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In this paper, we report results on two-colour above-threshold ionisation, where extreme ultraviolet pulses of femtosecond duration were synchronised to intense infrared laser pulses of picosecond duration, in order to study the laser-assisted photoelectric effect of atomic helium, neon, krypton and xenon which leads to the appearance of characteristic sidebands in the photoelectron spectra. The observed trends are found to be well described by a simple model based on the soft-photon approximation, at least for the relatively low optical intensities of up to 5x10$^{11}$ Wcm$^{-2}$ employed in these early experiments.

FLASH at DESY in Hamburg, the first and currently only free-electron laser for VUV and soft X-ray radiation, started user operation in the summer of 2005. Currently it covers a wavelength range from 13 nm to 50 nm with GW peak power and pulse durations between 10 fs and 50 fs. Approximately 20 weeks of beamtime are provided per year for scientific experiments that are not possible on any other radiation source.

In a free-electron laser (FEL) the lasing medium is a high-energy beam of electrons flying with relativistic speed through a periodic magnetic field. The interaction between the synchrotron radiation that is produced and the electrons in the beam induces a periodic bunching of the electrons, greatly increasing the intensity of radiation produced at a particular wavelength. Depending only on a phase match between the electron energy and the magnetic period, the wavelength of the FEL radiation can be continuously tuned within a wide spectral range. The FEL concept can be adapted to produce radiation wavelengths from millimetres to Angstroms, and can in principle produce hard x-ray beams with unprecedented peak brightness, exceeding that of the brightest synchrotron source by ten orders of magnitude or more. This paper focuses on short-wavelength FELs. It reviews the physics and characteristic properties of single-pass FELs, as well as current technical developments aiming for fully coherent x-ray radiation pulses with pulse durations in the 100 fs to 100 as range. First experimental results at wavelengths around 100 nm and examples of scientific applications planned on the new, emerging x-ray FEL facilities are presented.

A nondestructive radiation detector unit is under construction at the TESLA Test Facility at DESY providing monitoring of the radiation (pulse energy, transverse intensity distribution, and statistical properties). The concept of the detector design is based on experience obtained during operation of TTF FEL, Phase 1. Key element of the detector is a wide dynamic range micro-channel plate (MCP) which detects scattered radiation from a thin gold wire crossing photon beam. Operating wavelength range of the detector is from 6 to 100 nm . An important feature of the MCP-based detector is that it is capable to cover all dynamic range of the radiation intensity, from the level of spontaneous emission up to the saturation level of SASE FEL.

Gigawatt level radiation pulses at wavelengths between 80 and 120 nm and a pulse duration below 100 fs have recently been produced by a single-pass free electron laser at DESY. The radiation has been characterized with respect to pulse energy, statistical properties, spectral distribution and coherence. It has also been used for first experiments on solids and gases demonstrating the potential of the new source. The facility is currently upgraded to higher energy and is planned to start user operation in 2004. Based on this work a large free electron laser laboratory with several experimental stations for the hard X-ray regime down to 1 A will be built at DESY which could start user operation at the beginning of the next decade.

During the last few years free electron lasers (FELs) based on self-amplified spontaneous emission (SASE) have been demonstrated at wavelengths of 12 μm [1], 830 nm [2], 530 nm [3] and 385 nm [3], and around 100 nm [4]. Recently, saturation has been observed in the vacuum ultraviolet (VUV) spectral region between 82 nm and 125 nm at the TESLA Test Facility (TTF) at DESY. The radiation pulses have been characterized with respect to pulse energy, statistical fluctuations, angular divergence and spectral distribution, both in the linear gain and in the saturation regime of the FEL [5-6]. The results are in good agreement with theoretical simulations, providing a solid basis for other projects aiming at still shorter wavelengths down to the 0.1 nm range [7-8].

A beamline designed to reduce the spectral bandwidth of the DESY XUV FEL is described. The beamline is intended to cover the wavelength range from 6.4 to 50 nm with three variable line spacing gratings. A plane mirror in front of the grating is used to maintain constant magnification in the dispersion direction. The electric field generated by the first undulator at three wavelengths, 6.4, 13, and 25 nm, is propagated through the beamline. The results show that the beamline has the resolution and imaging properties required for seeding the second undulator at these wavelengths.

