Resonant Inelastic X-Ray Scattering from Valence Excitations in Insulating Copper-Oxides

We report resonant inelastic x-ray measurements of insulating La$_2$CuO$_4$ and Sr$_2$CuO$_2$Cl$_2$ taken with the incident energy tuned near the Cu K absorption edge. We show that the spectra are well described in a shakeup picture in 3rd order perturbation theory which exhibits both incoming and outgoing resonances, and demonstrate how to extract a spectral function from the raw data. We conclude by showing {\bf q}-dependent measurements of the charge transfer gap.Comment: minor notational changes, discussion of anderson impurity model fixed, references added; accepted by PR

Slowdown and compression of a strong X-ray free-electron pulse propagating through the Mg vapors

Here we study the propagation of a strong X-ray free-electron pulse through the resonant medium of atomic Mg accompanied by the self-seeded stimulated resonant X-ray Raman scattering. The X-ray pulse is decelerated by two orders of magnitude because of nonlinear interaction and experiences a 6-fold compression. The simulations are based on a strict numerical solution of the coupled Bloch and Maxwell equations for a 50 fs pulse tuned in the $2p_{3/2}$-$4s$ resonance (54.8 eV). The extensive ringing tail produced during propagation widens the power spectrum. This seed field triggers the Stokes channel $3s$-$2p_{3/2}$ (49.4 eV) of stimulated resonant X-ray Raman scattering and the weaker Stokes and anti-Stokes fields caused by four-wave mixing. The beating between the Stokes and pump fields quenches the population inversion at longer propagation distances where lasing without inversion enhances the Stokes component

Resonant X-ray second harmonic generation in atomic gases

International audienceWe explore the x-ray second-harmonic generation process induced by resonant two-photon absorption in systems with inversion symmetry. We show that this process becomes allowed in the x-ray region due to nondipole contributions. It is found that, although a plane-wave pump field generates only a longitudinal second-harmonic field, a Gaussian pump beam creates also a radially polarized transverse second-harmonic field which is stronger than the longitudinal one. Contrary to the longitudinal component, the transverse second-harmonic field with zero intensity on the axis of the pump beam can run in free space. Our theory is applied to Ar and Ne atomic vapors and predicts an energy conversion efficiency of x-ray second-harmonicgeneration of 3.2 × 10−11 and 1.3 × 10−12, respectively

Nonlocal resonant inelastic x-ray scattering

In the description of resonant inelastic x-ray scattering (RIXS) from inversion-symmetric molecules the small core-level splitting is typically neglected. However, the spacing Delta between gerade and ungerade core levels in homonuclear diatomic molecules can be comparable with the lifetime broadening of the intermediate core-excited state Gamma. We show that when Delta similar to Gamma the scattering becomes nonlocal in the sense that x-ray absorption at one atomic site is followed by emission at the other one. This is manifested in an unusual dependence of the RIXS cross section on the sum of the momenta of incoming and outgoing x-ray photons k + k', contrary to the normal k - k' dependence in the conventional local RIXS theory. The nonlocality of the scattering influences strongly the scattering angle and excitation energy dependence of the intensity ratio between parity forbidden and allowed RIXS channels. Numerical simulations for N-2 show that this effect can readily be measured at present-day x-ray radiation facilities

Simulating fluorine K-edge resonant inelastic x-ray scattering of sulfur hexafluoride and the effect of dissociative dynamics

We report on a computational study of resonant inelastic x-ray scattering (RIXS), at different fluorine K-edge resonances of the SF6 molecule, and corresponding nonresonant x-ray emission. Previously measured polarization dependence in RIXS is reproduced and traced back to the local σ and π symmetry of the molecular orbitals and corresponding states involved in the RIXS process. Also electron-hole coupling energies are calculated and related to experimentally observed spectator shifts. The role of dissociative S-F bond dynamics is explored to model detuning of RIXS spectra at the |F1s−16a11g⟩ resonance, which shows challenges to accurately reproduce the required steepness for core-excited potential energy surface. We show that the RIXS spectra can only be properly described by considering breaking of the global inversion symmetry of the electronic wave function and core-hole localization, induced by vibronic coupling. Due to the core-hole localization we have symmetry forbidden transitions, which lead to additional resonances and changing width of the RIXS profile

Spatial Quantum Beats in Vibrational Resonant Inelastic Soft X-Ray Scattering at Dissociating States in Oxygen

Resonant inelastic soft x-ray scattering (RIXS) spectra excited at the 1σg→3σu resonance in gas-phase O2 show excitations due to the nuclear degrees of freedom with up to 35 well-resolved discrete vibronic states and a continuum due to the kinetic energy distribution of the separated atoms. The RIXS profile demonstrates spatial quantum beats caused by two interfering wave packets with different momenta as the atoms separate. Thomson scattering strongly affects both the spectral profile and the scattering anisotropy.QC 20110426 QC 2011051