Autoionization Resonances Research Articles

In-source spectroscopy on astatine and radium for resonant laser ionization

At on-line isotope separator facilities, rare isotopes of radioactive elements such as astatine, radium or polonium are demanded for fundamental research on nuclear structure. These elements are generally suitable for a resonance ionization laser ion source, but more data on the atomic structure is necessary to develop efficient laser ionization schemes. Due to the missing stable reference isotopes spectroscopic investigation of the atomic structure can only be performed during on-line operation. At the Isotope Separator and ACcelerator (ISAC) facility at TRIUMF, the elements astatine and radium were investigated by in-source laser spectroscopy to optimize the laser ionization efficiency. For astatine, laser spectroscopy was performed to search for high lying bound states as well as for auto-ionizing resonances. This led to the identification of four new high lying bound states of odd parity, while no auto-ionizing resonances were observed in the investigated region. Furthermore, the feasibility and the impact of laser ionization on the yield of radium isotopes was investigated using an activated target after proton irradiation.

The sensitivities of high-harmonic generation and strong-field ionization to coupled electronic and nuclear dynamics.

The sensitivities of high-harmonic generation (HHG) and strong-field ionization (SFI) to coupled electronic and nuclear dynamics are studied, using the nitric oxide (NO) molecule as an example. A coherent superposition of electronic and rotational states of NO is prepared by impulsive stimulated Raman scattering and probed by simultaneous detection of HHG and SFI yields. We observe a fourfold higher sensitivity of high-harmonic generation to electronic dynamics and attribute it to the presence of inelastic quantum paths connecting coherently related electronic states [Kraus et al., Phys. Rev. Lett.111, 243005 (2013)]. Whereas different harmonic orders display very different sensitivities to rotational or electronic dynamics, strong-field ionization is found to be most sensitive to electronic motion. We introduce a general theoretical formalism for high-harmonic generation from coupled nuclear-electronic wave packets. We show that the unequal sensitivities of different harmonic orders to electronic or rotational dynamics result from the angle dependence of the photorecombination matrix elements which encode several autoionizing and shape resonances in the photoionization continuum of NO. We further study the dependence of rotational and electronic coherences on the intensity of the excitation pulse and support the observations with calculations.

The effect of vibrational autoionization on the H2+ X 2Σg+ state rotationally resolved photoionization dynamics

The effect of vibrational autoionization on the H2+ X 2Σg+ v+ = 3, N+ state rotationally resolved photoelectron angular distributions and branching ratios has been investigated with a velocity map imaging spectrometer and synchrotron radiation. In photon excitation regions free from the influence of autoionizing Rydberg states, where direct ionization dominates, the photoelectron anisotropy parameter associated with the X 1Σg+ v″ = 0, N″ = 1 → X 2Σg+ v+ = 3, N+ = 1 transition has a value close to the theoretical maximum. However, in the vicinity of a Rydberg state, vibrational autoionization leads to a substantial reduction in anisotropy. The value of the anisotropy parameter associated with the S-branch of the photoelectron spectrum is found to be considerably higher than that predicted under the assumption that the outgoing electron can be represented solely as a p-wave. This suggests that the f-wave contribution must be taken into account to obtain a proper description of the photoionization dynamics. The observed variations in the rotationally resolved branching ratios, in the vicinity of an autoionizing resonance, depend upon the rotational level of the Rydberg state. The rotationally averaged photoelectron anisotropy parameters have been compared with the corresponding, previously calculated, theoretical results and reasonable agreement has been found. The influence of vibrational autoionization on the H2+ X 2Σg+ v+ = 0, 1, 2, 3 vibrational branching ratios has also been investigated, and the experimental results show that, in energy regions encompassing Rydberg states, these ratios deviate strongly from the Franck–Condon factors for direct ionization.

Autoionization resonances in the partial photoionization cross sections of Al V

The photoionization cross sections for 2p or 2s sub-shell ionization of Al V are calculated using the R-matrix method in LS-coupling approximation. In the present work, the distorted-wave method is employed to calculate the orbital wave-functions, and the R-matrix method is employed to calculate the photoionization cross sections from ground state 2s22p5 2P and excited state 2s2p6 2S. The 12 series of autoionization resonances between the 2s22p4 3P and 1s22p6 1S threshold are identified.

Isotopically Resolved Photoelectron Imaging Unravels Complex Atomic Autoionization Dynamics by Two-Color Resonant Ionization

Angle-resolved electron spectroscopy in coincidence with high-resolution mass spectroscopy has been applied to study two-color resonant photoionization in atomic xenon. Separation of different isotopes enabled us to extract results for the electronic dynamics free from depolarization effects, which are generally introduced by the coupling of the electronic and nuclear angular momenta. The concerted experimental and theoretical analysis of the photoelectron angular distributions in the region of an autoionizing resonance emphasizes the strong sensitivity of the observed structures to the fine details of the treatment of the underlying dynamics.