First lasing of a single-pass free electron laser below 100 nm wavelength was recently observed at the TESLA Test Facility at DESY, after a singular experiment with very high gain at Los Alamos National Laboratory at 12 μm wavelength in 1997 and a very recent experiment at 530 nm at Argonne National Laboratory. These novel devices are based on the principle of self-amplified spontaneous emission (SASE), allowing very high gain up to saturation in a single pass through a long periodic magnetic structure. This concept does not require any optical components and promises to produce intense, sub-picosecond laser pulses for a very wide, continuous range of wavelengths down to approximately 0.1 nm. The experimental results at DESY and the other laboratories are in agreement with theory and indicate that SASE FELs will soon be available for experiments in the VUV and soft X-ray region. This paper gives an overview of the status of the present SASE projects and the anticipated research and development in the near future. It focuses particularly on the recent results at DESY and the current program towards a VUV user facility.

The local structure of the CuO2 planes in Nd2−xCexCuO4−δ (x=0, 0.15; δ∼0.01) has been examined by temperature-dependent Cu K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. Three different Cu–O distances can be extracted up to at least 100 K, indicating that the atomic structure of the CuO2 planes deviates from the T′ structure much more than it was expected. Positions of the displaced O1 atoms for x=0.15 were obtained from comparative analysis of the experimental and calculated polarized Cu K-edge X-ray absorption near-edge structure, taking into account many-body excitations in the CuO2 planes.

The angle-resolved N 1s photoelectron spectrum of N2 has been measured with high resolution in the threshold region. An analysis of the vibrational fine structure yields a vibrational energy of 314(4) meV and a lifetime broadening of 103(10) meV for the core-ionized molecule. As in the case of C 1s ionization of CO, the vibrational fine structure changes considerably with photon energy in the region of both the sigma* shape resonance and the double excitations. Both the vibrationally resolved partial photoionization cross sections and asymmetry parameters have been determined in this photon energy range. The calculated cross sections in the literature are in reasonable agreement with experiment. In addition, we have measured the cross sections and asymmetry parameters of the most intense direct and conjugate shake-up satellites. The behaviour of the pi-pi* satellites is found to differ significantly from that of the corresponding C 1s satellites of CO.

The partial photoionization cross sections and angular distributions of the C 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$ and O 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$ single-hole states of ${\mathrm{CO}}_{2}$ have been measured with high precision near threshold. The 4${\mathrm{\ensuremath{\sigma}}}_{\mathit{u}}^{\mathrm{*}}$ shape resonance is well reproduced in the 1s single-hole cross sections, but strong site-specific effects are observed with respect to both its energy and intensity. A striking feature of the present data is the presence of fine structure in the 1s single-hole cross sections ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{SH}}$ due to multielectron excitations: virtually all spectral details in the total 1s cross sections ${\mathrm{\ensuremath{\sigma}}}_{1\mathit{s}}$ are also present in ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{SH}}$ implying that multielectron excitations couple strongly to the underlying 1s continuum. The decay dynamics of the double-excitation feature at 303.5 eV in the C 1s cross section of ${\mathrm{CO}}_{2}$ have been characterized by Auger spectroscopy. Partial cross sections and angular distributions have also been measured for the first group of strong shake-up satellites in the C 1s photoelectron spectrum. Similar to the C 1s ionization of CO, a strong enhancement of intensity near threshold is observed for some satellite channels as well as the occurrence of features that are not present in the sudden limit. The latter are attributed to conjugate shake-up.

A substantial enhancement of intensity in the high-energy part of the ${\mathrm{N}}_{2}$ KLL Auger spectrum has been observed at photon energies a few tens of eV above the N 1s threshold. By monitoring the corresponding 1s photoelectron spectrum, we show that the effect is due to the emergence of strong conjugate shake-up satellites. As shown in previous e-2e studies for CO, such Auger spectra are useful in determining the intensity behavior of shake-up satellites in the immediate threshold regions.

FLASH has been a user facility since 2005, delivering radiation in the wavelength range between 7 and 47 nm using the SASE principle. After the present upgrade, the wavelength range is extended to 4.45 nm. With the third harmonic accelerating module in place to linearize the longitudinal phase space, the stability and reproducibility of the machine is substantially improved. The user requests for beam time by far exceeds the time available. In order to increase user beam time and to improve the radiation properties delivered to users, a major extension of the user facility called FLASH II has been proposed by DESY in collaboration with the HZB. FLASH II is a seeded FEL in the parameter range of FLASH. As logical continuation, the seeding with HHG which started with sFLASH will result in direct seeding. Because in the foreseeable future there will probably not be HHG seed lasers available at high repetition rates down to wavelengths of 4 nm, a cascaded HGHG scheme is proposed to produce short wavelengths. After a first design report, the project now enters its technical design phase. During this time, the FLASH beam parameters after the present upgrade 2009/2010 will be characterized and the present design will be re-evaluated and adjusted. In addition, start-to-end simulations will complete the simulations which have been performed so far, including a design of the extraction area.

