VUV Free Electron Laser Research Articles

Observation of Carbon-Carbon Coupling Reaction in Neutral Transition-Metal Carbonyls.

Neutral titanium-metal carbonyl complexes with the chemical formula Ti(CO)n (n = 4-7) are produced in the gas phase by the reactions of titanium atoms with carbon monoxide in a pulsed laser vaporization-supersonic expansion source. Their infrared absorption spectra in the carbonyl stretching frequency region are measured by infrared plus vacuum ultraviolet (IR+VUV) two-color ionization spectroscopy based on a tunable VUV free electron laser. Infrared spectroscopy in conjunction with quantum chemical calculations confirm that all of these complexes have unexpected titanium ketenylidene OTiCCO(CO)n-2 structures. Bonding analysis indicates that the OTiCCO core structure can be described by the bonding interactions between a TiO+ cation in the doublet ground state and a doublet ground state of CCO-. The results reveal that the C-O bond breaking and C-C bond formation proceed efficiently in the reactions between laser-vaporized titanium atoms and carbon monoxide.

Infrared + vacuum ultraviolet two-color ionization spectroscopy of neutral metal complexes based on a tunable vacuum ultraviolet free-electron laser.

This paper describes an experimental technique for studying neutral metal complexes using infrared + vacuum ultraviolet (IR+VUV) two-color ionization spectroscopy based on a tunable VUV free-electron laser (VUV-FEL). The preliminary IR spectroscopy results of mass-selected nickel tetracarbonyl are reported in this work. The results demonstrate that the tunable VUV-FEL light allows the selective ionization of a given neutral cluster free of confinement along with the recording of well-resolved IR spectra. As the ionization energies of many neutral clusters are accessible by a broadly tunable VUV-FEL (50-150 nm) and near-threshold ionization can be readily achieved, the proposed experimental method offers unique possibilities for the size-specific study of a wide variety of confinement-free neutral clusters.

Physical design of a wavelength tunable fully coherent VUV source using a self-seeding free electron laser

In order to meet the requirements of the synchrotron radiation users, a fully coherent VUV free electron laser (FEL) has been preliminarily designed. One important goal of this design is that the radiation wavelength can be easily tuned in a broad range (70–170 nm). In the light of the users' demand and our actual conditions, the self-seeding scheme is adopted for this proposal. Firstly, we attempted to fix the electron energy and only changed the undulator gap to vary the radiation wavelength; however, our analysis implies that this is difficult because of the great difference of the power gain length and FEL efficiency at different wavelengths. Therefore, we have considered dividing the wavelength range into three subareas. In each subarea, a constant electron energy is used and the wavelength tuning is realized only by adjusting the undulator gap. The simulation results show that this scheme has an acceptable performance.

Time-resolved photoelectron imaging using a femtosecond UV laser and a VUV free-electron laser

A time-resolved photoelectron imaging using a femtosecond ultraviolet (UV) laser and a vacuum UV free-electron laser is presented. Ultrafast internal conversion and intersystem crossing in pyrazine in a supersonic molecular beam were clearly observed in the time profiles of photoioinzation intensity and time-dependent photoelectron images.

Large-format, high-speed, X-ray pnCCDs combined with electron and ion imaging spectrometers in a multipurpose chamber for experiments at 4th generation light sources

Fourth generation accelerator-based light sources, such as VUV and X-ray Free Electron Lasers (FEL), deliver ultra-brilliant (∼10 12–10 13 photons per bunch) coherent radiation in femtosecond (∼10–100 fs) pulses and, thus, require novel focal plane instrumentation in order to fully exploit their unique capabilities. As an additional challenge for detection devices, existing (FLASH, Hamburg) and future FELs (LCLS, Menlo Park; SCSS, Hyogo and the European XFEL, Hamburg) cover a broad range of photon energies from the EUV to the X-ray regime with significantly different bandwidths and pulse structures reaching up to MHz micro-bunch repetition rates. Moreover, hundreds up to trillions of fragment particles, ions, electrons or scattered photons can emerge when a single light flash impinges on matter with intensities up to 10 22 W/cm 2. In order to meet these challenges, the Max Planck Advanced Study Group (ASG) within the Center for Free Electron Laser Science (CFEL) has designed the CFEL-ASG MultiPurpose (CAMP) chamber. It is equipped with specially developed photon and charged particle detection devices dedicated to cover large solid-angles. A variety of different targets are supported, such as atomic, (aligned) molecular and cluster jets, particle injectors for bio-samples or fixed target arrangements. CAMP houses 4π solid-angle ion and electron momentum imaging spectrometers (“reaction microscope”, REMI, or “velocity map imaging”, VMI) in a unique combination with novel, large-area, broadband (50 eV–25 keV), high-dynamic-range, single-photon-counting and imaging X-ray detectors based on the pnCCDs. This instrumentation allows a new class of coherent diffraction experiments in which both electron and ion emission from the target may be simultaneously monitored. This permits the investigation of dynamic processes in this new regime of ultra-intense, high-energy radiation—matter interaction. After an introduction into the salient features of the CAMP chamber and the properties of the redesigned REMI/VMI spectrometers, the new 1024×1024 pixel format pnCCD imaging detector system will be described in detail. Results of tests of four smaller format (256×512) devices of identical performance, conducted at FLASH and BESSY, will be presented and the concept as well as the anticipated properties of the full, large-scale system will be elucidated. The data obtained at both radiation sources illustrate the unprecedented performance of the X-ray detectors, which have a voxel size of 75×75×450 μm 3 and a typical read-out noise of 2.5 electrons (rms) at an operating temperature of −50 °C.

