Autoionizing Levels Research Articles

Resonance ionization spectroscopy of neodymium for determining a highly efficient two-step ionization scheme

Two-color two-step resonance ionization spectroscopy of neodymium (Nd) was performed to identify more than 120 even-parity autoionizing levels and their candidate J-values. The analysis of the observed autoionizing Rydberg series yielded a value of 44,560.11 (43)stat (2)sys cm−1 as a more accurate ionization potential of Nd. The saturation method was used to measure the transition cross-sections for some intense ionization transitions. From the measured cross-sections, the ionization efficiencies of some two-step ionization schemes were evaluated using the scheme cross-section formula to obtain several promising schemes.

Resonance ionization spectrum of autoionization states of lutetium atom

<sup>177</sup>Lu is an important medical isotope used in imaging-guided radiotherapy, and it can be produced by irradiating <sup>176</sup>Lu or <sup>176</sup>Yb with high abundance. With an increasing demand for medical isotopes, it is very essential to improve the supply capacity for <sup>177</sup>Lu. The multi-step multi-color photoionization method is an effective method to obtain isotopes, and the information of odd-parity autoionization levels is essential. Laser resonance ionization spectroscopy is one of a few spectroscopic experimental methods that can study autoionization levels. An experimental system is developed for the frontier spectroscopic research, and it consists of custom-made tunable lasers and a high-resolution time of flight mass spectrometer. The lifetime of the excited state 35274.5 cm<sup>–1</sup> is measured to be (31.6 ± 1.7) ns by the delayed photoionization method for the first time. A three-step three-color photoionization process is used to detect the autoionization levels, with a delay of 30 ns between <i>λ</i><sub>2</sub> – <i>λ</i><sub>1</sub> and <i>λ</i><sub>3</sub> – <i>λ</i><sub>2</sub> respectively, in order to avoid any unexpected transitions. Forty-seven odd-parity autoionization levels are obtained, of which 33 levels are discovered for the first time, and the <i>λ</i><sub>2</sub> and <i>λ</i><sub>1</sub> are blocked to exclude possible interference peaks, such as the <i>λ</i><sub>1</sub>+<i>λ</i><sub>3</sub>+<i>λ</i><sub>3</sub> transition. Several autoionization levels show asymmetrical peak shapes, and the Fano fitting is carried out for all the levels to determine the widths and relative transition strengths of the autoionizing transitions. This study provides critical data for the high-efficient photoionization of lutetium atoms in the visible range. The angular momenta of 21 odd-parity autoionization levels in an energy range of 50650–51650 cm<sup>–1</sup> are identified for the first time, which provides a reference for determining the forbidden state of electric dipole transitions from other excited states and ascertaining the electronic configuration.

Effect of the Breit interaction on the angular distribution of Auger electrons following electron-impact excitation of highly charged Be-like ions

The electron-impact excitation from the ground state to the autoionizing level $1s2{s}^{2}2{p}_{1/2}\phantom{\rule{0.16em}{0ex}}J=1$ and the subsequent nonradiative Auger decay $1s2{s}^{2}2{p}_{1/2}\phantom{\rule{0.16em}{0ex}}J=1\ensuremath{\rightarrow}1{s}^{2}2s\phantom{\rule{0.16em}{0ex}}J=1/2$ of Be-like ions have been studied by using the multiconfigurational Dirac-Fock method and the relativistic distorted-wave theory. Special attention has been paid to the effect of the Breit interaction on the angular distribution of the Auger electrons emitted. To do so, the partial cross sections, alignment parameters of the autoionizing level, and intrinsic anisotropy parameters of the Auger decay are evaluated for Be-like ${\mathrm{Mg}}^{8+}, {\mathrm{Fe}}^{22+}, {\mathrm{Mo}}^{38+}, {\mathrm{Nd}}^{56+}, {\mathrm{Au}}^{75+}$, and ${\mathrm{U}}^{88+}$ ions, from which the angular distribution of the Auger electrons is further obtained. It is found that the Breit interaction hardly contributes to the angular distribution for low-$Z$ ions such as ${\mathrm{Mg}}^{8+}$, especially at low impact energies, while for medium- and high-$Z$ ions the situation becomes fairly different. To be specific, the Breit interaction contributes to lowering the anisotropy of the angular distribution, which first becomes quickly more prominent with increasing atomic number up to certain high-$Z$ ions and then behaves very slowly less prominently for higher-$Z$ ions. Moreover, for high-$Z$ ions the effect of the Breit interaction on the angular distribution is found to be nearly independent of the atomic number $Z$. Owing to opposite effects of the Breit interaction on the alignment parameters and the intrinsic anisotropy parameters, such an ``abnormal behavior'' is speculated to be caused very likely by a ``competition'' between the opposite effects of the Breit interaction on both of them.