FLASH (free electron laser in Hamburg) is a unique, ultra-brilliant soft x-ray source providing highly coherent femtosecond pulses, currently in a wavelength range of 6.8–47 nm. Up to several 1012 coherent photons within a 10–70 fs pulse allow the study of dynamical changes in nanometer-sized structures. This is a big step towards the ultimate goal of observing femtosecond dynamics on the atomic length scale, for example 'watching bio-machines at work'. In this review, the properties of FLASH are summarized with a focus on coherence, and the potential of FLASH for structural investigations is illustrated with an overview of the recently performed coherent imaging experiments.

The X1 undulator on the X-ray ring of the NSLS at Brookhaven is an extremely bright source of soft X-ray photons. On the recently completed spectroscopy beamline X1B we have attempted to utilise the intrinsically narrow beam size and divergence to maximise the intensity throughput in a spherical grating monochromator (SGM). The high power density of the undulator radiation requires the use of water cooled mirrors, entrance slit and grating. Calculations predict that simple side cooled copper pads attached to optical elements will be sufficiently effective that resolution degradation due to thermal deformation of figure error will be negligible in the SGM design. This is confirmed by attainment of resolution superior to other SGMs and the BESSY SX700-II monochromator over the energy range 300–540 eV (C1s−1, N1s−1 and Ols−1). At a resolution comparable to the best so far obtained for the nitrogen N 1s π∗ excitation (401 eV) on other monochromators we have demonstrated an intensity increase, at the sample, of more than three orders of magnitude. The design consideration upon which beamline X1B is based are highly relevant to the construction of soft X-ray monochromators at future synchrotron radiation sources such as the ALS (Berkeley). Elettra (Trieste) and BESSYII (Berlin).

We describe an angle‐resolving photoelectron spectrometer for atoms and molecules which uses the magic angle geometry in combination with multidetection such that cross sections and asymmetry parameters can be determined simultaneously. The instrument is based on the cylindrical mirror analyzer (CMA) design with the cylinder axis and the light beam collinear. Only the photoelectrons which are emitted in the ‘‘reverse’’ direction at the magic angle reach the ring‐shaped position‐sensitive detector. The complete system also incorporates a conical effusive gas source, in order to maintain cylindrical symmetry, and very efficient differential pumping between target and electron spectrometer. Results from the C 1s photoionization of CO2 demonstrate the kind of precision attainable.

The layout and first data on the performance of the soft x‐ray undulator beam line on the x‐ray ring at Brookhaven are described. The undulator (X1) has 35 periods of 8 cm, resulting in a tuning range for the fundamental of about 200–700 eV at the nominal ring energy of 2.5 GeV. The monochromator is based on the Dragon concept with spherical grating, movable exit slit but with water‐cooled optical elements in order to deal with the maximum flux of ca. 400 W. This system represents one of the next generation of soft x‐ray undulator beam lines which will figure prominently at new synchrotron radiation facilities such as ALS, ELETTRA and BESSY II. During the first performance tests absorption spectra of simple molecules at the C, N, and O K edges have been recorded with resolution and flux so far unattained in the soft x‐ray region.

Using a noninvasive, electro-optically based electron bunch arrival time measurement at FLASH (free electron laser in Hamburg) the temporal resolution of two-color pump-probe experiments has been significantly improved. The system determines the relative arrival time of the extended ultraviolet pulse of FLASH and an amplified Ti:sapphire femtosecond-laser pulse at the interaction region better than 90 fs rms. In a benchmarking pump-probe experiment using two-color above threshold ionization of noble gases, an enhancement in the timing resolution by a factor of 4 compared to the uncorrected data is obtained.

Two-color multiphoton ionization of atomic helium was investigated by combining extreme ultraviolet (XUV) radiation from the Free Electron Laser in Hamburg with an intense synchronized optical laser. In the photoelectron spectrum, lines associated with direct ionization and above-threshold ionization show strong variations of their amplitudes as a function of both the intensity of the optical dressing field and the relative orientation of the linear polarization vectors of the two fields. The polarization dependence provides direct insight into the symmetry of the outgoing electrons in above-threshold ionization. In the high field regime, the monochromaticity of the XUV radiation enables the unperturbed observation of nonlinear processes in the optical field.