State selective photo-recombination cross sections in Be-like C and Al ions

We report results from detailed state selective photo-recombination study along with the doubly excited autoionizing resonances in Be-like C2+ and Al9+ ions. In the present investigation, the primary focus is on detailed energy profiles of the individual photo-recombination cross sections. The calculation was carried out for the excited Rydberg states of type 1s22sns(1Se) which interact with the odd-parity continua up to the C3+ and Al10+ 2p threshold limit. The numerical evaluation has been performed at a fine energy mesh across all the autoionizing Rydberg series of resonances 1s22pns(1P0) converging to Li-like ion 2p threshold. The method of calculation keeps the essential ingredients of the Feshbach projection operator approximation. The photo-ionization cross sections have been evaluated with and without relativistic effects included into the R-matrix numerical procedures, while the allowance for both quantum interference between dielectronic and radiative recombination, and overlapping resonances has been done utilizing results from the earlier R-matrix Floquet calculation. We discuss all these results with respect to the effect of quantum interference term on the energy dependence profile of photo-recombination cross section for studied transitions.

Near and Above Ionization Electronic Excitations with Non-Hermitian Real-Time Time-Dependent Density Functional Theory.

We present a real-time time-dependent density functional theory (RT-TDDFT) prescription for capturing near and post-ionization excitations based on non-Hermitian von Neumann density matrix propagation with atom-centered basis sets, tuned range-separated DFT, and a phenomenological imaginary molecular orbital-based absorbing potential to mimic coupling to the continuum. The computed extreme ultraviolet absorption spectra for acetylene (C2H2), water (H2O), and Freon 12 (CF2Cl2) agree well with electron energy loss spectroscopy (EELS) data over the range of 0-50 eV. The absorbing potential removes spurious high-energy finite basis artifacts, yielding correct bound-to-bound transitions, metastable (autoionizing) resonance states, and consistent overall absorption shapes.

Photoelectron imaging of XUV photoionization of CO2 by 13–40 eV synchrotron radiation

Valence band photoionization of CO2 has been studied by photoelectron spectroscopy using a velocity map imaging spectrometer and synchrotron radiation. The measured data allow retrieving electronic and vibrational branching ratios, vibrationally resolved asymmetry parameters, and the total electron yield which includes multiple strong resonances. Additionally, the spectrum of low kinetic energy electrons has been studied in the resonant region, and the evolution with photon energy of one of the forbidden transitions present in the slow photoelectrons spectrum has been carefully analyzed, indicating that in the presence of auto-ionizing resonances the vibrational populations of the ion are significantly redistributed.

Line-profile analysis of excitation spectroscopy in the even 4p5(2P1/2)nl′ [K′] J (l′ = 1,3) autoionizing resonances of Kr

The even-parity autoionizing resonance series 4p5 np′ [3/2]1,2, [1/2]1, and 4p5 nf′ [5/2]3 of krytpon have been investigated by laser excitation from the two metastable states 4p55s [3/2]2 and 4p55s′ [1/2]0 in the photon energy region of 29000–40000 cm−1 at experimental bandwidth of ∼0.1 cm−1. The excitation spectra of the even-parity autoionizing resonance series, most of which are experimentally studied for the first time in this work, show typical asymmetric line shapes. Complementary information on level energies, quantum defects, line profile indices and resonance widths, resonance lifetimes and reduced widths of the autoionizing resonances are derived by Fano-type line-shape analyses of the experimental results. Results from this work indicate that the line profile index (q) and the resonance width (Γ) are approximately proportional to the effective principal quantum number (n*); the line separation of the 4p5 np′ autoionizing resonances is also in good agreement with theoretical model.

Line-profile Analysis of Excitation Spectroscopy in Even 5p5(2P1/2)nl′ [K′]J (l′=1, 3) Autoionizing Resonances of Xe

The even-parity autoionizing resonance series 5p5np′ [3/2]1, [1/2]1, and 5p5nf′5/2]3 of xenon have been investigated, excited from the two metastable states 5p56s [3/2]2 and 5p56s′ [1/2]0 in the photon energy range of 28000–42000 cm−1 with experimental bandwidth of ̃0.1 cm−1. The excitation spectra of the even-parity autoionizing resonance series show typical asymmetric line shapes. New level energies, quantum defects, line profile indices and resonance widths, resonance lifetimes and reduced widths of the autoionizing resonances are derived by a Fano-type line-shape analysis. The line profile index and the resonance width are shown to be approximately proportional to the effective principal quantum number. The line separation of the 5p5np′ autoionizing resonances is discussed.