Terbium Medical Radioisotope Production: Laser Resonance Ionization Scheme Development.

Terbium (Tb) is a promising element for the theranostic approach in nuclear medicine. The new CERN-MEDICIS facility aims for production of its medical radioisotopes to support related R&D projects in biomedicine. The use of laser resonance ionization is essential to provide radioisotopic yields of highest quantity and quality, specifically regarding purity. This paper presents the results of preparation and characterization of a suitable two-step laser resonance ionization process for Tb. By resonance excitation via an auto-ionizing level, the high ionization efficiency of 53% was achieved. To simulate realistic production conditions for Tb radioisotopes, the influence of a surplus of Gd atoms, which is a typical target material for Tb generation, was considered, showing the necessity of radiochemical purification procedures before mass separation. Nevertheless, a 10-fold enhancement of the Tb ion beam using laser resonance ionization was observed even with Gd:Tb atomic ratio of 100:1.

Investigations of odd-parity autoionization levels of atomic samarium using three-color resonance ionization spectroscopy

Three-color three-step laser resonance ionization spectroscopy has been employed to investigate the autoionization spectra of atomic samarium in the energy region 50,330 - 51,160 cm−1. The measurements were carried out in an atomic beam coupled to a linear time of flight mass spectrometer. Fifty-five odd-parity autoionization levels of atomic samarium in the above-mentioned energy region have been observed using three excitation schemes. Among these, fifty autoionization levels are new. The total angular momenta of eighteen odd-parity autoionization levels have been assigned uniquely.

Odd-parity autoionizing levels of uranium observed by two-color two-step photoionization optogalvanic spectroscopy

Two-color two-step photoionization optogalvanic spectroscopy was performed using high-repetition-rate titanium sapphire lasers and a uranium hollow cathode lamp to find the two-step resonance ionization schemes of uranium. Many ionization transitions were observed by exciting uranium atoms in a ground state into five, even parity, excited levels with the first-step laser and by scanning the second-step laser wavelengths. By blocking the first-step laser, single-color, two-photon ionization transitions were also identified. From these results, we have found more than 50 odd-parity autoionizing levels of uranium in the energy, ranging from the ionization potential (49958.4 cm−1) to 51 150 cm−1. The determined energy levels are within ∼±1 cm−1 of previously reported values.

Odd parity autoionization levels of Lu I by three-step photoionization

Resonance ionization mass spectroscopy is employed in a time of flight mass spectrometer to explore atomic structure of lutetium in continuum using three- step photoionization. Nineteen new odd parity autoionization levels of neutral lutetium are identified in the range 50650 – 51650 cm−1 above ground level. The work provides three photon ionization pathways for neutral lutetium with all transitions in the visible range.

Excitation of the lowest autoionizing level of caesium atoms by positron impact

We have obtained differential and integral cross sections for the excitation of caesium atom to the lowest autoionizing level by positron impact using the distorted wave approximation. We have also done calculations for electron impact excitation of the same level using the same method and compared our present positron/electron impact results with the available theoretical positron/ electron impact results and experimental electron impact results. It is found that the present positron impact integral cross sections are higher than the cross sections of Pangantiwar and Srivastava at low and intermediate energies, but the results converge as the energy increases in the high energy range. The disagreement at near threshold energies is due to the different distortion potentials used in the two calculations. The charge on the projectile is also a determining factor in the shape of the scattering cross section curves.

