Enhanced nonlinear double excitation of He in intense extreme ultraviolet laser fields

Nonlinear, three-photon double excitation of He in intense extreme ultraviolet free-electron laser fields (∼24.1  eV, ∼5  TW/cm2) is presented. Resonances to the doubly excited states converging to the He+ N=3 level are revealed by the shot-by-shot photoelectron spectroscopy and identified by theoretical calculations based on the time-dependent Schrödinger equation for the two-electron atom under a laser field. It is shown that the three-photon double excitation is enhanced by intermediate Rydberg states below the first ionization threshold, giving a greater contribution to the photoionization yields than the two-photon process by more than 1 order of magnitude.

Second-order autocorrelation of XUV FEL pulses via time resolved two-photon single ionization of He

Second-order autocorrelation spectra of XUV free-electron laser pulses from the Spring-8 Compact SASE Source (SCSS) have been recorded by time and momentum resolved detection of two-photon single ionization of He at 20.45 eV using a split-mirror delay-stage in combination with high-resolution recoil-ion momentum spectroscopy (COLTRIMS). From the autocorrelation trace we extract a coherence time of 8 ± 2 fs and a mean pulse duration of 28 ± 5 fs, much shorter than estimations based on electron bunch-length measurements. Simulations within the partial coherence model [Opt. Lett. 35, 3441 (2010)] are in agreement with experiment if a pulse-front tilt across the FEL beam diameter is taken into account that leads to a temporal shift of about 6 fs between both pulse replicas.

Wavefront analysis of nonlinear self-amplified spontaneous-emission free-electron laser harmonics in the single-shot regime

The single-shot spatial characteristics of the vacuum ultraviolet self-amplified spontaneous emission of a free electron laser (FEL) is measured at different stages of amplification up to saturation with a Hartmann wavefront sensor. We show that the fundamental radiation at 61.5 nm tends towards a single-mode behavior as getting closer to saturation. The measurements are found in good agreement with simulations and theory. A near diffraction limited wavefront was measured. The analysis of Fresnel diffraction through the Hartmann wavefront sensor hole array also provides some further insight for the evaluation of the FEL transverse coherence, of high importance for various applications.

Multiphoton double ionization of Ar in intense extreme ultraviolet laser fields studied by shot-by-shot photoelectron spectroscopy

Photoelectron spectroscopy has been performed to study the multiphoton double ionization of Ar in an intense extreme ultraviolet laser field (hν ∼ 21  eV, ∼ 5  TW/cm²), by using a free electron laser (FEL). Three distinct peaks identified in the observed photoelectron spectra clearly show that the double ionization proceeds sequentially via the formation of Ar(+): Ar+hν→Ar (+) + e⁻ and Ar²(+) + 2hν→Ar(+) + e⁻. Shot-by-shot recording of the photoelectron spectra allows simultaneous monitoring of FEL spectrum and the multiphoton process for each FEL pulse, revealing that the two-photon ionization from Ar(+) is significantly enhanced by intermediate resonances in Ar(+).

The extreme ultraviolet split and femtosecond delay unit at the plane grating monochromator beamline PG2 at FLASH

An extreme ultraviolet split and femtosecond delay unit based on grazing incidence Mach-Zehnder geometry has been designed and implemented on the plane grating monochromator beamline PG2 at FLASH, the Free Electron Laser at DESY. This device splits the FLASH radiation into two beams, which can independently be steered, filtered and temporally delayed between -5.1 and +5.1 ps with uncertainty in the temporal accuracy of 210 as. To demonstrate the performance of this device, we have performed longitudinal coherence studies of FLASH radiation as well as measured the pulse length by nonlinear two-photon double-ionization in helium.

Note: Measurement of saturable absorption by intense vacuum ultraviolet free electron laser using fluorescent material

Advances in free electron lasers (FELs) which generate high energy photons are expected to open novel nonlinear optics in the x-ray and vacuum ultraviolet (VUV) regions. In this paper, we report a new method for performing VUV-FEL focusing experiments. A VUV-FEL was focused with Kirkpatrick-Baez optics on a multilayer target, which contains fused silica as a fluorescent material. By measuring the fluorescence, a 5.6x4.9 microm(2) focal spot was observed in situ. Fluorescence was used to measure the saturable absorption of VUV pulses in the tin layer. The transmission increases nonlinearly higher with increasing laser intensity.

Longitudinal coherence measurements of an extreme-ultraviolet free-electron laser

We have measured the average single-pulse longitudinal coherence characteristics of FLASH, a self amplified spontaneous emission free electron laser, at extreme UV wavelengths. Electric field autocorrelation measurements in the time domain were enabled by a wavefront division beam splitter applied to a tunable delay Mach-Zehnder interferometer. These data agree with the spectral bandwidth measurements made in the frequency domain. They exhibit two correlation time scales and the measured coherence curves have relevant implications for single-shot measurements.