Giant autoionization resonance in Compton scattering of an x-ray photon by an open-shell atom

The existence of the giant autoionization resonance in a double differential cross section of the nonresonant Compton scattering of hard x-ray photons by an atom with an open shell in the ground state is theoretically predicted on the example of the Mn atom. The developed mathematical formalism is based on the multiconfiguration generalization of the nonrelativistic quantum theory of contact inelastic scattering of a photon by an atom, constructed in the work by Hopersky and Nadolinsky (2012 J. Exp. Theor. Phys. 115 402).

A critical re-assignment of the Rydberg states of iodomethane based on new polarization data

2- and 3-photon excitation of components of the lower Rydberg states of iodomethane (CH3I) using linearly and circularly polarized light, followed by ionization with one more photon, is used to determine their molecular term symbol, Ω, values as well as quantum defects. These Ω values, together with a detailed theoretical analysis, require a re-assignment of the 7s and 8s states to various components of the 5d and 6d states, but there is evidence of (n+2)s∕nd hybridization in the pairs of Ω = 1 states. Predissociation sets in for all Rydberg states beyond 6d based on the ground ((2)Π(3∕2)) state of the core, but sharp autoionizing resonances based on the (2)Π(1∕2) core state are assigned to the 9s, 7d, and 5f states. The dominant effect of the singlet∕triplet character of the Rydberg states on their accessibility from the ground state, seen in bromomethane and chloromethane, is again apparent and a concordant interpretation of the Rydberg spectra of CH3I can now be presented. Evidence for coupling of some Ω = 1 and Ω = 0(+) Rydberg states with a repulsive valence state and an ion-pair state, respectively, is also put forward.

In-source laser spectroscopy developments at TRILIS—towards spectroscopy on actinium and scandium

Resonance Ionization Laser Ion Sources (RILIS) have become a versatile tool for production and study of exotic nuclides at Isotope SeparatorOn-Line (ISOL) facilities such as ISAC at TRIUMF. The recent development and addition of a grating tuned spectroscopy laser to the TRIUMF RILIS solid state laser system allows for wide range spectral scans to investigate atomic structures on short lived isotopes, e.g., those from the element actinium, produced in uranium targets at ISAC. In addition, development of new and improved laser ionization schemes for rare isotope production at ISAC is ongoing. Here spectroscopic studies on bound states, Rydberg states and autoionizing (AI) resonances on scandium using the existing offline capabilities are reported. These results allowed to identify a suitable ionization scheme for scandium via excitation into an autoionizing state at 58,104 cm−1 which has subsequently been used for ionization of on-line produced exotic scandium isotopes.

Theoretical spectroscopy of autoionization resonances in spectra of lanthanide atoms

A theoretical study of the autoionization resonances in the spectra of lanthanide atoms (ytterbium) was carried out within the relativistic many-body perturbation theory and the generalized relativistic energy approach (the Gell-Mann and Low S-matrix formalism). The accurate results on the autoionization resonance energies and widths in ytterbium are presented with correctly accounting for the exchange correlation and relativistic corrections and are compared with the other available theoretical and experimental data.

Observation of even-parity autoionization states of uranium by three-colour photoionization optogalvanic spectroscopy in U–Ne hollow cathode discharges

Three-colour three-step photoionization spectroscopy of uranium has been performed in a U–Ne hollow cathode discharge tube by temporally resolving three-colour photoionization optogalvanic (PIOG) signal from the normal optogalvanic (OG) signal using three tunable pulsed dye lasers. U–Ne hollow cathode discharge tube has been used as a source of uranium atomic vapours and photoionization detector. Using this technique, photoionization spectra of uranium have been investigated systematically in the energy region 52,150–52,590cm−1, through three different excitation pathways, originating from its ground state, 0cm−1(5Lo6). By analysing the three-colour photoionization spectra sixty new even-parity autoionization resonances of uranium have been identified and their probable total angular momentum (J) values have been assigned according to the J-momentum selection rule. The J-value of five autoionization resonances, which have been observed either through all three excitation pathways or through two different excitation pathways where J-value of the second excited levels differs by two, has been assigned uniquely.