Line Identification and Excitation of Autoionizing States in a Late-type, Low-mass Wolf–Rayet Star

Abstract Identifications of a large fraction of previously unidentified lines in the complex spectrum of the low mass, late-type LMC [WC11] star J060819.93−715737.4 have been made utilizing electronic databases. There are an exceptionally large number of C ii emission lines originating from autoionizing (AI) levels. Resonance fluorescence between the C ii ground state and excited AI levels is shown to be an important photoabsorption process that is competitive with dielectronic recombination in exciting AI emission lines in stellar winds, and has broad application to many types of emission-line stars. In addition, numerous C ii quartet multiplets appear in emission that are not excited directly by recombination or resonance fluorescence, signifying high wind densities in the emission region that enhance collisional transfer between doublet and quartet states.

Multi-channel Decay Partial Widths of <sup>1,3</sup>L° Autoionizing Levels of O<sup>6+</sup> Below N=5 Threshold

The motivation of the diagonalization method is to take into consideration the coupling between closed and opened channels in term of perturbation theory and to neglect the indirect coupling as well but also the autoionisation states through the opened channels. This procedure leads to a relatively simple mathematical problem consisting of solving a system of linear algebraic equations instead of a system of coupled differential equations or integro-differential equations. In this paper, we will focus on the Resonance positions, Partial and total widths for the autoionization into various decay channels of some ^1,3L° doubly excited states of helium-like oxygen ion O⁶⁺ converging on the N=5 hydrogenic thresholds are reported for L=1, 2, 3, 4, 5, 6 and 7. The calculation was made in the framework of the diagonalization method approximation in LS coupling scheme. The partial widths for multi-channel autoionizing levels to sublevels of O⁷⁺ are calculated by neglecting the direct coupling between the open channels. We were able to find results consistent with some in the literature. It is about the configuration P, D, F, G, H, I and K. We could see that the decays are relatively dependent on the ionization thresholds of the residual ion O⁷⁺. Some analyses are made on these resultast and only according to the level of decay focused in this paper then made a surplus in the framework of nuclear spectroscopy.

The effect of exchange and absorption potential in the distorted wave calculation of electron impact excitation of the autoionizing state of caesium

We have calculated the differential and integral cross sections for electron impact excitation of the lowest autoionizing level of caesium atoms in the energy range 13 – 500 eV using the distorted wave method. In this study we have used a complex distortion potential as the distortion potential both at the initial and the final channels. Our results are compared amongst themselves to establish the effect of exchange and imaginary absorption potentials. We also compared with other available theoretical and experimental results to determine the suitability of our present method. From the results it is seen that when an absorption potential is included in the distortion potential, the integral cross section moves closer to the experimental result in the intermediate energy range 19 – 40 eV.

Discovery of a Rare Late-type, Low-mass Wolf–Rayet Star in the LMC

Abstract We report the serendipitous discovery of an object, UVQS J060819.93−715737.4, with a spectrum dominated by extremely intense, narrow C ii emission lines. The spectrum is similar to those of the very rare, late-type [WC11] low-mass Wolf–Rayet stars. Despite the recognition of these stars as a distinct class decades ago, there remains barely a handful of Galactic members, all of which are also planetary-nebula central stars. Although no obvious surrounding nebulosity is present in J0608, [O ii], [N ii], and [S ii] emission suggest the presence of an inconspicuous, low-excitation nebula. There is low-amplitude incoherent photometric variability on timescales of days to years, as well as numerous prominent P Cygni profiles, implying mass loss. There are indications of a binary companion. The star is located on the outskirts of the LMC, and the observed radial velocity (∼+250 km s−1) and proper motion strongly suggest membership. If indeed an LMC member, this is the first extragalactic late [WC] star, and the first with an accurately determined luminosity, as the Galactic examples are too distant for precise parallax determinations. A high-quality, broad-coverage spectrum of the prototype of the late [WC] class, CPD −56° 8032, is also presented. We discuss different excitation mechanisms capable of producing the great strength of the C ii emission. Numerous autoionizing levels of C ii are definitely populated by processes other than dielectronic recombination. Despite the spectacular emission spectra, observational selection makes objects such as these difficult to discover. Members of the [WC11] class may in fact be considerably more common than the handful of previously known late [WC] stars.

