Spectroscopic characterization of vacuum ultraviolet free electron laser pulses

Abstract

Because of the stochastic nature of self-amplified spontaneous emission (SASE), it is crucial to measure for single pulses the spectral characteristics of ultrashort pulses from the vacuum ultraviolet free electron laser (FLASH) at DESY, Germany. To meet this particular challenge, we have employed both photon and photoelectron spectroscopy. Each FEL pulse is composed of an intense and spectrally complex fundamental, centered at a photon energy of about 38.5 eV, with a bandwidth of 0.5% accompanied by higher harmonics, each carrying an intensity of typically 0.3 to 0.6% of that of the fundamental. Free electron lasers (FELs), operating on the principle of self-amplified spontaneous emission 1 (SASE), open up completely new vistas in intense lasermatter interaction, as they have the potential to provide ultrashort pulses of coherent laser radiation with photon energies far above the ionization thresholds of matter. As a result, nonlinear optics and spectroscopy can be extended into the vacuum ultraviolet (VUV) region of the electromagnetic spectrum for the first time to our knowledge. In contrast with traditional visible lasers, the predominant interaction of VUV-FEL radiation is expected to be with inner-shell electrons. Hence new physical phenomena, in which electrons in resonant autoionizing 2 and continuum states 3 play an important role, are expected to be observed.Established VUV sources, such as storage-ringbased insertion devices and high harmonics of optical lasers, lack the enormous brilliance of the FLASH facility, but they do enjoy stable conditions of operation. In contrast, the statistical character of the SASE process gives rise to intensity and frequency fluctuations for each individual shot. Hence it is absolutely essential to have the capacity to determine the salient properties of the FEL beam, such as spectral distribution and intensity variations, on a shot-to-shot basis. Similar to SASE FELs in the visible frequency range, 4,5 SASE VUV-FELs emit intense odd 6 and even 7 harmonics up to the percent level. The ratio of the third harmonic to the fundamental intensity grows strongly within the linear regime of operation and is limited to a theoretical maximum of 2% when the FEL output reaches saturation.6 To comprehensively characterize the FEL spectral distribution, we combined a high-resolution grazing incidence spectrometer to record single-shot spectra, with a time-of-flight (TOF) photoelectron spectrometer capable of recording electrons produced by the fundamental, as well as the harmonics, of the FEL radiation within one single pulse.Kinetic energy analysis of the photoelectrons was obtained by using a magnetic bottle electron analyzer similar to that described in Ref. 8. Electrons produced by the interaction of an effusive gas jet and the FEL beam (focus ϳ30 m FWHM) in the acceptance volume of the analyzer are directed by a strong permanent magnet ͑0.5 T͒ towards a 65 cm long TOF tube. A solenoid ͑0.5 mT͒ then guides the electrons further. The total collection efficiency of this configurat...

Radiation characteristics of extreme UV and soft X-ray sources

“…The beam was attenuated by two filters, a 200 nm thick, meshless Zr filter and a 198 nm thick, meshless Al filter to prevent saturation of the phosphor screen in the experimental setup, shown in figure 6.1 and described below. Thus, based on previous studies on the spectral composition of the FLASH beam [153,93] and the theoretical filter transmissions [138], 80% of the beam energy are attributed to the fundamental wavelength, 18% to the third harmonic and 2% to the fifth harmonic. Since the fundamental dominates for this attenuation scheme, the beam is treated as monochromatic at λ = 24.7 nm.…”

Non-thermal desorption/ablation of molecular solids induced by ultra-short soft x-ray pulses

“…The operation of FLASH and process of ultra-intense soft x-ray radiation generation in this nearly 300m long laser system is described in [1][2][3]. Exiting the undulator, the laser beam propagates along an ultra-high vacuum (UHV) beamline involving several diagnostic devices such as pulse energy measuring gas monitor detector (GMD) [23], a variable line spacing grating spectrometer (VLS) [24], optional ionizing gas monitor measuring the beam profile, etc. A gas attenuator is used to vary the average pulse energy and a pair of selectable circular apertures allows the suppression of beam pointing instability and reduces imperfections of the beam profile in the far field.…”

A femtosecond X-ray/optical cross-correlator

“…The pulse energies reached up to 16µJ, as measured through the facility gas monitor detector 29 and independently by an intensity monitor within the PG2 beamline. With a spot size of (395 ± 23)µm x (274 ± 14)µm 30 the fluence stays well below the optical damage threshold of the GaAs surface (50 mJ/cm 2 for 30 fs pulse length at 800 nm) 31 .…”