Autoionization widths of open-M-shell germanium ions: effects on inner-shell absorptions

Large-scale configuration interaction calculations were carried out to obtain the level-by-level autoionization (AI) rates and resonance widths of the inner-shell excited configurations for the open-M-shell germanium ions. Detailed results are presented for Ge5+ as an example. The dominant ionization channels of the autoionized levels 2p53/23s23p63d10 and 2p51/23s23p63d10 of Ge5+ were determined, and it is shown that the channel from level 3s23p6(3d−13/23d−15/2)4 contributes the largest fraction of AI widths of 0.195 and 0.188 eV to the total width of 0.577 and 0.544 eV, respectively, from all possible channels. For typical experimental plasmas at temperatures of several tens and hundreds of eV, the AI resonance broadening is the dominant mechanism among the Doppler, electron impact and natural lifetime broadenings. Among the physical effects such as relativistic and the interaction between fine-structure levels belonging to the same non-relativistic configuration and different configurations, particular attention is paid to the effect of AI resonance broadening on the L-shell absorption. Different features of the inner-shell absorption due to AI broadening are presented in detail for Ge5+, Ge10+ and Ge17+. The total AI widths of levels from the configuration of one 2p electron being excited to 3d orbital were systematically investigated for Ge6+–Ge21+. Except for Ge20+ and Ge21+, the AI widths of the open-M-shell germanium ions dominate over all other line broadening mechanisms.

Ab initiostudy of high-lying doubly excited states of helium in static electric fields: Complex-scaling generalized pseudospectral method in hyperspherical coordinates

We present a complex-scaling (CS) generalized pseudospectral (GPS) method in hyperspherical coordinates (HSC) for $\mathit{ab}$ initio and accurate treatment of the resonance energies and autoionization widths of two-electron atomic systems in the presence of a strong dc electric field. The GPS method allows nonuniform and optimal spatial discretization of the two-electron Hamiltonian in HSC with the use of only a modest number of grid points. The procedure is applied for the first precision calculation of the energies and autoionization widths for the high-lying ${}^{1}\phantom{\rule{-0.16em}{0ex}}{S}^{e}$, ${}^{1}\phantom{\rule{-0.16em}{0ex}}{P}^{o}$, ${}^{1}\phantom{\rule{-0.16em}{0ex}}{D}^{e}$, and ${}^{1}\phantom{\rule{-0.16em}{0ex}}{F}^{o}$ ($n=10$--20) doubly excited resonance states of He atoms. In addition, we present a theoretical prediction of the energies and widths of high-lying doubly excited resonance states of ${}^{1}\phantom{\rule{-0.16em}{0ex}}{P}^{o}$ ($n=$ 8--15) in external dc electric field strengths of 3.915--10.44 kV/cm. The calculated dc-field perturbed high-lying resonance energies are in good agreement with the latest experimental data.

Branching ratio between resonant and non-resonant ionization of xenon evaluated from photoelectron angular distributions

Photoelectron imaging of xenon gases ionized with a femtosecond 388 nm pulse reveals three ionization processes: resonant and non-resonant multi-photon ionization and autoionization. The relative yield of the first two was evaluated from the deconvolution of their angular distribution for the specific electron energy, which includes the electron production via the 3hν–5p5(2P°1/2)6s resonance. The non-resonant process gains in importance with increasing laser intensity and accounts for 50% of the ionization yield at 30 TW cm−2.

Study of even-parity autoionization resonances of atomic uranium by three-color optogalvanic spectroscopy

We report our investigation on the even-parity autoionization (AI) resonances of atomic uranium in the energy region 52,850–53,350  cm−1, using the three-color optogalvanic spectroscopy technique in a U–Ne hollow cathode discharge lamp with three pulsed dye lasers. To the best of our knowledge, this is the first report of observation of even-parity AI states in atomic uranium lying more than 2000  cm−1 above the ionization limit. We have used four different excitation schemes, starting from the lowest metastable state of uranium at 620  cm−1 (K505). We have identified 102 new even-parity AI resonances in atomic uranium and assigned probable total angular momentum (J) values to these resonances. By observing 25 out of these 102 AI resonances through more than one excitation pathway, the ambiguity in the assignment of their J values has been reduced considerably.

Autoionization resonances in the argon iso-electronic sequence

We report results of theoretical studies of 3s → np and 2s → np autoionization resonances, respectively, in the 3p and the 2p photoionization cross sections of a few members (Ar, K+, Ca2+, Fe8+, Zn12+, and Kr18+) of the argon isoelectronic sequence, within the framework of the relativistic random phase approximation and using the relativistic multichannel quantum defect theory. The present approach would enable analysis of complex atomic spectra especially of atomic and (or) ionic species that are found in stellar gases in a computationally economical way. The shape profiles of the autoionization resonances have been analyzed in the present work using appropriate Fano parameters. The quantum defects determined for some of the lower or intermediate excited states are found to be in good agreement with the experimental and (or) theoretical results.