Autoionization Lines Research Articles

A Missing Piece of the E‐Region Puzzle: High‐Resolution Photoionization Cross Sections and Solar Irradiances in Models

AbstractMost ionospheric models cannot sufficiently reproduce the observed electron density profiles in the E‐region ionosphere, since they usually underestimate electron densities and do not match the profile shape. Mitigation of these issues is often addressed by increasing the solar soft X‐ray flux which is ineffective for resolving data‐model discrepancies. We show that low‐resolution cross sections and solar spectral irradiances fail to preserve structure within the data, which considerably impacts radiative processes in the E‐region, and are largely responsible for the discrepancies between observations and simulations. To resolve data‐model inconsistencies, we utilize new high‐resolution (0.001 nm) atomic oxygen (O) and molecular nitrogen (N2) cross sections and solar spectral irradiances, which contain autoionization and narrow rotational lines that allow solar photons to reach lower altitudes and increase the photoelectron flux. This work improves upon Meier et al. (2007, https://doi.org/10.1029/2006gl028484) by additionally incorporating high‐resolution N2 photoionization and photoabsorption cross sections in model calculations. Model results with the new inputs show increased O+ production rates of over 500%, larger than those of Meier et al. (2007, https://doi.org/10.1029/2006gl028484) and total ion production rates of over 125%, while production rates decrease by ∼15% in the E‐region in comparison to the results obtained using the cross section compilation from Conway (1988, https://apps.dtic.mil/sti/pdfs/ADA193866.pdf). Low‐resolution molecular oxygen (O2) cross sections from the Conway compilation are utilized for all input cases and indicate that is a dominant contributor to the total ion production rate in the E‐region. Specifically, the photoionization contributed from longer wavelengths is a main contributor at ∼120 km.

Spectra of Low Energy Electrons Emitted in the Interaction of Slow Ne+ Ions with Mg Surfaces

We measured spectra of low energy electrons emitted in the interaction of singly charged Ne+ ions with the Mg surface at incident ion energies ranging from 50 eV to 4 keV. The study examines issues related to the excitation of both the surface and the bulk plasmons of the target. We will also focus on the dynamics of the production of the singlet Ne2p4(1D)3s2 and triplet Ne2p4(3P)3s2 autoionizing states of projectiles scattered in a vacuum. The threshold behavior of the autoionization lines show that double excitation occurs simultaneously in a single scattering. The predominant excitation of the triplet state indicates the importance of charge rearrangement and the electron correlation effects during the collisional excitation.

The Behavior of BpSi Stars in the Far UV—Paper I: HD 22920

The analysis of five spectra of the Bp Si star HD 22920 recorded by the SWP camera on board the International Ultraviolet Explorer reveals variations of the far-ultraviolet flux of this star throughout its recently derived rotation period of 3.947 days. Spectra separated by half of the rotation period show variations of the FUV flux shortwards of 1700 Å. The Si ii autoionization lines in the region 1350 to 1460 Å clearly vary in phase with the Si ii doublet near 1530 Å. The FUV variations are caused by variations of the FUV Si ii continuous and line opacity as spots of overabundant silicon come into the line of sight while the star rotates.

The Silicon Lines in the Far Ultraviolet Spectrum of HD 172883

The comparison of the mean high resolution spectrum of the chemically peculiar star HD 172883 recorded by the SWP camera on board the International Ultraviolet Explorer with the mean spectrum of HD 38899, a normal late B-type star having similar fundamental parameters, reveals the large flux deficiency of HD 172883 shortwards of 1700 Å. This flux deficiency is due to the strong lines, photoionization edges and autoionization transitions of Si ii. The three strongest autoionization lines of Si ii are present The strong resonance line of Al ii conspicuous in HD 38899 is very ewak in HD 172883 highlighting a strong aluminum deficiency in HD 172883. Conversely, the Hg ii resonance line at 1649.94 Å is much stronger in HD 172883, confirming the large overabundance of mercury in this star.

