Direct observation of double-core-hole shake-up States in photoemission.

Direct measurements of Ar^{+} 1s^{-1}2p^{-1}nl double-core-hole shake-up states are reported using conventional single-channel photoemission, offering a new and relatively easy means to study such species. The high-quality results yield accurate energies and lifetimes of the double-core-hole states. Their photoemission spectrum also can be likened to 1s absorption of an exotic argon ion with a 2p core vacancy, providing new information about the spectroscopy of both this unusual ionic state as well as the neutral atom.

Selecting core-hole localization or delocalization in CS2 by photofragmentation dynamics.

Electronic core levels in molecules are highly localized around one atomic site. However, in single-photon ionization of symmetric molecules, the question of core-hole localization versus delocalization over two equivalent atoms has long been debated as the answer lies at the heart of quantum mechanics. Here, using a joint experimental and theoretical study of core-ionized carbon disulfide (CS2), we demonstrate that it is possible to experimentally select distinct molecular-fragmentation pathways in which the core hole can be considered as either localized on one sulfur atom or delocalized between two indistinguishable sulfur atoms.

Partial-ion-yield studies of SOCl2 following x-ray absorption around the S and Cl K edges.

We present a series of photoabsorption and partial-ion-yield experiments on thionyl chloride, SOCl(2), at both the sulfur and chlorine K edges. The photoabsorption results exhibit better resolution than previously published data, leading to alternate spectral assignments for some of the features, particularly in the Rydberg-series region. Based on measured fragmentation patterns, we suggest the LUMO, of a(') character, is delocalized over the entire molecular skeleton.

A new method to derive electronegativity from resonant inelastic x-ray scattering.

Electronegativity is a well-known property of atoms and substituent groups. Because there is no direct way to measure it, establishing a useful scale for electronegativity often entails correlating it to another chemical parameter; a wide variety of methods have been proposed over the past 80 years to do just that. This work reports a new approach that connects electronegativity to a spectroscopic parameter derived from resonant inelastic x-ray scattering.

Thomson-resonant interference effects in elastic x-ray scattering near the Cl K edge of HCl.

We experimentally observed interference effects in elastic x-ray scattering from gas-phase HCl in the vicinity of the Cl K edge. Comparison to theory identifies these effects as interference effects between non-resonant elastic Thomson scattering and resonant Raman scattering. The results indicate the non-resonant Thomson and resonant Raman contributions are of comparable strength.

Resonant inelastic x-ray scattering of methyl chloride at the chlorine K edge.

We present a combined experimental and theoretical study of isolated CH(3)Cl molecules using resonant inelastic x-ray scattering (RIXS). The high-resolution spectra allow extraction of information about nuclear dynamics in the core-excited molecule. Polarization-resolved RIXS spectra exhibit linear dichroism in the spin-orbit intensities, a result interpreted as due to chemical environment and singlet-triplet exchange in the molecular core levels.

K-shell x-ray spectroscopy of atomic nitrogen.

Absolute K-shell photoionization cross sections for atomic nitrogen have been obtained from both experiment and state-of-the-art theoretical techniques. Because of the difficulty of creating a target of neutral atomic nitrogen, no high-resolution K-edge spectroscopy measurements have been reported for this important atom. Interplay between theory and experiment enabled identification and characterization of the strong 1s → np resonance features throughout the threshold region.

Partial ion yield spectroscopy around the Cl 2p and C 1s ionization thresholds in CF3Cl.

We present a partial ion yield experiment on freon 13, CF(3)Cl, excited in the vicinity of the C 1s and Cl 2p ionization thresholds. We have collected a large amount of cationic fragments and a few anionic fragments at both edges. We have observed a strong intensity dependence of Rydberg transitions with ion fragment size for the CF(n)Cl(+) and CF(n)(+)/F(+) (n=0-3) series at both the Cl 2p and C 1s ionization edges.

Fragmentation properties of three-membered heterocyclic molecules by partial ion yield spectroscopy: C(2)H(4)O and C(2)H(4)S.

We investigated the photofragmentation properties of two three-membered ring heterocyclic molecules, C(2)H(4)O and C(2)H(4)S, by total and partial ion yield spectroscopy. Positive and negative ions have been collected as a function of photon energy around the C 1s and O 1s ionization thresholds in C(2)H(4)O, and around the S 2p and C 1s thresholds in C(2)H(4)S. We underline similarities and differences between these two analogous systems.

Linear dichroism in resonant inelastic x-ray scattering to molecular spin-orbit states.

Polarization-dependent resonant inelastic x-ray scattering (RIXS) is shown to be a new probe of molecular-field effects on the electronic structure of isolated molecules. A combined experimental and theoretical analysis explains the linear dichroism observed in Cl 2p RIXS following Cl 1s excitation in HCl and CF3Cl as due to molecular-field effects, including singlet-triplet exchange, indicating polarized-RIXS provides a direct probe of spin-orbit-state populations applicable to any molecule.

Investigation of nanostructure and thermal behavior of zinc-substituted fluorapatite.

The incorporation of various cations such as Zn (2+) into the structure of fluorapatite, Ca 5(PO 4) 3F, is governed by the effectiveness of the cations to substitute for Ca (2+) ions. In this work different concentrations of zinc were used to substitute for calcium. Microscopic characterization was done by observing the nanostructural variations induced by these zinc substitutions and by relating these observations to thermal behavior of the zinc-substituted fluorapatite.