Techniques to characterize attosecond harmonic pulses based on thecross-correlation between harmonic and IR laser pulses are presented. The selfcompressionof an attosecond harmonic pulse in the harmonic generation itself has been demonstrated, obtaining near transform-limited attosecond harmonic pulses.

We describe the layout and expected performance of the soft X-ray spectroscopy beamline, which is currently being installed at the 35 pole undulator on the X-ray ring at NSLS Brookhaven. We present our rationale for the choice of monochromator and discuss some of the experimental program planned for this new high resolution instrument.

Two-color above threshold ionization of helium and xenon has been used to analyze the synchronization between individual pulses of the femtosecond extreme ultraviolet (XUV) free electron laser in Hamburg and an independent intense 120 fs mode-locked Ti:sapphire laser. Characteristic sidebands appear in the photoelectron spectra when the two pulses overlap spatially and temporally. The cross-correlation curve points to a 250 fs rms jitter between the two sources at the experiment. A more precise determination of the temporal fluctuation between the XUV and infrared pulses is obtained through the analysis of the single-shot sideband intensities.

We describe an experimental system designed for single-shot photoelectron spectroscopy on free atoms and molecules at the Free Electron Laser in Hamburg (FLASH at DESY). The combination of the extreme ultra-violet (EUV) Free Electron Laser and a temporally synchronized optical fs laser (Ti:Sapphire) enables a variety of two-color pump-probe experiments. The spectral, temporal and spatial characteristics of both the EUV FEL and the optical laser pulses, the experimental procedure to control their overlap as well as the performance of an electron spectrometer used to obtain single-shot photoelectron spectra are discussed. As an illustration of the capabilities of this set-up, some results on two-photon two-color ionization of rare gases are presented.

Two-photon ionization of atomic helium has been measured by combining femtosecond extreme-ultraviolet pulses from the free-electron laser in Hamburg (FLASH at DESY) with intense light pulses from a synchronized neodymium-doped yttrium lithium fluoride laser. Sidebands appear in the photoelectron spectra when the two laser pulses overlap in both space and time. Their intensity exhibits a characteristic dependence on the relative time delay between the ionizing and the dressing pulses and provides an inherent time marker for time-resolved pump-probe experiments. The measurements of the sidebands are in good agreement with theoretical predictions and allow for a direct analysis of two-photon ionization, free from processes related to interference between multiple quantum paths.

Photon beam diagnostics for vacuum‐ultraviolet free electron lasers (VUV FEL) are critical to monitoring and understanding their performance characteristics. Due to the shot‐to‐shot fluctuations inherent in FELs based on the self amplified spontaneous emission (SASE) process, it is mandatory to use pulse‐resolved diagnostics. We have designed a spectrograph based on a variable‐line‐spacing (VLS) plane grating and a phosphor/CCD to monitor single shot spectra of the free electron laser at DESY. The basic concept is to allow most of the beam to be reflected towards an experimental station while the first order light is dispersed and focused by the VLS grating onto the CCD. The spectrograph will cover the wavelength range 6.4–60 nm with the CCD accepting a bandwidth of ∼10%. The grazing angle of incidence on the grating is 2°, the central line density is 1200 l/mm, and the distance grating‐CCD is approximately 2 m. The linear variation of the grating line spacing combined with positioning the detector at the focal curve, allows zeroing the defocus in the full spectrograph wavelength range. The correction of higher order grating aberrations yields a theoretical resolving power greater than 20000 over the full length of the 20 mm CCD when the CCD is positioned tangent to the focal plane. Based on power considerations, a shallow blazed grating is the preferred profile. Efficiency calculations over the spectrograph range show that with a carbon coating the absolute efficiency for zeroth order is higher than 0.85 and the first order efficiency varies between 0.5% and 8%.

Absolute inner‐shell photoabsorption cross‐section measurements were obtained from a variety of simple gas‐phase molecules containing low Z elements (C, N, O, F), independent of the effects of stray light, high‐order light, and photoabsorption from the various optical elements. The experimental apparatus consists of (1) a compact high‐order light suppressor based on the photon energy dependence of the grazing angle reflectivity of various materials which provides an adjustable cut‐off energy, and (2) a gas‐phase absorption cell which monitors the photon flux before and after the gas cell to yield measurements independent of the absorptions of various optical elements, response functions of the photon detectors used, and variations of the photon flux during the scans.