Dynamics of electron-phonon scattering: crystal- and angular-momentum transfer probed by resonant inelastic x-ray scattering

Experimentally, we observe angular-momentum transfer in electron-phonon scattering, although it is commonly agreed that phonons transfer mostly linear momentum. Therefore, the incorporation of angular momentum to describe phonons is necessary already for simple semiconductors and bears significant implications for the formation of new quasiparticles in correlated functional materials. Separation of linear and angular-momentum transfer in electron-phonon scattering is achieved by highly selective excitations on the femtosecond time scale of resonant inelastic x-ray scattering.

Spin-orbit mediated interference in the radiative and nonradiative channels of the La 4d core resonances

Symmetrical fluorescence yield profiles and asymmetrical electron yield profiles of the preresonances at the La N_{IV,V} x-ray absorption edge are experimentally observed in LaPO_{4} nanoparticles. Theoretical studies show that they are caused by interference effects. The spin-orbit interaction and the giant resonance produce symmetry entangled intermediate states that activate coherent scattering and alter the spectral distribution of the oscillator strength. The scattering amplitudes of the electron and fluorescence decays are further modified by the spin-orbit coupling in the final 5p;{5}epsilonl and 5p;{5}4f;{1} states.

Cold-target recoil-ion momentum spectroscopy for diagnostics of high harmonics of the extreme-ultraviolet free-electron laser light source at SPring-8

We have developed a cold-target recoil-ion momentum spectroscopy apparatus dedicated to the experiments using the extreme-ultraviolet light pulses at the free-electron laser facility, SPring-8 Compact SASE Source test accelerator, in Japan and used it to measure spatial distributions of fundamental, second, and third harmonics at the end station.

Probing the electron delocalization in liquid water and ice at attosecond time scales

We determine electron delocalization rates in liquid water and ice using core-hole decay spectroscopy. The hydrogen-bonded network delocalizes the electrons in less than 500 as. Broken or weak hydrogen bonds--in the liquid or at the surface of ice--provide states where the electron remains localized longer than 20 fs. These asymmetrically bonded water species provide electron traps, acting as a strong precursor channel to the hydrated electron.

Nonadiabatic effects in resonant inelastic x-ray scattering

We have studied the spectral features of resonant inelastic x-ray scattering of condensed ethylene with vibrational selectivity both experimentally and theoretically. Purely vibrational spectral loss features and coupled electronic and vibrational losses are observed. The one-step theory for resonant soft x-ray scattering is applied, taking multiple vibrational modes and vibronic coupling into account. Our investigation of ethylene underlines that the assignment of spectral features observed in resonant inelastic x-ray scattering of polyatomic systems requires an explicit description of the coupled electronic and vibrational loss features.

Ultrahigh-vacuum reaction apparatus to study synchrotron-radiation-stimulated processes

An ultrahigh-vacuum reaction apparatus to study synchrotron-radiation-stimulated processes has been constructed and placed on beamline 4B of the synchrotron radiation storage ring (UVSOR) at the Institute for Molecular Science. The apparatus is designed so that multiple synchrotron radiation processes such as etching and chemical vapour deposition can be carried out successively without breaking the high vacuum. It is equipped with IR reflection absorption spectroscopy (IRRAS) apparatus and reflective high-energy electron diffraction (RHEED) apparatus for in situ observations. The basic parameters of the apparatus including etching and deposition rates have been measured. IRRAS using buried metal layer substrates has been confirmed to be a very useful method of analyzing the reaction mechanisms of the synchrotron-radiation-stimulated processes.

In Situ Detection of Surface SiH(n) in Synchrotron-Radiation-Induced Chemical Vapor Deposition of a-Si on an SiO(2) Substrate

The sensitivity and linearity of infrared reflection absorption spectroscopy (IRAS) has been significantly improved by using a buried-metal-layer (BML) substrate having an SiO(2)(15 nm)/Al(200 nm)/Si(100) structure, instead of a plain Si(100) substrate. By applying this BML-IRAS technique to the in situ observation of synchrotron-radiation-induced chemical vapor deposition of amorphous Si (a-Si) on an SiO(2) surface using Si(2)H(6) gas, the vibrational spectra of surface SiH(n) species in this reaction system have been observed for the first time with sufficient sensitivity for submonolayer coverage. The main silicon hydride species after deposition at 423 K are surface SiH(2) and SiH. Surface SiH(3) and SiH(2) are observed to be easily decomposed by synchrotron radiation irradiation. The decomposition rate of SiH by synchrotron radiation irradiation is much slower than those of SiH(2) and SiH(3).