The Behavior of HgMn Stars in the Far-UV—Paper 12: HD 172883

The analysis of high resolution spectra of the chemically peculiar star HD 172883 recorded by the SWP camera on board the International Ultraviolet Explorer reveals no significant variations of the far-ultraviolet flux of this star. The far-ultraviolet resonance and autoionization lines of Si ii, characteristic of the Bp Si-rich stars, are strong absorbers in the spectrum of HD 172883. The comparison of the mean SWP spectrum of HD 172883 with that of HD 38899, a normal late B-type star having similar fundamental parameters, reveals the presence of a strong resonance line of Hg ii line at 1942.27 Å for the first time. The TESS lightcurve of HD 172883 does not show a clear pattern of variability.

Single versus double 2p excitation in neon projectiles scattered from surfaces

We scattered 0.45--5 keV ${\mathrm{Ne}}^{+}$ ions at Al surfaces, performing angle resolved measurements of electron spectra from autoionizing decay of excitations in the scattered projectiles, produced by electron promotion in close atomic encounters with target atoms. The shift and the broadening of the autoionization lines due to the motion of the emitting atoms in vacuum show that the double $2p$ excitation in neon projectiles occurs simultaneously in a single scattering event and not in two consecutive collisions, as commonly assumed. Implications of our findings are commented upon for their relevance in research on charge exchange and energy deposition in ion-solid interactions, which are expected to be of interest for many fields.

The Large Far-ultraviolet Flux Deficiency of α Doradus (B8 IIIp Si)

Abstract The far-UV spectral energy distribution of α Dor recorded by IUE is compared to that of the constant and normal star ζ Dra, which has similar fundamental parameters. α Dor has much less flux than ζ Dra shortwards of 1800 Å, due to the accumulation of very strong absorption lines of Si ii, in particular autoionization lines around 1400 Å. Two spectra of α Dor separated by 9 hr (i.e., much less than the rotational period) show evidence of modest variability shortwards of 1800 Å.

Branching Ratios of the N(2D03/2) and N(2D05/2) Spin-Orbit States Produced in the State-Selected Photodissociation of N2 Determined Using Time-Sliced Velocity-Mapped-Imaging Photoionization Mass Spectrometry (TS-VMI-PI-MS).

Branching ratios for N(2D03/2) and N(2D05/2) produced by predissociation of state selected excited nitrogen molecules in the vacuum ultraviolet region have been measured for the first time. The quantum numbers of the excited nitrogen molecule are defined by selective excitation of the nitrogen molecule in the Franck-Condon region from the ground electronic, 1Σg+, vibrational, v″, and rotational, J″ state to an excited Eu', v', J' state with a tunable vacuum ultraviolet, VUV1, laser. The neutral atoms produced by predissociation from this excited state are then selectively ionized with a second tunable VUV2 laser. Measurement of the relative populations of these two atoms formed in their spin-orbit states defines the quantum states for the atomic products. This means that the wave functions of the initial state and knowledge of the relative yields define all the experimental parameters for this series of unimolecular reactions. The ions formed by VUV2 are mass analyzed with a time-of-flight mass spectrometer and detected with a time slice velocity ion imaging mass spectrometer. In this manner, we can determine the recoil velocity associated with the predissociation process. Two different techniques are used to determine the spin-orbit ratios, namely, resonant VUV photoionization (RVUV-PI) spectroscopy and total kinetic energy release (TKER) spectroscopy determined from the image produced when the atoms are selectively ionized by VUV2 in the interaction region. The TKER spectra obtained from the lines at 110 296.25 and 110 304.96 cm-1 that couple to a newly discovered autoionization line at 129 529.4255 ± 0.0015 cm-1 prove that the lines observed in this region originate from the N(2D03/2) and N(2D05/2) atoms. Two other lines in this region at 110 286.20 and 110 299.89 cm-1 originate from the nitrogen N(4S03/2) that is photoionized in a 1+ 1 VUV-UV resonant multiphoton ionization process. The spin-orbit branching ratios have been evaluated for valence and Rydberg electronic excited states from 104 129.4 to 118 772.1 cm-1, and it shows that they are independent of the rotational and vibrational quantum numbers. They are not appreciably affected by the symmetry properties of the wave function in the Franck-Condon region of the excited states. In the energy region below 117 153.8 cm-1 the pathways at long internuclear distances appear to determine [N(2D03/2)]/[N(2D05/2)] branching ratios of ∼0.38, ∼0.62, and ∼1.04. At higher energies, TKER and RVUV-PI spectroscopy have been used to show that the average fraction of the N(2D03/2) and N(2D05/2) atoms produced in the spin-allowed channels that produce two N(2D0J) is 0.85 versus 0.15 for spin-forbidden channels. The importance and need for this information for comparison with theory and applications in astrochemistry are briefly discussed.