RF conditioning studies of input power couplers for superconducting cavities operating in pulsed mode

We have performed systematic studies of RF conditioning on a series of 30 power couplers (the so-called TTF-III coupler) to be used with the superconducting cavities of the FLASH VUV Free Electron Laser (FEL) and, eventually, the European X-ray FEL. We have investigated the reduction in conditioning time resulting from the effects of baking-out the couplers. We show that preservation of conditioning after lengthy storage of the couplers under vacuum is conditional on this bake-out. We confirm the beneficial effect of DC bias on this coupler to counteract multipactoring. By carrying out successive variations of the control algorithm for the conditioning procedure (vacuum thresholds and interlock levels, loop control speed, pulse repetition rate), we have demonstrated a factor of three reduction in the time required to fully condition the couplers at ambient temperature on a test stand. This result has been confirmed on tests of the couplers on cavities at high gradient in cryogenic conditions.

Thomson scattering from near-solid density plasmas using soft X-ray free electron lasers

We discuss a collective Thomson scattering experiment at the VUV free electron laser facility at DESY (FLASH) to diagnose warm dense matter at near-solid density. The plasma region of interest marks the transition from an ideal plasma to a correlated and degenerate many-particle system and is of current interest, e.g., in ICF experiments or laboratory astrophysics. Plasma diagnosis of such plasmas is a longstanding issue which is addressed here using a pump–probe scattering experiment to reveal the collective electron plasma mode (plasmon) using the high-brilliance radiation to probe the plasma. Distinctive scattering features allow one to infer basic plasma properties. For plasmas in thermal equilibrium the electron density and temperature are determined from scattering off the plasmon mode.

Design considerations for table-top, laser-based VUV and X-ray free electron lasers

A recent breakthrough in laser-plasma accelerators, based upon ultrashort high-intensity lasers, demonstrated the generation of quasi-monoenergetic GeV-electrons. With future Petawatt lasers ultra-high beam currents of ∼100 kA in ∼10 fs can be expected, allowing for drastic reduction in the undulator length of free-electron-lasers (FELs). We present a discussion of the key aspects of a table-top FEL design, including energy loss and chirps induced by space-charge and wakefields. These effects become important for an optimized table-top FEL operation. A first proof-of-principle VUV case is considered as well as a table-top X-ray-FEL which may also open a brilliant light source for new methods in clinical diagnostics.

Measurements of the transverse emittance at the FLASH injector at DESY

The VUV and soft x-ray free electron laser FLASH (former VUV-FEL) is a user facility at DESY (Hamburg). In order to optimize the performance of the facility, an accurate characterization of the electron beam properties is essential. The transverse projected emittance, one of the important parameters characterizing the quality of an electron beam, is measured using a four monitor method with optical transition radiation monitors. A normalized rms emittance below 2 mm mrad for a 1 nC beam has been measured. In this paper we describe the experimental setup, data analysis methods, and present experimental results.

Reply to “Comment on ‘Photoionization of helium atoms irradiated with intense vacuum ultraviolet free-electron laser light. Part I. Experimental study of multiphoton and single-photon processes’ ”

We do not agree with the conclusion of the Comment by Charalambidis et al. questioning our observation of two-photon ionization of helium by intense radiation with $13\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ photons from a vuv free-electron laser. Two-photon ionization is clearly established by the detection of low-energy photoelectrons at $\ensuremath{\sim}1.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, which agrees very well with the expected energy for a two-photon ionization process.

