Yong Ma

Magnetic circular dichroism (MCD) has been observed at the ${\mathit{L}}_{2,3}$ absorption edges of ferromagnetic nickel by use of circularly-polarized soft-x-ray synchrotron radiation. The MCD intensity ratio between the ${\mathit{L}}_{2}$ and ${\mathit{L}}_{3}$ edges is found to differ appreciably from that predicted by a simple exchange-split-valence-band model. Fine MCD features, imperceptible in the absorption spectra, are also observed, and a tentative interpretation is given. This work, demonstrating the feasibility of MCD measurements in the soft-x-ray region, provides a new approach to study 3d and 4f ferromagnetic systems with their respective dipole-permitted 2p\ensuremath{\rightarrow}3d and 3d\ensuremath{\rightarrow}4f transitions.

Oxygen K-edge absorption spectra of carefully characterized ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$ samples were measured using a bulk-sensitive fluoresence-yield-detection method. They reveal two distinct pre-edge peaks which evolve systematically as a function of Sr concentration. The measured spectra are quantitatively described by calculations based on the Hubbard model, including local Coulomb interactions and core-hole excitonic correlations. The absorption data are consistent with a description of electronic states based on a doped charge-transfer insulator.

K-shell photoabsorption spectra of CO, $^{13}\mathrm{C}^{18}$O, NO, ${\mathrm{O}}_{2}$, ${\mathrm{CO}}_{2}$, ${\mathrm{N}}_{2}$O, ${\mathrm{C}}_{2}$${\mathrm{H}}_{2}$, ${\mathrm{C}}_{2}$${\mathrm{D}}_{2}$, ${\mathrm{C}}_{2}$${\mathrm{H}}_{4}$, ${\mathrm{C}}_{2}$${\mathrm{D}}_{4}$, ${\mathrm{C}}_{2}$${\mathrm{H}}_{6}$, ${\mathrm{C}}_{2}$${\mathrm{D}}_{6}$ measured with unprecedented energy resolution and signal-to-noise ratio are presented. These spectra reveal many new features in core-excited valence and Rydberg states. Detailed vibrational structures are observed in these spectra, providing valuable information on the geometrical and vibrational properties of the core-excited molecules. In addition, C 1s and N 1s core-hole lifetimes are found to be \ensuremath{\sim}120 and \ensuremath{\sim}135 meV in these molecules with little dependence on their bonding environments. These results and the tentative peak assignments are discussed briefly in terms of the equivalent core model, multielectron excitations, exchange interactions, and the geometry of the excited molecules.

The K-shell photoabsorption spectrum of the ${\mathrm{N}}_{2}$ molecule, recorded with unprecedented energy resolution and statistical accuracy, reveals several new Rydberg transitions and double-excitation states. The nearly identical term values, vibrational frequencies and internuclear separations of the 1s-excited states of ${\mathrm{N}}_{2}$ and of the 2\ensuremath{\pi}-excited states of NO, allow for a complete peak assignment for the ${\mathrm{N}}_{2}$ Rydberg series and strongly support both the core-hole localization picture and equivalent-core model. Subtle differences are related to the final-state charge distribution in the vicinity of the nuclei and to the bonding character of the valence orbitals.

The K-shell excitation spectra of the hydrides water, ammonia, and methane have been measured in photoabsorption experiments using synchrotron radiation in combination with a high-resolution monochromator. For the case of methane, in particular, a wealth of spectral detail is observed which was not accessible in previous studies. The measured excitation energies and relative intensities compare well with values calculated using a complete second-order approximation for the polarization propagator. In order to determine the extent of admixing of valence excitations (i.e., transitions into virtual ${\mathrm{\ensuremath{\sigma}}}^{\mathrm{*}}$ orbitals) to the Rydberg manifolds, the X-H bond lengths have been varied in the calculations. In the case of ${\mathrm{H}}_{2}$O, the two lowest-energy bands are due to the O 1s-4${\mathit{a}}_{1}$/3s and O 1s-2${\mathit{b}}_{2}$/3p transitions and have strong valence character; their width indicates that both excitations are dissociative. The ${\mathrm{NH}}_{3}$ and ${\mathrm{ND}}_{3}$ spectra are also broad which is not only due to possible dissociation but also to unresolved vibrational fine structure (${\ensuremath{\nu}}_{2}$ mode) and a Jahn-Teller instability. Valence character is concentrated in the lowest excited state in the Rydberg ns manifold, but is distributed more uniformly over the np(e) manifold. The weak dipole-forbidden C 1s-3s(${\mathit{a}}_{1}$) transition in ${\mathrm{CH}}_{4}$ and ${\mathrm{CD}}_{4}$ is accompanied by vibrational structure due to the ${\ensuremath{\nu}}_{4}$ mode, indicating that it derives its intensity from vibronic coupling with the C 1s-3p(${\mathit{t}}_{2}$) transition. The structure on the latter band is extremely complicated due to Jahn-Teller coupling and cannot be assigned at present, as is the case for the Rydberg transitions at higher energies. The higher np Rydberg excitations contain considerable valence character.