Resonance Parameters, Autoionisation of <sup>2S+1</sup>L<sup>o</sup> Doubly Excited N<sup>5+</sup> Ion States Associated with n=3 and 4, N<sup>6+</sup> Threshold

The motivation of the diagonalization method is to take into consideration the coupling between closed and opened channels in term of perturbation theory and to neglect the indirect coupling as well but also the autoionisation states through the opened channels. This procedure leads to a relatively simple mathematical problem consisting of solving a system of linear algebraic equations instead of a system of coupled differential equations or integro-differential equations. Diagonalization method under LS coupling scheme for the states ^1,3P^o; ^1,3D^o; ^1,3F^o; ^1,3G^o; ^1,3H^o was performed. The partial widths for multi-channel autoionizing levels to sublevels of N⁶⁺ were carried out by neglecting the direct coupling between opened channels. The calculations of total and partial widths of the (nln'l') ^1,3L^o (L=1, 2, 3, 4 and 5) multiple-decay-channel system N⁵⁺ were performed. From L=1 to 2, the (4l4l') ^1,3Lo states located under n=3 and the (3lnl') ^1,3Lo</sup> states follow the same rules. All the (4l4l') ^1,3Go and (4l4l') ^1,3H^o states located under n=3 observe the rule 1.

Three-step resonance ionization of zirconium with Ti:Sapphire lasers

Three-step resonance ionization of atomic zirconium using Ti:Sapphire lasers is investigated for the first time. We have located eight new excited states between 41,160 and 41,824 cm−1 that could serve as the intermediate state for the second-step transition. Three-step ionization paths via two of the newly observed states have been studied and numerous high-lying and autoionizing levels are observed. Eight new Rydberg series of odd-parity are identified in the photoionization spectra. The convergence limits of these Rydberg series allow us to determine the first ionization potential of Zr to be 53,507.832(35)stat(20)sys cm−1 with an order of magnitude improvement in uncertainty over the previous measurements. In addition, our measurements for one of the selected three-step paths show that the transitions can be saturated with low to moderate laser powers.

Enhancement of photoionization by applying polarization-based common level excitation scheme for the selective photoionization of odd isotopes of samarium

Due to the importance of 149Sm isotope in the nuclear industry, isotope-selective photoionization of the odd isotopes of samarium has been studied using multi-color photoionization technique. The present work has been carried out in an atomic beam coupled to a linear time-of-flight mass spectrometer using multimode dye lasers. A new photoionization scheme: 292.58 cm−1 →17,769.71 cm−1→34,929.9 cm−1→ continuum with the J-value sequence 1-1-0-continuum has been identified for the enrichment of only odd isotopes using linearly polarized lasers in the same direction. This photoionization scheme in combination with the first-step transition: 0 cm−1 →17,769.71 cm−1 (J = 0→J = 1) has been utilized to realize a common level excitation scheme which has significantly increased the photoionization yield of the odd isotopes and suppressed the ionization of the even isotopes of samarium using suitable choice of laser polarization. To enhance the photoionization yield further, a search for the third-step transition terminating into an autoionization level has been carried out. A strong autoionization resonance at the third-step laser wavelength 644.14 nm, corresponding to the transition 34,929.9 cm−1→ 50,450.3 cm−1 has been identified. The J-value of the newly discovered autoionization level at 50,450.3 (±0.5) cm−1 has been uniquely assigned as J = 1. Finally, the photoexcitation/photoionization cross sections of all the transitions involved in the common level excitation scheme including that of the third-step transition terminating into the autoionization level have been measured by saturation method.

Ab initio dielectronic recombination rate coefficients for highly-charged Ar-like ions

Employing the independent-processes isolated-resonance distorted-wave approximation, we have made a comprehensive study on dielectronic recombination (DR) for 19 highly-charged Ar-like ions with nuclear charge number Z=23–30, 36, 42, 50, 54, 60, 64, 70, 74, 79, 85 and 92. For the convenience of usage, the ab initio level-by-level total rate coefficients for DR initially from both the ground state 3s23p6 1S0 and the metastable state 3s23p53d 3P0 are presented in a simple compact fitting form. We include the 3 → 3 (Δn=0), 2 → 3 and 3 → 4 (Δn=1), and 3 → 5 (Δn=2) core-excitations resonances as well as the decays from the resonances toward low-lying autoionizing levels followed possibly by radiative cascade effects in our ab initio DR results. The detailed configuration interaction analysis and comparisons with both theoretical and experimental results show that present total DR rate coefficients are reliable at both low (with a typical uncertainty of a factor of 2 in the photoionized plasma zone) and high (with an uncertainty of about 20% in the collisional plasma zone) temperature. To our knowledge, these are the first ab initio DR results for most of ions with Z > 30.