Absorption of VUV-FEL radiation in warm dense matter

The interaction of ultra-intense 50 fs VUV-beams with solid-density metal and plasma targets has been simulated, solving Maxwell's equations in one-dimensional geometry. An explicit dielectric function is derived describing photo-ionization in addition to collisional absorption by conduction electrons. As an example, heating of thin aluminum foils up to electron temperatures of 450 eV is discussed, including results on emission spectra during and after the heating. The simulations correspond to experiments now in progress at DESY in Hamburg, using the new VUV Free Electron Laser.

The design and performance of CW and pulsed power couplers—A review

The design of input power couplers represents one of the most important challenges of accelerators that use super-conducting RF technology. These devices must fulfill several functions while being subject to mechanical, electromagnetic, vacuum and cryogenic constraints. The rapidly increasing number of projects, planned or under construction, which propose to use super-conducting cavities has prompted developments in power couplers for both CW and pulsed applications. Amongst the projects for which couplers have been, or are being, developed one finds VUV and X-ray free electron lasers (based on self-amplified spontaneous emission), spallation neutron sources (SNS), energy recovery linacs, and high energy colliders. We will review the design requirements and performances obtained for several of these couplers. Particular attention will be paid to the couplers which have been used on the TESLA Test Facility at DESY, the variations of this coupler which are under consideration for future synchrotron light sources and the coupler chosen for use on the SNS.

Facility Updates: Fermi @ Elettra: A Free Electron Laser for EUV and Soft X-ray Radiation

Sincrotrone Trieste has received funding and has begun the final refinement of technical parameters as well as the construction of a new Free Electron Laser (FEL) called Fermi. The new light source will be located adjacent to the existing Elettra storage ring and will use the linac that presently injects electrons into this light source. The linac will shortly become fully available to the Fermi project as a new dedicated full energy injection system is also being built for Elettra. Why are VUV and X-ray FELs so important and how do they work? Most readers will know that in a bending magnet, synchrotron radiation is created by the incoherent emission of each electron as it moves in a magnetic field.

Optical configurations for pump and probe experiments with a VUV free-electron laser

We discuss strategies for color separation and recombination, regarding 2-color pump-and-probe experiments planned for the DESY XUV FEL Phase 2. We show that efficient handling of the pump and probe components, with good spectral and spatial separation, is possible with either filters or gratings; the former are cheap while the latter are continuously tunable over a range. Nice spatial overlap of the pump and probe on a spot 5×8 μm 2 FWHM, limited by surface slope errors on the optics, can be achieved with a Kirkpatrick-Baez pair of cylindrical elliptic mirrors placed about 500 mm away from the sample position. Thermal loading is also discussed. It is found that the filters cannot survive more than a few dozen FEL pulses in a train, while a grating on a water-cooled silicon thin blade will present unacceptable surface slope errors after about 1800 FEL pulses on a train.

Multiphoton ionization of xenon in the vuv regime

In a recent experiment at the VUV free-electron laser facility at DESY in Hamburg, the generation of multiply charged ions in a gas of atomic xenon was observed. This paper develops a theoretical description of multiphoton ionization of xenon and its ions. The numerical results lend support to the view that the experimental observation may be interpreted in terms of the nonlinear absorption of several VUV photons. The method rests on the Hartree-Fock-Slater independent-particle model. The multiphoton physics is treated within a Floquet scheme. The continuum problem of the photoelectron is solved using a complex absorbing potential. Rate equations for the ionic populations are integrated to take into account the temporal structure of the individual VUV laser pulses. The effect of the spatial profile of the free-electron laser beam on the distribution of xenon charge states is included. An Auger-type many-electron mechanism may play a role in the VUV multiphoton ionization of xenon ions.

4GLS—the UK’s fourth generation light source at Daresbury: new prospects in biologicalsurface science

4GLS is a suite of accelerator-based light sources planned to provide state-of-the-artradiation in the low energy photon regime. Superconducting energy recovery linac (ERL)technology will be utilized in combination with a variety of free electron lasers (IR toXUV), undulators and bending magnets. The 4GLS undulators will be optimized togenerate spontaneous high flux, high brightness radiation, of variable polarization, from 3to 100 eV. The ERL technology of 4GLS will allow shorter bunches and higherpeak photon fluxes than possible on storage ring sources. It also provides pulsestructure flexibility and an effectively infinite beam lifetime. The XUV and VUV freeelectron lasers will be used to generate short (femtosecond regime) pulses of extremeultraviolet light that is broadly tunable and more than a million times more intensethan the equivalent spontaneous undulator radiation. A strong feature of thescientific programme planned for 4GLS is dynamics experiments in a wide range offields. Pump–probe experiments will allow the study of chemical reactions andshort-lived intermediates on the timescale of bond breaking and bond making,even from very dilute species. The high intensity of the FEL radiation will allowvery high resolution in imaging applications, using near-field approaches. Forexample, in the IR regime, resolution of the order of 30–50 nm should becomepossible, allowing the sub-cellular structure of live cells to be examined. Thecombination of high brilliance with short pulse lengths from multiple sources willallow development of techniques that probe the nature of biomolecule interactionswith surfaces. These include methods for probing conformational changes onbinding such as time-resolved sum-frequency spectroscopy and reflection anisotropyspectroscopy.

