed the dipole-dipole (DD) interaction effects between [1 1 1]Pb centers (∗Si ≡ Si3 defects with unpaired sp3 hybrid ‖ [1 1 1]) at the 2 dimensional (1 1 1)Si/SiO2 interface. This has been enabled by the perfectly reversible H2 passivation of Pb, which affects the defect's spin state. Sequential hydrogenation at 253–353°C and degassing treatments in high vacuum at 743–835°C allowed to vary the Pb density in the range 5 × 1010 < [Pb] ⩽ (1.14 ± 0.06) × 1013 cm-2. With increasing [Pb] fine structure doublets are clearly resolved. It is found that (1 1 1)Si/SiO2 interfaces, dry thermally grown at ≈920°C, naturally comprise a ∗Si ≡ Si3 defect density — passivated or not — of 1.14 × 1013 cm-2.

Observation of dipolar interactions between Pb0 defects at the (111) Si/SiO2 interface.

Dipole-dopole (DD) interactions between ${\mathit{P}}_{\mathit{b}0}$ (Si?${\mathrm{Si}}_{3}$) defects at the two-dimensional (2D) (111) Si/${\mathrm{SiO}}_{2}$ interface are revealed by electron-spin resonance. A DD fine-structure doublet develops with increasing [${\mathit{P}}_{\mathit{b}0}$] resulting in a line shape characteristic of DD interactions within a 2D dilute spin system, for which the observations are of fundamental interest. The residual line shape, devoid of DD effects, has been deduced. It is shown that a DD line broadening previously calculated for ${\mathit{P}}_{\mathit{b}0}$ is too small, thus questioning the assumed random-distribution model.