Investigation of Optical Ionization of Strontium via 5s21S0 ⟶ 5s5p 3P1° ⟶ 5s5d 3D2 ⟶ 4dnp (or 4ndf, n = 39) for Isotope Selectivity Enhancement

We investigated a promising three-step resonance ionization scheme of strontium (Sr) 5s21S0 ⟶ 5s5p 3P1° ⟶ 5s5d 3D2 ⟶ 4dnp (or 4dnf, n = 39) using the first-step intercombination transition for the enhancement of isotope selectivity. The power broadening observed in the 2nd transition indicates that laser power of less than 0.2 mW is sufficient for saturating this transition. Isotope separation can be successfully expected with application of the 3rd transition due to the narrow width of the 4dnp (or 4dnf, n = 39) autoionization level, paving the way for analysis of trace 90Sr in environmental applications.

Broadband laser-driven electromagnetically induced transparency in three-level systems with a double Fano continuum

We discuss the electromagnetically induced transparency (EIT) phenomenon for a model in which a structured continuum is described by a so-called double Fano structure; instead of one autoionizing (AI) state, two such states are embedded in a flat continuum. Such a Fano structure is the upper level of a Λ-like three-level system and is coupled to two lower ones by an external laser field involving δ-correlated, Gaussian, Markov process (white noise), simulating realistic conditions of the experiment. For such a system we derived and solved a set of coupled stochastic integrodifferential equations in the stationary regime, obtaining exact formulas determining the electric susceptibility of the system. Dispersion and absorption spectra of the medium susceptibility were calculated and compared with those already discussed in the literature. We have shown that the presence of an additional AI level considerably changes the structure of transparency windows and how noisy excitation influences the EIT processes.

Out-of-plane ( e,2e ) measurements and calculations on He autoionizing levels as a function of incident-electron energy

Out-of-scattering-plane $(e,2e)$ measurements and calculations are reported for the three singlet helium $2\ensuremath{\ell}2{\ensuremath{\ell}}^{\ensuremath{'}}$ autoionizing levels, with 80, 100, 120, 150, and 488 eV incident-electron energies, and scattering angles ${60}^{\ensuremath{\circ}}, 50.{8}^{\ensuremath{\circ}}, {45}^{\ensuremath{\circ}}, 39.{2}^{\ensuremath{\circ}}$, and $20.{5}^{\ensuremath{\circ}}$, respectively. The kinematics are the same in all cases: the momentum transfer is $K=2.1$ a.u., and ejected electrons are detected in a plane that contains the momentum-transfer direction and is perpendicular to the scattering plane. The results are presented as $(e,2e)$ angular distributions energy integrated over each level. They are compared with fully nonperturbative $B$-spline $R$-matrix and hybrid second-order distorted-wave + $R$-matrix calculations.

Even-parity autoionizing levels of atomic uranium and their total angular momenta

Employing three-colour three-step resonance ionization spectroscopy, photoionization spectra of atomic uranium have been investigated extensively in the energy region 49,959–51,504 cm−1. A detailed analysis of the photoionization spectra has resulted in the observation of 346 even-parity autoionizing levels including 199 new levels and a Rydberg series converging to the second-lowest ionic level ( $${}^{6}{\text{L}}_{11/2}^{\text{o}}$$ ) of uranium. Using the method of laser polarization combinations as well as the method of total angular momentum (J) selection rules, the J values of 72 of these autoionizing levels have been assigned unambiguously. Out of these 72 levels, the unique J values of 56 autoionizing levels have been determined for the first time, while it has been confirmed for the remaining 16 reported previously.