S.-W. Yu

Anionic and cationic photofragmentation of core-excited N2O

We have measured all detectable cationic and anionic fragments in singlechannel mode from N2O as a function of photon energy in the vicinity of the nitrogen 1s core-level threshold. Due to the high degree of localization of the core electrons, the two excitations Nt1s → 3π∗ and Nc1s → 3π∗ show high levels of site-selective behaviour. The observed partial ion yield for the sole anionic fragment,O−, in conjunctionwith the partial cation yields,confirms our previous demonstration of anion-yield spectroscopy as a unique tool to identify core-level shape resonances.

Fragmentation processes following core excitation in acetylene and ethylene by partial ion yield spectroscopy

Partial ion yield spectroscopy provides a very detailed picture of fragmentation processes following core excitation in isolated molecules. We exploit this potential in the analysis of decay processes following C1s→π and C1s→Rydberg excitations in ethylene and acetylene. We show that the relative intensity of spectral features related to the excitation to empty molecular orbitals or to Rydberg states is a function of the time variation of the fragmentation process. Namely, we see an intensity increase in the Rydberg states compared to the molecular orbitals as the fragmentation process becomes more extensive, a result attributable to the diffuse nature of the Rydberg virtual orbitals, which makes spectator decay more likely than participator decay. Therefore, the number of dissociative final states that can be reached from Rydberg excitation is higher than for excitation to empty molecular orbitals. In acetylene, we obtain the first direct evidence of the presence of a σ* excitation embedded in the Rydber...

Fragmentation of methyl chloride studied by partial positive and negative ion-yield spectroscopy

The authors present partial-ion-yield experiments on the methyl chloride molecule excited in the vicinity of the Cl2p and C1s inner shells. A large number of fragments, cations produced by dissociation or recombination processes, as well as anionic species, have been detected. Although the spectra exhibit different intensity distributions depending on the core-excited atom, general observations include strong site-selective fragmentation along the C-Cl bond axis and a strong intensity dependence of transitions involving Rydberg series on fragment size.

Evidence of dynamical spin shielding in Ce from spin-resolved photoelectron spectroscopy

Using Fano effect measurements upon polycrystalline Ce, we have observed a phase reversal between the spectral structure at the Fermi edge and the other 4f derived feature near a binding energy of 2 eV. The Fano effect is the observation of spin polarized photoelectron emission from nonmagnetic materials, under chirally selective excitation, such as circularly polarized photons. The observation of phase reversal between the two peaks is a direct experimental proof of Kondo shielding in Ce, confirming the predictions of Gunnarsson and Shoenhammer, albeit with a small modification.

An Alternative Model for Electron Correlation in Pu

Using a density functional theory based approach that treats the 5f electrons relativistically, a Pu electronic structure with zero net magnetic moment is obtained, where the 5f orbital and 5f spin moments cancel each other. By combining the spin and orbital specific densities of states with state, spin and polarization specific transition moments, it is possible to reconstruct the experimentally observed photoemission spectra from Pu. Extrapolating to a spin-resolving Fano configuration, it is shown how this would resolve the extant controversy over Pu electronic structure.

Anion and cation-yield spectroscopy of core-excited SF6

We report an extensive study on total and partial-ion-yield spectroscopy around both the S 2p and F 1s thresholds in SF(6). All positive and negative single-ion channels have been measured. Below the F 1s threshold we detect a large variation in relative intensity of the resonant structures according to the specific channel monitored, indicating selective fragmentation. Above threshold, at variance with previous cases described by us, we detect high-intensity structures related to shape resonances not only in the cation channels but also for the anions. We discuss the applicability and limits of a model we have developed for the analysis of shape resonances in anion yields as a function of molecular size.

Photofragmentation study of core-excited NO

The production of anionic and cationic fragments of NO has been measured after photoexcitation in the vicinity of the nitrogen 1s threshold. The partial ion yield for the anionic fragment O−, in conjunction with the partial cation yields, can clearly distinguish the shape resonances from doubly excited states. On the N 1s → π* resonances, which are composed of the three intermediate states 2Δ, 2Σ− and 2Σ+ in the order of increasing photon energy, the intensity ratios for the NO++, O+ and O++ fragments are very different from the total ion yield. An explanation for this behaviour in terms of symmetry is proposed.

An instrument for the investigation of actinides with spin resolved photoelectron spectroscopy and bremsstrahlung isochromat spectroscopy.

A new system for spin resolved photoelectron spectroscopy and bremsstrahlung isochromat spectroscopy has been built and commissioned at Lawrence Livermore National Laboratory for the investigation of the electronic structure of the actinides. Actinide materials are very toxic and radioactive and therefore cannot be brought to most general user facilities for spectroscopic studies. The technical details of the new system and preliminary data obtained therein will be presented and discussed.

Photofragmentation of HCl near the chlorine L2,3 ionization threshold: new evidence of a strong ultrafast dissociation channel

We report a study using partial-ion-yield spectroscopy around the Cl 2p ionization threshold of HCl. All cation channels have been measured, while no evidence for anion production was obtained. The absence of any detectable H− can be related to the electronegativity difference between the two atoms and the observed low probability of directly creating Cl++ and Cl+++. In the photon-energy region around the Cl 2p3/2,1/2 → 6σ* excitation process, production of H+ is almost completely suppressed, which indicates a dominant ultrafast dissociation process leading to neutral H.

Fragmentation dynamics of H2S following S 2p photoexcitation.

The fragmentation dynamics of core-excited H(2)S has been studied by means of partial anion and cation yield measurements around the S L(2,3)-subshell ionization thresholds. All detectable ionic fragments are reported, and significant differences between partial ion yields are observed. Possible dissociation pathways are discussed by comparison to previous studies of electron spectra.