I. Dumitriu

Identification of absolute geometries of cis and trans molecular isomers by Coulomb Explosion Imaging

An experimental route to identify and separate geometric isomers by means of coincident Coulomb explosion imaging is presented, allowing isomer-resolved photoionization studies on isomerically mixed samples. We demonstrate the technique on cis/trans 1,2-dibromoethene (C2H2Br2). The momentum correlation between the bromine ions in a three-body fragmentation process induced by bromine 3d inner-shell photoionization is used to identify the cis and trans structures of the isomers. The experimentally determined momentum correlations and the isomer-resolved fragment-ion kinetic energies are matched closely by a classical Coulomb explosion model.

Measurements of molecular-frame Auger electron angular distributions at the CO C 1s−1 2π* resonance with high energy resolution

The molecular-frame Auger electron angular distributions (MFAEADs) of resonantly excited CO 1s → π molecules in the gas phase were determined with high energy resolution using a novel experimental approach. We investigated the process of excitation, Auger decay and fragmentation in unprecedented detail. We confirmed theoretical predictions for the different MFAEADs of close lying Auger final states. By examining the dependence of the MFAEADs on the vibrational state of the excitation and on the fragmentation energy we found that the measured MFAEADs can be considered independent of the vibrational excitation and the fragmentation process. The method used to obtain molecular-frame angular distributions of Auger electrons is based on electron–ion coincidence measurements using two high-resolution electron spectrometers with limited acceptance angles mounted at fixed positions.

Inner-shell photodetachment from Se− negative ions at the ALS

The photodetachment spectrum of Se− from 50 – 62 eV has been investigated using the merged ion-photon beam photodetachment technique. Se+ and Se++ ions formed by double and triple detachment were detected as a function of photon energy. Correlations in short-lived negative ion resonances formed by completely filling the valence 4p shell in Se- by photoexcitation of 3d electrons lead to three resonance structures. The resonances are observed in both the Se+ and Se++ decay channels bound by several eV below the 3d detachment threshold near 54 eV.

Inner-shell photodetachment from Ru{sup -}

Inner-shell photodetachment from Ru{sup -} was studied near and above the 4p excitation region, 29-to-91-eV photon energy range, using a merged ion-photon-beam technique. The absolute photodetachment cross sections of Ru{sup -} ([Kr]4d{sup 7}5s{sup 2}) leading to Ru{sup +}, Ru{sup 2+}, and Ru{sup 3+} ion production were measured. In the near-threshold region, a Wigner s-wave law, including estimated postcollision interaction effects, locates the 4p{sub 3/2} detachment threshold between 40.10 and 40.27 eV. Additionally, the Ru{sup 2+} product spectrum provides evidence for simultaneous two-electron photodetachment (likely to the Ru{sup +} 4p{sup 5}4d{sup 6}5s{sup 2} state) located near 49 eV. Resonance effects are observed due to interference between transitions of the 4p electrons to the quasibound 4p{sup 5}4d{sup 8}5s{sup 2} states and the 4d{yields}{epsilon}f continuum. Despite the large number of possible terms resulting from the Ru{sup -} 4d open shell, the cross section obtained from a 51-state LS-coupled R-matrix calculation agrees qualitatively well with the experimental data.

Promoting a core electron to fill a d-shell in negative ions: Shape versus Feshbach resonances and a novel threshold law

Two new results emerging from inner-shell photodetachment of transition metal negative ions are obtained, following studies in Pt−. First, the d-wave form of the Wigner threshold law is observed for the first time in single-photon measurements. Second, single-vacancy valence shells are filled with a core electron. While this should result in stabilization of the core excited state, producing Feshbach resonances, we find that stabilization does not occur for some core excitations, dramatically demonstrating the importance of core-valence interactions.

Shape resonances in inner-shell photodetachment of transition metal negative ions

Inner-shell photodetachment of transition metal negative ions was studied using photoion spectroscopy. We focused on shape resonances, their behavior, and their decay mechanisms produced by np → (nd+d) (n = 3, 4, 5) photoexcitation for Fe, Ru, Ni, Pd, and Pt transition metal negative ions.

Calculation of Atomic Data for NASA Missions

The interpretation of cosmic spectra relies on a vast sea of atomic data which are not readily obtainable from analytic expressions or simple calculations. Rather, their evaluation typically requires state-of-the-art atomic physics calculations, with the inclusion of weaker effects (spin-orbit and configuration interactions, relaxation, Auger broadening, etc.), to achieve the level of accuracy needed for use by astrophysicists. Our NASA-supported research program is focused on calculating data for three important atomic processes, 1) dielectronic recombination (DR), 2) inner-shell photoabsorption, and 3) fluorescence and Auger decay of inner-shell vacancy states. Some additional details and examples of our recent findings are given below.

Multi‐Auger Decay in Negative Ion Photodetachment

Inner‐shell photoexcitation and detachment experiments on He− and S− ions are discussed. In both systems, negative ion Feshbach resonances produced by excitation into the partially filled valence p orbital are observed to decay via many‐electron processes, such as multi‐Auger decay. Absolute cross sections, near‐threshold behavior, and relative product channel strengths (branching ratios) are also discussed.