Photofragmentation of SiF4 upon Si 2p and F 1s core excitation: cation and anion yield spectroscopy.

We have studied the fragmentation dynamics of core-excited SiF4 by means of soft-x-ray photoexcitation and partial positive and negative ion yield measurements around the Si L2,3-shell and F K-shell ionization thresholds. All detectable ionic fragments are reported and we observe significant differences between the various partial ion yields near the Si 2p threshold. The differences are similar to our previous results from CH3Cl showing more extended fragmentation in correspondence to transitions to Rydberg states.

Micro-structural characterization of precipitation-synthesized fluorapatite nano-material by transmission electron microscopy using different sample preparation techniques.

Fluorapatite is a naturally occurring mineral of the apatite group and it is well known for its high physical and chemical stability. There is a recent interest in this ceramic to be used as a radioactive waste form material due to its intriguing chemical and physical properties. In this study, the nano-sized fluorapatite particles were synthesized using a precipitation method and the material was characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM).

Design and performance of a curved-crystal x-ray emission spectrometer.

A curved-crystal x-ray emission spectrometer has been designed and built to measure 2-5 keV x-ray fluorescence resulting from a core-level excitation of gas phase species. The spectrometer can rotate 180 degrees, allowing detection of emitted x rays with variable polarization angles, and is capable of collecting spectra over a wide energy range (20 eV wide with 0.5 eV resolution at the Cl K edge) simultaneously.

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.

Low-energy nondipole effects in molecular nitrogen valence-shell photoionization.

Observations are reported for the first time of significant nondipole effects in the photoionization of the outer-valence orbitals of diatomic molecules. Measured nondipole angular-distribution parameters for the 3sigma(g), 1pi(u), and 2sigma(u) shells of N2 exhibit spectral variations with incident photon energies from thresholds to approximately 200 eV which are attributed via concomitant calculations to particular final-state symmetry waves arising from (E1)multiply sign in circle(M1,E2) radiation-matter interactions first-order in photon momentum. Comparisons with previously reported K-edge studies in N2 verify linear scaling with photon momentum, accounting in part for the significantly enhanced nondipole behavior observed in inner-shell ionization at correspondingly higher momentum values in this molecule.

Near-edge X-ray absorption fine structure study of disordering in Gd2(Ti1-yZry)2O7 pyrochlores.

Disorder in Gd2(Ti(1-y)Zry)2O7 pyrochlores, for y = 0.0-1.0, is investigated by Ti 2p and O 1s near-edge X-ray absorption fine structure spectroscopy.

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.

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.

Nondipole effects in the photoionization of xe 4d5/2 and 4d3/2: evidence for quadrupole satellites.

Measurements of nondipole parameters in spin-orbit-resolved Xe 4d photoionization demonstrate dynamical differences arising from relativistic effects. The experimental data do not agree with relativistic random-phase approximation calculations of single ionization dipole and quadrupole channels. It is suggested that the discrepancy is due to the omission of multiple-excitation quadrupole channels, i.

Nearest-neighbor-atom core-hole transfer in isolated molecules.

A new phenomenon sensitive only to next-door-neighbor atoms in isolated molecules is demonstrated using angle-resolved photoemission of site-selective core electrons. Evidence for this interatomic core-to-core electron interaction is observable only by measuring nondipolar angular distributions of photoelectrons. In essence, the phenomenon acts as a very fine atomic-scale sensor of nearest-neighbor elemental identity.

Dramatic nondipole effects in low-energy photoionization: experimental and theoretical study of Xe 5s.

The Xe 5s nondipole photoelectron parameter gamma is obtained experimentally and theoretically from threshold to approximately 200 eV photon energy. Significant nondipole effects are seen even in the threshold region of this valence shell photoionization. In addition, contrary to previous understanding, clear evidence of interchannel coupling among quadrupole photoionization channels is found.

Nondipolar electron angular distributions from fixed-in-space molecules.

The first indication of nondipole effects in the azimuthal dependence of photoelectron angular distributions emitted from fixed-in-space molecules is demonstrated in N (2). Comparison of the results with angular distributions observed for randomly oriented molecules and theoretical derivations for the nondipole correction first order in photon momentum suggests that higher orders will be needed to describe distributions measured in the molecular frame.

Photoexcitation of a dipole-forbidden resonance in helium.

We have observed photoexcitation of the dipole-forbidden 1s(2) 1S0-->2p(2) 1D2 resonance in helium by measuring the nondipolar forward-backward asymmetry of photoelectron angular distributions in the 2l2l(') autoionizing region. By exploiting the electric dipole-quadrupole interference in the excitation of both the 2s2p 1P1 and 2p(2) 1D2 levels, we have observed the quadrupole resonance in photoabsorption and extracted its Fano line shape parameters and the relative phase of the 1sEp and 1sEd continua. We find the quadrupole line profile index q(2) to be markedly different from theoretical expectations.

Large nondipole effects in the angular distributions of K-shell photoelectrons from molecular nitrogen.

Measurements of angular distributions of K-shell electrons photoejected from molecular nitrogen are reported which reveal large deviations at relatively low photon energies (Planck's omega < or = 500 eV) from emission patterns anticipated from the dipole approximation to interactions between radiation and matter. A concomitant theoretical analysis incorporating the effects of electromagnetic retardation attributes the observed large nondipole behaviors in N2 to bond-length-dependent terms in the E1 [multiply sign in circle] (E2,M1) photoelectron emission amplitudes which are indicative of a potentially universal nondipole behavior in molecular photoionization.