Two-photon ionization of atomic helium has been measured by combining femtosecond extreme-ultraviolet pulses from the free-electron laser in Hamburg ͑FLASH at DESY͒ 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. DOI: 10.1103/PhysRevA.74.011401 PACS number͑s͒: 32.80.Rm, 32.80.Fb., 42.50.Hz The achievement of short-wavelength free-electron-laser ͑FEL͒ action at DESY in the year 2000 ͓1͔, based on the process of self-amplified spontaneous emission ͑SASE͒, represented a synergistic tour de force as optical and accelerator technologies were combined to produce ultrashort laser pulses at high fundamental photon energies with high peak and average power. The first experiments on rare-gas clusters revealed new insights into intense laser-matter interactions ͓2͔. In contrast to visible and infrared laser-matter interactions, where valence electrons are the primary participants, the fundamental FEL photon energy lies far above the ionization potential of all stable matter. Under these conditions, inner-shell electrons can be excited into resonant and nonresonant continuum states ͑e.g., ͓3,4͔͒ and will be the predominant mediators of the underlying photoprocesses ͑linear and nonlinear͒.Starting in mid-2005 the free-electron laser in Hamburg ͑FLASH͒ covers a much larger wavelength range compared to the first lasing in 2000. In view of its unprecedented characteristics ͓5͔, an associated time-synchronized optical laser facility opens up new and particularly exciting research opportunities allowing the investigation of fundamental photoionization processes in intense bichromatic laser fields where one field can directly ionize valence and/or inner-shell electrons in a single step. The dynamics of various processes can be investigated, including ultrafast electronic relaxation of autoionization states ͓6͔, coupling between two autoionization states ͓7,8͔, wave-packet formation of high-lying Rydberg states ͓9͔, fast dissociation of molecules upon inner-and outer-shell photoexcitation, etc. Such a pump-probe setup has recently been implemented at the FEL facility at DESY, which provides either femtosecond or picosecond infraredvisible pulses, synchronized with a rms jitter of less than 1 ps to the femtosecond xuv pulses from the FEL ͓10͔. Here, we present experimental results obtained with this system, combined with a corresponding theoretical analysis, on the photoionization of the most prototypical of atoms for such studies-helium-in the presence of a strong o...

The experimental study of molecular dissociation of H2+ by intense laser pulses is complicated by the fact that the ions are initially produced in a wide range of vibrational states, each of which responds differently to the laser field. An electrostatic storage device has been used to radiatively cool HD+ ions enabling the observation of above threshold dissociation from the ground vibrational state by 40 fs laser pulses at 800 nm. At the highest intensities used, dissociation through the absorption of at least four photons is found to be the dominant process.

It is now well established that energetic electron emission, nonsequential ionization, and high harmonic generation, produced during the interaction of intense, femtosecond laser pulses with atoms (and atomic positive ions), can be explained by invoking rescattering of the active electron in the laser field, the so-called rescattering mechanism. In contrast for negative ions, the role of rescattering has not been established experimentally. By irradiating F ÿ ions with ultrashort laser pulses, F ion yields as a function of intensity for both linearly and circularly polarized light have been measured. We find that, at intensities well below saturation for F production by sequential ionization, there is a small but significant enhancement in the yield for the case of linearly polarized light, providing the first clear experimental evidence for the existence of the rescattering mechanism in negative ions.

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...

An electrostatic trapping scheme for use in the study of light-induced dissociation of molecular ions is outlined. We present a detailed description of the electrostatic reflection storage device and specifically demonstrate its use in the preparation of a vibrationally cold ensemble of deuterium hydride (HD + ) ions. By interacting an intense femtosecond laser with this target and detecting neutral fragmentation products, we are able to elucidate previously inaccessible dissociation dynamics for fundamental diatomics in intense laser fields. In this context, we present new results of intense field dissociation of HD + which are interpreted in terms of recent theoretical calculations.