In this paper, we describe optical feedback system of VUV Regenerative FEL Amplifier (RAFEL) at the TESLA test facility at DESY. The aim of the RAFEL experiment is to construct fully coherent, tunable VUV radiation source by means of applying narrow-band optical feedback in the VUV SASE FEL operating currently at DESY. One of the problem of the realization of the RAFEL is severe requirements for the angular stability of the optical elements (about few microradians). This problem has been solved by means of installation of active alignment system with reference laser. Another problem is alignment of optical elements separated by 65 m within complicated experimental conditions connected with aperture limitations (down to 6 mm ) . This problem has been solved in two steps. Preliminary alignment with an accuracy of about 80 μrad has been performed with laser alignment system and OTR screens used at the TTF accelerator for electron beam diagnostics. Final alignment has been performed with VUV SASE FEL radiation. Measured feedback coefficient is about 1 percent and is in agreement with the designed value.

We propose to use Thomson backscattering of far-infrared (FIR) pulses (100–300 μm wavelength range) by a 500 MeV electron beam to generate femtosecond X-rays at the TESLA Test Facility (TTF) at DESY. Using the parameters of the photocathode rf gun and the magnetic bunch compressors of the TESLA Test Facility (TTF), it is shown that electron pulses of 100-fs (FWHM) duration can be generated. Passing the short electron bunches through an undulator (after the conversion point) can provide a FIR high-power source with laser-like characteristics. On the basis of the TTF parameters we expect to produce X-ray pulses with 100-fs duration, an average brilliance of nearly 1013 photons s−1 mrad−2 mm−2 per 0.1% BW at a photon energy 50 keV. The total number of Thomson backscattered photons, produced by a single passage of the electron bunch through the mirror focus, can exceed 107 photons/pulse. We also describe the basic ideas for an upgrade to shorter X-ray pulse duration. It is demonstrated that the TTF has the ca...

We present the first observation of Self-Amplified Spontaneous Emission (SASE) in a free-electron laser (FEL) in the Vacuum Ultraviolet regime at 109 nm wavelength (11 eV). The observed free-electron laser gain (approx. 3000) and the radiation characteristics, such as dependency on bunch charge, angular distribution, spectral width and intensity fluctuations all corroborate the existing models for SASE FELs., 6 pages including 6 figures; e-mail: joerg.rossbach@desy.de

19th International Conference on Free Electron Lasers, FEL'97, Beijing, China, 18 Aug 1997 - 21 Aug 1997; TESLA-FEL Reports 36-39 (1997).

The C 1s partial photoionization cross section and photoelectron angular distribution of methane (${\mathrm{CH}}_{4}$) have been measured with high-energy resolution between threshold and 385 eV photon energy. From the analysis of the vibrational fine structure on the C 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$ photoelectron line a vibrational energy of 396\ifmmode\pm\else\textpm\fi{}2 meV and an equilibrium bond length of 1.039(\ifmmode\pm\else\textpm\fi{}0.001) \AA{} for the ${\mathrm{CH}}_{4}^{+}$ ion have been determined. The lifetime broadening was found to be 83(\ifmmode\pm\else\textpm\fi{}10) meV. The weak feature in the photoabsorption cross section just above threshold does not influence the vibrational fine structure in a way typical for a shape resonance. We therefore suggest that it is due to doubly excited states of the type C (1s${)}^{\mathrm{\ensuremath{-}}1}$(Val${)}^{\mathrm{\ensuremath{-}}1}$(Ryd${)}_{\mathit{a}}^{1}$(Ryd${)}_{\mathit{b}}^{1}$, an assignment which is supported by recent Auger decay studies. Measurements of the shakeup structure revealed six satellite lines, one of which increases strongly in intensity at threshold, thus pointing to the existence of a conjugate shakeup process. \textcopyright{} 1996 The American Physical Society.

Core hole decay spectra of the free N{sub 2} molecule show evidence for hitherto unobserved molecular resonances both below and above the {ital K}-shell photoionization threshold. Based on earlier calculations they are assigned to doubly excited neutral states which could not be seen below threshold in recent high resolution absorption spectra because of the more intense core-to-Rydberg excitations. By calculating the Auger spectrum of core-excited nitrogen atoms, we show that the features are atomiclike. {copyright} {ital 1996 The American Physical Society.}