The 5p autoionization spectra of Ba atoms excited by electron impact: identification of lines

The ejected-electron spectra corresponding to the radiationless decay of 5p5 states in Ba atoms have been measured precisely for different incident electron energies ranging from the appearance of the first autoionizing line at 15.68 eV up to 140 eV. The spectra have been obtained at an observation angle of 54.7° and with incident-electron and ejected-electron energy resolutions of 0.2 eV and 0.07 eV, respectively. In total, 63 lines with excitation thresholds below 22 eV have been observed between 9.8 and 16.6 eV ejected-electron kinetic energy. Based on measured excitation energies, intensity behavior of lines, calculated excitation energies, cross sections and decay rates of states in 5p56s, 5p55d and 5p55d6snl configurations, assignments for all 63 lines are proposed. The excitation and decay processes for classified autoionizing states have been examined and compared with existing data. The excitation threshold of the subshell has been established at 15.61 ± 0.05 eV.

Collision induced phenomena in the autoionizing spectrum of Ba and Sr

Broadening and shift of spectral lines in dense Ba and Sr vapor have been studied below and above their first ionization limits. The latter are well-known autoionizing spectral lines that exhibit many different spectral features. However, at larger pressures of noble gases some previously unobserved or very weak spectral phenomena become visible and their studies might be also linked to energy conversion purposes. Originally, we used nanosecond laser which was scanned in a certain spectral region where the two-photon excitation of autoionization lines occurred. This pulsed laser was overlapped by a mode-locked femtosecond laser and a very sensitive thermionic detection was used to observe autoionizing spectral lines. We also presented our preliminary results with hot strontium vapor excited to the resonance level at 460.7 nm.

Laser power dependence of the Beutler-Fano profiles for the autoionizing Rydberg states of Ba

The change of the Beutler-Fano profiles of the asymmetric autoionizing lines of barium as a function of the excitation laser power was investigated. Utilizing two-photon absorption, the J = 0 and 2 levels of a few members of the 5d3/2 nd3/2 autoionizing Rydberg series above the first ionization limit of barium were excited in a heat-pipe oven. As the laser power was changed, the transitions with relatively symmetric lineshapes showed the expected power broadening. However, the transitions with asymmetric lineshapes experienced a dramatic profile change while displaying a less significant power broadening. The remarkable profile changes are attributed to the interference between different ionization channels even at the highest excitation laser power.

Electron reemission processes following photoelectron recapture due to post-collision interaction in inner-shell photoionization of water molecules

Electron reemission following photoelectron recapture due to post-collision interaction has been studied at 0.7 eV the inner-shell photoionization threshold of water molecules, using a multi-electron coincidence method. Electron reemissions after single Auger decay occur from O and OH fragments which are produced by the dissociations of high-n Rydberg H2O(+) states populated through photoelectron recapture. In addition, electron reemissions after double Auger decay are identified in triple coincidence events, where autoionization lines from O and O(+) fragments are observed.

Spectroscopic studies of5d3/2nd1D0,2autoionization lines of barium under collision with rare gases

The spectroscopic behavior of 5${d}_{3/2}nd$ (${}^{1}\phantom{\rule{-0.16em}{0ex}}$${D}_{0}$ and ${}^{1}\phantom{\rule{-0.16em}{0ex}}$${D}_{2}$) autoionizing Rydberg series of barium was studied under collision with rare gases. The series members from $n$ $=$ 8 to $n$ $=$ 64 were observed using two-photon excitation of the two valence electrons in the 6${s}^{2\phantom{\rule{0.16em}{0ex}}1}\phantom{\rule{-0.16em}{0ex}}$${S}_{0}$ ground state of barium. The barium vapor was produced in a heat-pipe-like oven, and a tunable dye laser pumped by an excimer laser was used as the excitation source. The obtained spectral data have Beutler-Fano profiles. These spectral lines were investigated when inert gases Ar, Kr, and Xe at different pressures were introduced into the oven as perturbing gases. The collision-induced line shifts were measured and the shift parameters for the even-parity 5${d}_{3/2}nd$ ${}^{1}\phantom{\rule{-0.16em}{0ex}}$${D}_{0}$ and 5${d}_{3/2}nd$ ${}^{1}\phantom{\rule{-0.16em}{0ex}}$${D}_{2}$ ($n$ $=$ 8--35) autoionizing states were extracted from the data. The collision-induced change in the spectral line shape at different Xe pressure was also explored.