Recent developments in X-UV optics and X-UV diagnostics

Metrology of XUV beams (X-ray lasers, high-harmonic generation and VUV free-electron lasers) is of crucial importance for the development of applications. We have thus developed several new optical systems enabling us to measure the optical properties of XUV beams. By use of a Michelson interferometer working as a Fourier-transform spectrometer, the line shapes of different X-ray lasers have been measured with a very high accuracy (Δλ/λ∼10-6). Achievement of the first XUV wavefront sensor has enabled us to measure the beam quality of laser-pumped as well as discharge-pumped X-ray lasers. A capillary discharge X-ray laser has demonstrated a very good wavefront allowing us to achieve an intensity as high as 3×1014 W cm-2 by focusing with a f=5 cm mirror. The sensor accuracy has been measured using a calibrated spherical wave generated by diffraction. The accuracy has been estimated to be as good as λ/120 at 13 nm. Commercial developments are underway. At Laboratoire d’Optique Appliquée, we are setting up a new beamline based on high-harmonic generation in order to start the femtosecond, coherent XUV optic .

Progress at the Heidelberg EBIT

Two years after the relocation of the Heidelberg EBIT, several experiments are alreadyin operation. Spectroscopic measurements in the optical region have delivered the most precisereported wavelengths for highly charged ions, in the case of the forbidden transitions of Ar XIV andAr XV. The lifetimes of the metastable levels involved in those transitions has been determined withan error of less than 0.2%. A new, fully automatized x-ray crystal spectrometer allows systematicmeasurements with very high precision and reproducibility. Absolute measurements of the Lymanseries of H-like ions are currently underway. Dielectronic recombination studies have yieldedinformation on rare processes, as two-electron-one photon transitions in Ar16+, or the interferenceeffects between dielectronic and radiative recombination in Hg77+. The apparatus can now operateat electron beam currents of more than 500 mA, and energies up to 100 keV. A further beam energyincrease is planned in the near future. Ions can be extracted from the trap and transported to externalexperiments. Up to 4 × 107 Ar16+ ions per second can be delivered to a 1 cm diameter target at 10 m distance. Charge-exchange experiments with U64+ colliding with a cold He atomic beam havebeen carried out, as well as experiments aiming at the optimization of the charge state distributionof the extracted via dielectronic recombination. Two new EBITs, currently in advanced state ofconstruction in Heidelberg, will be used for experiments at the VUV free electron laser at TESLA(Hamburg) and for the charge breeding of short-lived radioactive isotopes at the TRIUMF ISACfacility.

Pump–probe experiments in the femtosecond regime, combining first and third harmonics of SASE FEL radiation

Two-color pump–probe experiments combining optical femtosecond lasers with short wavelength radiation from a free electron laser (FEL) are very attractive for sub-picosecond time-resolved studies. Since the synchronization between the two light sources to an accuracy of 100fs is not yet solved, it is proposed to derive both radiation pulses from the same electron bunch. In the present work we focus on the special case where pump and probe beams are generated by the same electron bunch in the same insertion device. Specifically we propose to combine GW-level VUV FEL pulses between 150 and 90nm wavelength and 10MW-level third-harmonic radiation between 50 and 30nm. This scheme does not require any special synchronization nor additional FEL hardware components since the nonlinear third-harmonic generation occurs naturally in the planar FEL undulator. Reflection optics are used for beam splitting and tunable delay, and the two harmonics are separated by using notch filters. The effect on the pulse duration is negligible.

Undulator system for the VUV FEL at the TESLA test facility phase-2

The Phase-1 of the VUV Free Electron Laser at the TESLA Test Facility finishes in fall 2002. Phase-2, an extension of Phase-1 towards shorter wavelengths is under construction and will be ready for operation in 2003. A radiation wavelength as low as 6nm will be obtained by raising the electron energy to 1GeV. There will be only minor changes to the undulator system. Compared to Phase-1, six instead of three undulator segments will be installed. The integrated focusing system will be replaced by an electromagnetic doublet structure. We report about the changes of the undulator, the undulator vacuum system, the separated quadrupoles including a stretched wire alignment systems and the modifications to the beam diagnostic system consisting of pick up monitors and wire scanners.