A new design for a single pass X-ray Self-Amplified Spontaneous Emission (SASE) FEL is proposed. The scheme consists of two undulators and an X-ray monochromator located between them. The first stage of the FEL amplifier operates in the SASE linear regime. After the exit of the first undulator the electron bunch is guided through a non-isochronous bypass and the X-ray beam enters the monochromator. The main function of the bypass is to suppress the modulation of the electron beam induced in the first undulator. This is possible because of the finite value of the natural energy spread in the beam. At the entrance to the second undulator the radiation power from the monochromator dominates significantly over the shot noise and the residual electron bunching. As a result the second stage of the FEL amplifier operates in the steady-state regime when the input signal bandwidth is small with respect to that of the FEL amplifier. Integral losses of the radiation power in the monochromator are relatively small because grazing incidence optics can be used. The proposed scheme is illustrated for the example of the 6 nm option SASE FEL at the TESLA Test Facility under construction at DESY. As shown in this paper the spectral bandwidth of such a two-stage SASE FEL (Δλ/λ⋍ 5 × 10 −5 ) is close to the limit defined by the finite duration of the radiation pulse. The average brilliance is equal to 7 × 10 24 photons/(s × mrad 2 × mm 2 × 0.1% bandw.) which is by two orders of magnitude higher than the value which could be reached by the conventional SASE FEL. The monochromatization of the radiation is performed at a low level of radiation power (about 500 times less than the saturation level) which allows one to use conventional X-ray optical elements (grazing incidence grating and mirrors) for the monochromator design.

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.

In vast regions of the universe highly charged ions (HCI [1, 2]) are the predominant form of visible matter. Their importance extends to high-temperature terrestrial plasmas, such as those used in fusion research. Yet, accurate prediction of their electronic structure remains a challenge for theory due to the strong electromagnetic field in which the remaining bound electrons dwell. Experimental accuracy has now reached the performance limits of conventional photon spectroscopy in the soft and hard x-ray regions. In this work [3], we report on the resonant laser excitation of the 22S1/2—22P1/2 transition of the Li-like Fe22P1/223+ ion at 48.6 eV, an energy range hitherto unattainable with powerful lasers. The HCI stored in an electron beam ion trap (EBIT [4]) were resonantly excited by ultra-brilliant radiation generated at the Free electron LASer in Hamburg (FLASH [5]). While yielding a relative statistical error of only 2.2·10−5, and extending laser spectroscopy on HCI from the near ultraviolet [6] to the soft X-ray region, this novel experiment demonstrates immediate potential to push the current limits of precision by orders of magnitude. Such experiments allow to verify predictions of quantum electrodynamics (QED) in a strong field environment where perturbation theory [7, 8] fails. Future EBIT experiments at upcoming x-ray free electron lasers (X-FEL) like the Stanford Linear Coherent Light Source (LCLS) or the European X-FEL will pave the way for laser spectroscopy into the hard x-ray region.

The authors present the first simultaneous studies of fragment ion production and electron emission of a molecule following deep core-level excitation. Using synchrotron radiation they tune into various absorption resonances and correlate certain electron-hole states in the decay of the x-ray excitation with the appearance of specific ionic fragments. For N/sub 2/ specifically they observe a 1:1 correspondence between single valence hole states and undissociated N/sub 2//sup +/ after N 1s..--> pi..( excitation and relate the production of N/sup + +/ ions to the generation of holes in the 2sigma/sub u/ orbital.

The cross sections and asymmetry parameters of the two $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}^{2}\ensuremath{\Sigma}^{+}$ shakeup satellites on $\mathrm{C} 1s$ in the photoelectron spectrum of CO are found to differ significantly in the threshold region. The first satellite (due to triplet $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ coupling) shows intensity enhancement near threshold whereas the second satellite (singlet $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ coupling) decreases in intensity as expected for the adiabatic limit. Moreover, the data indicate that a shape resonance in the first-satellite channel is responsible for the observed effect rather than a conjugate shakeup process.

Core level photoemission spectra of CO and N2 using snyhrotron radiation excitation have been measured in order to study the near threshold photon energy range of the K-shell satellites. The C 1s and N 1s π- π* satellites show very similar behavior characterized by increasing intensity of the first satellite and decreasing intensity of the second satellite towards threshold. In contrast to this characteristic behavior there is nothing comparable in the O 1s satellite spectrum. The results are discussed on the basis of the equivalent core model, potential theory and charge transfer effects.

We report on a 96 kHz repetition rate sub-10 fs, optical parametric chirped pulse amplifier system with 6.7 W average power and >5 GW peak power. Furthermore the oscillator is CEP stabilized and the CEP of the amplified pulses is measured.