Time-slice velocity-map ion imaging studies of the photodissociation of NO in the vacuum ultraviolet region

The time-slice velocity-map ion imaging and the resonant four-wave mixing techniques are combined to study the photodissociation of NO in the vacuum ultraviolet (VUV) region around 13.5 eV above the ionization potential. The neutral atoms, i.e., N((2)D(o)), O((3)P(2)), O((3)P(1)), O((3)P(0)), and O((1)D(2)), are probed by exciting an autoionization line of O((1)D(2)) or N((2)D(o)), or an intermediate Rydberg state of O((3)P(0,1,2)). Old and new autoionization lines of O((1)D(2)) and N((2)D(o)) in this region have been measured and newer frequencies are given for them. The photodissociation channels producing N((2)D(o)) + O((3)P), N((2)D(o)) + O((1)D(2)), N((2)D(o)) + O((1)S(0)), and N((2)P(o)) + O((3)P) have all been identified. This is the first time that a single VUV photon has been used to study the photodissociation of NO in this energy region. Our measurements of the angular distributions show that the recoil anisotropy parameters (β) for all the dissociation channels except for the N((2)D(o)) + O((1)S(0)) channel are minus at each of the wavelengths used in the present study. Thus direct excitation of NO by a single VUV photon in this energy region leads to excitation of states with Σ or Δ symmetry (ΔΩ = ±1), explaining the observed perpendicular transition.

A cold pulsed source of hydrogen and hydrogen clusters: development and extreme-ultraviolet studies

We describe the development and application of a differentially pumped pulsed cold H2 and (H2)2 beam source. The nozzle was cooled with liquid nitrogen and, actuated by a piezoelectric valve with pressures up to 5 bar, produced gas pulses 250 μs long. Strong limitations by collisions with the fast hydrogen molecules returning from the walls of the source chamber could be overcome by surrounding the volume around nozzle and skimmer with cold surfaces. As a consequence, the beam intensities for both the monomer and the clusters could be greatly enhanced. The valve was operated without any realignment for more than one year. The beam source was characterized by a number of photo-ionization studies. As a part of these efforts the presumably first laser-based single-photon ionization spectra of H2 and (H2)2 were obtained. We estimate that more than ten $\mathrm{H}_{3}^{+}$ ions could be detected per laser shot at a wavelength of 74.2 nm and a pressure of 5 bar. The auto-ionization lines of H2 were recorded with a resolution of about 0.54 cm−1. A line width of about 629 MHz was obtained in the B←X band system of hydrogen observed by resonance-enhanced two-photon ionization. Further potential improvements of the experiment are discussed.

Calibration of electron spectrometer resolution in attosecond streak camera

We report a new method for determining the energy resolution of time-of-flight spectrometers for detecting photoelectrons produced with attosecond XUV pulses. By measuring the width of the 2s2p autoionization line of helium, we found the resolution of our spectrometer to be approximately 0.6 eV for electrons at 35.5 eV. Furthermore, the resolution in the 10 to 35 eV range was determined by applying a retarding potential at the entrance of the drift tube.

