Efficient frequency doubler for the soft X-ray SASE FEL at the TESLA Test Facility

This paper describes an effective frequency doubler scheme for SASE free electron lasers. It consists of an undulator tuned to the first harmonic, a dispersion section, and a tapered undulator tuned to the second harmonic. The first stage is a conventional soft X-ray SASE FEL. Its gain is controlled in such a way that the maximum energy modulation of the electron beam at the exit is about equal to the local energy spread, but still far away from saturation. When the electron bunch passes through the dispersion section this energy modulation leads to effective compression of the particles. Then the bunched electron beam enters the tapered undulator and produces strong radiation in the process of coherent deceleration. We demonstrate that a frequency doubler scheme can be integrated into the SASE FEL at the TESLA Test Facility at DESY, and will allow to reach 3 nm wavelength with GW-level of output peak power. This would extend the operating range of the FEL into the so-called water window and significantly expand the capabilities of the TTF FEL user facility.Comment: 17 pages, 13 figure

Core Hole Double-Excitation and Atomiclike Auger Decay in N₂

Core hole decay spectra of the free N2 molecule show evidence for hitherto unobserved molecular resonances both below and above the 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

Angle-resolved photoelectron spectroscopy of the core levels of N_2O

We have measured photoionization cross sections and photoelectron asymmetry parameters for each of the core levels of N_2O. We have also carried out frozen‐ and relaxed‐core Hartree–Fock studies of these cross sections so as to better understand the underlying shape resonant structure and the role of electronic relaxation in these processes. A broad shape resonance is observed in each of the core‐hole cross sections at 10‐20 eV kinetic energy and there is some evidence of a second shape resonance near the thresholds, an energy region which is not accessible experimentally. The cross sections also exhibit site‐specific behavior with maxima at widely separated photoelectron kinetic energies. These differences probably arise from the fact that photoelectron matrix elements for different core orbitals probe different regions of the shape resonant orbital which extends over the entire molecule. Although the higher energy shape resonances appear quite similar, Hartree–Fock studies show that the central nitrogen resonance is more sensitive to effects of electronic relaxation than the terminal nitrogen or oxygen resonances. Large differences are also seen between the photoelectron asymmetry parameters for the central and terminal atoms

Angle‐resolved photoelectron spectroscopy of the core levels of N₂O

We have measured photoionization cross sections and photoelectron asymmetry parameters for each of the core levels of N2O. We have also carried out frozen‐ and relaxed‐core Hartree–Fock studies of these cross sections so as to better understand the underlying shape resonant structure and the role of electronic relaxation in these processes. A broad shape resonance is observed in each of the core‐hole cross sections at 10‐20 eV kinetic energy and there is some evidence of a second shape resonance near the thresholds, an energy region which is not accessible experimentally. The cross sections also exhibit site‐specific behavior with maxima at widely separated photoelectron kinetic energies. These differences probably arise from the fact that photoelectron matrix elements for different core orbitals probe different regions of the shape resonant orbital which extends over the entire molecule. Although the higher energy shape resonances appear quite similar, Hartree–Fock studies show that the central nitrogen resonance is more sensitive to effects of electronic relaxation than the terminal nitrogen or oxygen resonances. Large differences are also seen between the photoelectron asymmetry parameters for the central and terminal atoms

High-resolution C 1s photoelectron spectra of methane

The C 1s partial photoionization cross section and photoelectron angular distribution of methane (CH4) 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 1s−1 photoelectron line a vibrational energy of 396±2 meV and an equilibrium bond length of 1.039(±0.001) Å for the CH+4 ion have been determined. The lifetime broadening was found to be 83(±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)−1(Val)−1(Ryd)1a(Ryd)1b, 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

Velocity-selective sublevel resonance of atoms with an array of current-carrying wires

Resonance transitions between the Zeeman sublevels of optically-polarized Rb atoms traveling through a spatially periodic magnetic field are investigated in a radio-frequency (rf) range of sub-MHz. The atomic motion induces the resonance when the Zeeman splitting is equal to the frequency at which the moving atoms feel the magnetic field oscillating. Additional temporal oscillation of the spatially periodic field splits a motion-induced resonance peak into two by an amount of this oscillation frequency. At higher oscillation frequencies, it is more suitable to consider that the resonance is mainly driven by the temporal field oscillation, with its velocity-dependence or Doppler shift caused by the atomic motion through the periodic field. A theoretical description of motion-induced resonance is also given, with emphasis on the translational energy change associated with the internal transition.Comment: 7 pages, 3 figures, final versio

First Observation of Self-Amplified Spontaneous Emission in a Free-Electron Laser at 109 nm Wavelength

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.Comment: 6 pages including 6 figures; e-mail: [email protected]

Genetically Engineered Alginate Lyase-PEG Conjugates Exhibit Enhanced Catalytic Function and Reduced Immunoreactivity

Alginate lyase enzymes represent prospective biotherapeutic agents for treating bacterial infections, particularly in the cystic fibrosis airway. To effectively deimmunize one therapeutic candidate while maintaining high level catalytic proficiency, a combined genetic engineering-PEGylation strategy was implemented. Rationally designed, site-specific PEGylation variants were constructed by orthogonal maleimide-thiol coupling chemistry. In contrast to random PEGylation of the enzyme by NHS-ester mediated chemistry, controlled mono-PEGylation of A1-III alginate lyase produced a conjugate that maintained wild type levels of activity towards a model substrate. Significantly, the PEGylated variant exhibited enhanced solution phase kinetics with bacterial alginate, the ultimate therapeutic target. The immunoreactivity of the PEGylated enzyme was compared to a wild type control using in vitro binding studies with both enzyme-specific antibodies, from immunized New Zealand white rabbits, and a single chain antibody library, derived from a human volunteer. In both cases, the PEGylated enzyme was found to be substantially less immunoreactive. Underscoring the enzyme's potential for practical utility, >90% of adherent, mucoid, Pseudomonas aeruginosa biofilms were removed from abiotic surfaces following a one hour treatment with the PEGylated variant, whereas the wild type enzyme removed only 75% of biofilms in parallel studies. In aggregate, these results demonstrate that site-specific mono-PEGylation of genetically engineered A1-III alginate lyase yielded an enzyme with enhanced performance relative to therapeutically relevant metrics.Cystic Fibrosis Foundation (Research Development Program)National Center for Research Resources (U.S.) (P20RR018787-06