Electric-octupole and pure-electric-quadrupole effects in soft-x-ray photoemission

Second-order [O(k^2), k=omega/c] nondipole effects in soft-x-ray photoemission are demonstrated via an experimental and theoretical study of angular distributions of neon valence photoelectrons in the 100--1200 eV photon-energy range. A newly derived theoretical expression for nondipolar angular distributions characterizes the second-order effects using four new parameters with primary contributions from pure-quadrupole and octupole-dipole interference terms. Independent-particle calculations of these parameters account for a significant portion of the existing discrepancy between experiment and theory for Ne 2p first-order nondipole parameters.Comment: 4 pages, 3 figure

Nondipole Resonant X-ray-Raman Spectroscopy: Polarized Inelastic Scattering at the K Edge of Cl2

Experimental and theoretical studies are reported on the inelastic (Raman) scattering of wavelength-selected polarized x rays from the K edge of gas-phase chlorine molecules. The polarized emission spectra exhibit prominent nondipole features consequent of phase variations of the incident and emitted radiation over molecular dimensions, as predicted by the Kramers-Heisenberg scattering formalism. Issues pursuant to the detection of core-hole localization by resonant Raman scattering from homonuclear diatomic molecules are critically examined. [S0031-9007(97)03486-8

Reply to Comment on ‘Nondipole Resonant X-ray-Raman Spectroscopy: Polarized Inelastic Scattering at the K Edge of Cl2,’

Mills et al. Reply: In their Comment on our Letter [1], Gel’mukhanov and Ågren [2] reiterate recent assertions [3] based on their earlier theoretical studies [4]. The primary purpose of their Comment is apparently to refute our stated conclusion that core-excited-state localization/ delocalization mechanisms are irrelevant to interpretations of reported Raman scattering experiments on homonuclear diatomic molecules

Field-Dependent Tilt and Birefringence of Electroclinic Liquid Crystals: Theory and Experiment

An unresolved issue in the theory of liquid crystals is the molecular basis of the electroclinic effect in the smectic-A phase. Recent x-ray scattering experiments suggest that, in a class of siloxane-containing liquid crystals, an electric field changes a state of disordered molecular tilt in random directions into a state of ordered tilt in one direction. To investigate this issue, we measure the optical tilt and birefringence of these liquid crystals as functions of field and temperature, and we develop a theory for the distribution of molecular orientations under a field. Comparison of theory and experiment confirms that these materials have a disordered distribution of molecular tilt directions that is aligned by an electric field, giving a large electroclinic effect. It also shows that the net dipole moment of a correlated volume of molecules, a key parameter in the theory, scales as a power law near the smectic-A--smectic-C transition.Comment: 18 pages, including 9 postscript figures, uses REVTeX 3.0 and epsf.st

Direct Observation of Double-Core-Hole Shake-Up States in Photoemission

Direct measurements of Ar+ 1s−12p−1nl 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

Post-Collision-Interaction Effects in HCl Following Photofragmentation Near the Chlorine K Edge

Ion time-of-flight mass spectroscopy was used to study the relaxation dynamics of HCl following photoexcitation in the vicinity of the Cl K threshold (~2.8 keV). Detailed observations of molecular fragmentation mediated by postcollision interaction between a photoelectron and an Auger electron are presented, evidenced by the recapture of Cl K photoelectrons by either Cln+ or H+ dissociation fragments. [S1050-2947(98)51206-1

Photofragmentation of Third-Row Hydrides Following Photoexcitation at Deep-Core Levels

The relaxation dynamics of HCl, DCl, H2S, and D2S following photoexcitation in the vicinities of the Cl and S K-shell thresholds (∼2.8keV for Cl, ∼2.5 keV for S) were studied by means of ion time-of-flight mass spectroscopy. In all cases, the onset of pre-edge core-shell photoionization precedes the formation on resonance of a significant amount of neutral hydrogen as well as postcollision-interaction effects above threshold. Examination of the width of the H+ peak in spectra taken with the analyzer parallel and perpendicular to the polarization vector of the incident light indicates that on resonance, the photofragmentation asymmetry parameter, β, is approximately two for HCl, and is clearly positive for H2S

Probing Cation Antisite Disorder in Gd2Ti2O7 Pyrochlore by Site-specific NEXAFS and XPS

Disorder in Gd2Ti2O7 is investigated by near-edge x-ray-absorption fine structure (NEXAFS) and x-ray photoelectron spectroscopy (XPS). NEXAFS shows Ti4+ ions occupy octahedral sites with a tetragonal distortion induced by vacant oxygen sites. O 1s XPS spectra obtained with a charge neutralization system from Gd2Ti2O7(100) and the Gd2Ti2O7 pyrochlore used by Chen et al. [Phys. Rev. Lett. 88, 105901 (2002)], both yielded a single peak, unlike the previous result on the latter that found two peaks. The current results give no evidence for an anisotropic distribution of Ti and O. The extra features reported in the aforementioned communication resulted from charging effects and incomplete surface cleaning. Thus, a result confirming the direct observation of simultaneous cation-anion antisite disordering and lending credence to the split vacancy model has been clarified

Breakdown of the Independent Particle Approximation in High-Energy Photoionization

The independent particle approximation is shown to break down for the photoionization of both inner and outer nℓ (ℓ\u3e0) electrons of all atoms, at high enough energy, owing to interchannel interactions with the nearby ns photoionization channels. The effect is illustrated for Ne 2p in the 1 keV photon energy range through a comparison of theory and experiment. The implications for x-ray photoelectron spectroscopy of molecules and condensed matter are discussed