Allowed and forbidden transition parameters for Fe XXII

Radiative transitions for photo-excitations and de-excitations in Fe XXII are studied in the relativistic Breit–Pauli approximation. A comprehensive set of fine structure energy levels, oscillator strengths ( f), line strengths ( S), and radiative decay rates ( A) for electric dipole (E1), same spin multiplicity and intercombination, fine structure transitions is presented. These are obtained from the first calculations in the close coupling approximation using the Breit–Pauli R-matrix method for this ion, all existing theoretical results having been obtained from various other atomic structure calculations. The present work obtains a set of 771 fine structure energy levels with n ⩽ 10, l ⩽ 9, and 1/2 ⩽ J ⩽ 17/2, only 52 of which have been observed. The f, S, and A values are reported for 70,372 allowed E1 transitions, exceeding by far those published previously. The calculated fine structure levels have been identified spectroscopically using a procedure based on quantum defect analysis. The energies agree with the available observed energies to within less than one to a few percent. The A values for E1 transitions are in good agreement with other existing values for most transitions. Using the atomic structure code SUPERSTRUCTURE (SS), S and A values are also presented for 38,215 forbidden transitions of the types electric quadrupole (E2), electric octupole (E3), magnetic dipole (M1), and magnetic quadrupole (M2) among 274 fine structure levels formed from 25 configurations with orbitals ranging from 1 s to 4 f. Some of these levels lie above the ionization limit and hence can form autoionizing lines. Such lines for 1 s–2 p K α transitions have been observed in experiments. The energies from the SS calculations agree with observed energies within a few percent. The A values for E2 and M1 transitions agree very well with the available values. The atomic parameters for both allowed and forbidden transitions should be applicable for diagnostics as well as complete spectral modeling in the X-ray, ultraviolet, and optical regimes of astrophysical and laboratory plasmas.

Autoionization of N q+ ( q = 1–3) Rydberg states produced in high-energy collisions with He

Energy spectra of electrons ejected through autoionization decay of high-Rydberg states in high-energy collisions of N q + ( q = 1–3) with He have been measured with high-resolution by using zero-degree electron spectroscopy. Several series of autoionizing lines were observed, corresponding to decays from N 3+ 1s 22p( 2P) nl Rydberg states produced in N 3+ + He collisions, from N 2+ 1s 22s2p( 3P) nl Rydberg states produced in N 2+ + He and from N + 1s 22s2p 2( 4P) nl Rydberg states produced in N + + He, respectively. Angular momentum distributions for the first or second peak of three series of Coster–Kronig electron transitions for N q + ( q = 1–3) projectiles are also discussed, where the highly excited states are formed by electron excitation.

Autoionization of the Ne+ Rydberg states formed via valence photoemission

Autoionization decay from Ne+ Rydberg states has been investigated by using multi-electron spectroscopy at a photon energy of 200.4 eV, where the autoionizing states have been produced through valence photoemission with simultaneous excitation into Rydberg orbitals. The coincidence measurements enable one to resolve individual autoionization lines corresponding to decays from the Ne+ 2p−2(1D)nl, 2p−2(1S)nl, 2s−12p−1(1,3P)nl and 2s−2(1S)nl Rydberg states. The present investigation revealed the autoionization decay features of the Rydberg states: branching ratios, which show strong selectivity towards the Ne2+ final states and asymmetric line shapes in the electron spectra, which have been analysed in terms of the interaction between the discrete state and each continuum. In addition, it is demonstrated that the autoionization decay feature of the Ne+ 2s−12p−1(1P)3p 2S state is characterized by a strong interaction with 2s−12p−1(3P)nl autoionizing states.

Emission Characteristics of the Dielectronic Recombination Line of Atomic Cu and Selectively Excited Ionic Cu Line by Resonance Charge-Transfer Collision in a Low-Pressure Laser-Induced Plasma

Emisson spectra and time-resolved two-dimensional (2D) emission images of the electron-ion dielectronic recombination (i.e. a reversal process of auto-ionization) line of neutral Cu atoms, the selectively excited Cu ionic line, and normal Cu atomic line were observed for understanding the excitation mechanisms of Cu neutral and ionic lines in a low-pressure laser-induced plasma (LP-LIP) of Ar. From the observations, the number of charged particles around the emitting species seems to increase with increasing Ar pressure. Different time-resolved 2D emission images were observed among the selectively excited Cu ionic line and other Cu emission lines resulting from the different excitation mechanisms of the respective emission lines. Collisions of the second kind and electron-ion recombinations were found to be one of the major excitation mechanisms of Cu in Ar LP-LIP.