R.C. Bilodeau

Experimental photoionization cross-section measurements in the ground and metastable state threshold region of Se+

Absolute photoionization cross-section measurements are reported for Se+ in the photon energy range 18.0–31.0 eV, which spans the ionization thresholds of the 4S3/2 ground state and the low-lying 2P3/2, 1/2 and 2D5/2, 3/2 metastable states. The measurements were performed using the Advanced Light Source synchrotron radiation facility. Strong photoexcitation–autoionization resonances due to 4p → nd transitions are seen in the cross-section spectrum and identified with a quantum-defect analysis.

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.

Absolute single photoionization cross-section measurements of Se3+ and Se5+

Absolute single photoionization cross-section measurements of Se3+ and Se5+ ions were performed using monochromatized synchrotron radiation and the photo-ion merged-beams technique. Se3 + measurements were made at a photon energy resolution of 20 ± 3 meV from 41.90 to 54.56 eV, spanning the 2P1/2 ground state ionization threshold. Numerous autoionizing resonances arising from both the 2P1/2 ground state and 2P3/2 metastable state are identified using quantum defect analysis. For Se5 +, measurements were made at a photon energy resolution of 60 ± 10 meV from 81 to 101 eV, spanning the 2S1/2 ground state ionization threshold, and at a photon energy resolution of 28 ± 3 meV from 100.175 to 106.675 eV in a region rich with autoionizing resonances. The direct ionization threshold of the Se5 + 2S1/2 ground state is experimentally determined to be 81.780 ± 0.010 eV.

Size effects in van der Waals clusters studied by spin and angle-resolved electron spectroscopy and multi-coincidence ion imaging

We have studied the valence and inner-shell photoionization of free rare-gas clusters by means of angle and spin resolved photoelectron spectroscopy and momentum resolving electron-multi-ion coincidence spectroscopy. The electron measurements probe the evolution of the photoelectron angular distribution and spin polarization parameters as a function of photon energy and cluster size, and reveal a strong cluster size dependence of the photoelectron angular distributions in certain photon energy regions. In contrast, the spin polarization parameter of the cluster photoelectrons is found to be very close to the atomic value for all covered photon energies and cluster sizes. The ion imaging measurements, which probe the fragmentation dynamics of multiply charged van der Waals clusters, also exhibit a pronounced cluster size dependence.

Photo-multidetachment and Fragmentation of C60 Anions

Absolute single-photon multi-electron ionization and fragmentation cross sections for C−60 ions are measured for the first time. It is determined that the excess electron in the anion causes a significant increase in cross section and an overall energy scaling, but otherwise has little effect on the observed structure. Appearance energies (apparent thresholds) of the fragmentation products are also observed to be significantly lower for the anion, as compared to the neutral molecule.

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.

High-resolution photoionization of Xe+ ions: Experiment and theory

High-resolution photoionization measurements of Xe+ ions have been performed at the Advanced Light Source in Berkeley, California, USA. The experimental cross sections are compared with results from Dirac-Coulomb R-matrix calculations.

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.

Enhanced charge transfer in doped nano-systems measured via electron-ion coincidence technique

Interatomic energy and charge transfer in a Xe doped Ar cluster have been investigated. Besides interatomic Coulombic decay (ICD), electron transfer mediated decay (ETMD) seems to play a significant role in the relaxation process of heterogeneous systems which is in strong contrast to pristine clusters. Ar 2p electrons have been measured in coincidences with cluster fragments. The high resolution in time and energy of the detectors enabled us to determine the site of ionization on the cluster and the kind of the resulting fragments.