Mitsuru Nagasono

Extreme ultraviolet free electron laser seeded with high-order harmonic of Ti:sapphire laser

The 13th harmonic of a Ti:sapphire (Ti:S) laser in the plateau region was injected as a seeding source to a 250-MeV free-electron-laser (FEL) amplifier. When the amplification conditions were fulfilled, strong enhancement of the radiation intensity by a factor of 650 was observed. The random and uncontrollable spikes, which appeared in the spectra of the Self-Amplified Spontaneous Emission (SASE) based FEL radiation without the seeding source, were found to be suppressed drastically to form to a narrow-band, single peak profile at 61.2 nm. The properties of the seeded FEL radiation were well reproduced by numerical simulations. We discuss the future precept of the seeded FEL scheme to the shorter wavelength region.

Towards time resolved core level photoelectron spectroscopy with femtosecond x-ray free-electron lasers

We have performed core level photoelectron spectroscopy on a W(110) single crystal with femtosecond XUV pulses from the free-electron laser at Hamburg (FLASH). We demonstrate experimentally and through theoretical modelling that for a suitable range of photon fluences per pulse, time-resolved photoemission experiments on solid surfaces are possible. Using FLASH pulses in combination with a synchronized optical laser, we have performed femtosecond time-resolved core-level photoelectron spectroscopy and observed sideband formation on the W 4f lines indicating a cross correlation between femtosecond optical and XUV pulses.

Compact XFEL and AMO sciences: SACLA and SCSS

The concept, design and performance of Japans compact free-electron laser (FEL) facilities, the SPring-8 Compact SASE Source test accelerator (SCSS) and SPring-8 Angstrom Compact free electron LAser (SACLA), and their applications in mainly atomic, molecular and optical science are reviewed. At SCSS, intense, ultrafast FEL pulses at extreme ultraviolet (EUV) wavelengths have been utilized for investigating various multi-photon processes in atoms, molecules and clusters by means of ion and electron spectroscopy. The quantum optical effect superfluorescence has been observed with EUV excitation. A pump?probe technique combining FEL pulses with near infrared laser pulses has been realized to study the ultrafast dynamics of atoms, molecules and clusters in the sub-picosecond regime. At SACLA, deep inner-shell multi-photon ionization by intense x-ray FEL pulses has been investigated. The development of seeded FEL sources for producing transversely and temporally coherent light, as well as the expected impact on advanced science are discussed.

Development of electron-ion coincidence spectroscopy for the study of surface dynamics combined with synchrotron radiation

Energy-selected electron-ion coincidence spectroscopy for the study of surface dynamics combined with synchrotron radiation (SR) was developed. The equipment consists of an electron gun, a cylindrical mirror analyzer (CMA), and a time-of-flight (TOF) ion mass spectrometer. A sample surface was excited by SR, and energy of the emitted electron was analyzed by the CMA. The TOF spectrum of the desorbed ions was measured taking the energy-analyzed electron signal as the starting trigger. The ions coincidently desorbed with the electron gave a characteristic peak in the TOF spectrum. The apparatus was evaluated on the basis of photoelectron–photoion coincidence (PEPICO) and Auger electron–photoion coincidence (AEPICO) measurements of H2O condensed on gold foil. The results demonstrate that PEPICO and AEPICO combined with SR are powerful methods for investigating the ion desorption induced by core-level excitations.

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.

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.

Study of ion desorption induced by a resonant core-level excitation of condensed H2O using Auger electron photo-ion coincidence (AEPICO) spectroscopy combined with synchrotron radiation

Ion desorption induced by a resonant excitation of O 1s of condensed amorphous H2O has been studied by total ion and total electron yield spectroscopy, nonderivative Auger electron spectroscopy (AES) and Auger electron photo-ion coincidence (AEPICO) spectroscopy. The spectrum of total ion yield divided by total electron yield exhibits a characteristic threshold peak at hν = 533.4 eV, which is assigned to the 4a1 ← O 1s resonant transition. The AES at the 4a1 ← O 1s resonance is interpreted as being composed of the spectator-AES of the surface H2O, and the normal-AES of the bulk H2O, where the 4a1 electron is delocalized before Auger transitions. H+ is found to be the only ion species in AEPICO spectra measured at the 4a1 ← O 1s resonance and at the O 1s ionization (hν = 560 eV). The electron kinetic energy dependence of the AEPICO yield (AEPICO yield spectrum) at the 4a1 ← O 1s resonance is found to be greatly different from that at the O 1s ionization. The peak positions of the AEPICO yield spectrum at the 4a1 ← O 1s resonance are found to correspond to those of the spectator-AES of the surface H2O, which is extracted from the AES at the 4a1 ← O 1s resonance. Furthermore, the AEPICO yield is greatly enhanced at the 4a1 ← O 1s resonance as compared with that at the O 1s ionization. On the basis of these results, a spectator-Auger-stimulated ion desorption mechanism and/or ultra-fast ion desorption mechanism are concluded to be responsible for the H+ desorption at the 4a1 ← O 1s resonance. The enhancement of the H+ yield is ascribed to the OH anti-bonding character of the 4a1 orbital.

Site-specific fragmentation following Si:2p core-level photoionization of F3SiCH2CH2Si(CH3)3 condensed on a Au surface

We used photoelectron spectroscopy and the energy-selected-photoelectron photoion coincidence method to study site-specific fragmentation following Si:2p photoionization of 1-trifluorosilyl-2-trimethylsilylethane [F3SiCH2CH2Si(CH3)3, FSMSE] condensed on a Au surface. The photoelectron spectrum of FSMSE has two peaks for 2p-electron emission: One for the Si atom bonded to three methyl groups (Si [Me]) and one for the Si atom bonded to three F atoms (Si [F]). H+ and F+ ions are predominantly desorbed coincidentally with the Si[Me]:2p and Si[F]:2p electrons.

Dissociative two-photon ionization of N2 in extreme ultraviolet by intense self-amplified spontaneous emission free electron laser light

Dissociative multiple ionization processes of N2 were investigated by irradiating N2 with an intense extreme ultraviolet (XUV) light at 50.3nm generated by a compact self amplified spontaneous emission free electron laser light source. From the analysis of the momentum distribution of N+ ejected through the Coulomb explosion of N2 and by the single-shot correlation between the yields of N2+ and N+, it was confirmed that double ionization of N2 occurred by the two-photon absorption of the XUV light.

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.