L. F. DiMauro

Ultra-fast and ultra-intense x-ray sciences: first results from the Linac Coherent Light Source free-electron laser

X-ray free-electron lasers (FELs) produce femtosecond x-ray pulses with unprecedented intensities that are uniquely suited for studying many phenomena in atomic, molecular, and optical (AMO) physics. A compilation of the current developments at the Linac Coherent Light Source (LCLS) and future plans for the LCLS-II and Next Generation Light Source (NGLS) are outlined. The AMO instrumentation at LCLS and its performance parameters are summarized. A few selected experiments representing the rapidly developing field of ultra-fast and peak intensity x-ray AMO sciences are discussed. These examples include fundamental aspects of intense x-ray interaction with atoms, nonlinear atomic physics in the x-ray regime, double core-hole spectroscopy, quantum control experiments with FELs and ultra-fast x-ray induced dynamics in clusters. These experiments illustrate the fundamental aspects of the interaction of intense short pulses of x-rays with atoms, molecules and clusters that are probed by electron and ion spectroscopies as well as ultra-fast x-ray scattering.

Femtosecond x-ray pulse length characterization at the Linac Coherent Light Source free-electron laser

Two-color, single-shot time-of-flight electron spectroscopy of atomic neon was employed at the Linac Coherent Light Source (LCLS) to measure laser-assisted Auger decay in the x-ray regime. This x-ray-optical cross-correlation technique provides a straightforward, non-invasive and on-line means of determining the duration of femtosecond (>40?fs) x-ray pulses. In combination with a theoretical model of the process based on the soft-photon approximation, we were able to obtain the LCLS pulse duration and to extract a mean value of the temporal jitter between the optical pulses from a synchronized Ti-sapphire laser and x-ray pulses from the LCLS. We find that the experimentally determined values are systematically smaller than the length of the electron bunches. Nominal electron pulse durations of 175 and 75?fs, as provided by the LCLS control system, yield x-ray pulse shapes of 120?20?fs full-width at half-maximum (FWHM) and an upper limit of 40?20?fs FWHM, respectively. Simulations of the free-electron laser agree well with the experimental results.

Strong field physics with long wavelength lasers

The generation of short, intense, mid-infrared laser pulses allows for the exploration of atom–laser interactions deep in the tunnelling regime as well as providing the ability to explore scaled interactions. In this paper we present recent experimental and theoretical results for this largely unexplored parameter space.

Molecular frame Auger electron energy spectrum from N2

Here we present the first angle-resolved, non-resonant (normal) Auger spectra for impulsively aligned nitrogen molecules. We have measured the angular pattern of Auger electron emission following K-shell photoionization by 1.1 keV photons from the Linac Coherent Light Source (LCLS). Using strong-field-induced molecular alignment to make molecular frame measurements is equally effective for both repulsive and quasi-bound final states. The capability to resolve Auger emission angular distributions in the molecular frame of reference provides a new tool for spectral assignments in congested Auger electron spectra that takes advantage of the symmetries of the final diction states. Based on our experimental results and theoretical predictions, we propose the assignment of the spectral features in the Auger electron spectrum.

Mid-infrared strong field ionization angular distributions

We present a study of the photoelectrons energy distribution from ionization of Argon by a linearly polarized, intense, mid-infrared laser field with special attention to the recently discovered Low Energy Structure (LES) [1] whose origin is not yet fully understood. In this paper we will go deeper in the analysis of the LES by studying its angular distribution and examine its behavior in circularly polarized light.

Progress report on the LCLS XFEL at SLAC

The Linac Coherent Light Source (LCLS) Project will be an x-ray free-electron laser. It is intended to produce pulses of 800-8,000 eV photons. Each pulse, produced with a repetition frequency of up to 120 Hz, will provide >1012 photons within a duration of less than 200 femtoseconds. The project employs the last kilometer of the SLAC linac to provide a low-emittance electron beam in the energy range 4-14 GeV to a single undulator. Two experiment halls, located 100 m and 350 m from the undulator exit, will house six experiment stations for research in atomic/molecular physics, pump-probe dynamics of materials and chemical processes, x-ray imaging of clusters and complex molecules, and plasma physics. Engineering design activities began in 2003, and the project is to be completed in the middle of 2010. The project design permits straightforward expansion of the LCLS to multiple undulators.

Inelastic scattering of broadband electron wave packets driven by an intense midinfrared laser field.

Intense, 100 fs laser pulses at 3.2 and 3.6 μm are used to generate, by multiphoton ionization, broadband wave packets with up to 400 eV of kinetic energy and charge states up to Xe(+6). The multiple ionization pathways are well described by a white electron wave packet and field-free inelastic cross sections, averaged over the intensity-dependent energy distribution for (e, ne) electron impact ionization. The analysis also suggests a contribution from a 4d core excitation, or giant resonance, in xenon.

Driving-frequency scaling of high-harmonic quantum paths

We have developed an analytical theory explaining how the single- atom efficiency of high-harmonic generation scales with laser frequency, and verified this by numerically solving the time-dependent Schr¨ odinger equation in three spatial dimensions. According to our saddle-point analysis of quantum paths, the imaginary part of the action has a significant impact on the scaling law. Furthermore, we found that the scaling law depends on the analytical properties of the ground-continuum transition matrix element. Our analysis elucidates how the relative contributions of different quantum orbits and their relative phases vary with the driving laser frequency and how the resulting quantum- path interferences in high-harmonic spectra can be controlled with an attosecond accuracy. 7 Author to whom any correspondence should be addressed. Content from this work may be used under the terms of the Creative Commons Attribution-NonCommercial- ShareAlike 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Strong-field physics using mid-infrared lasers

This document reports recent theoretical and experimental investigations of strong field ionization and high harmonic generation from mid-infrared lasers at 2 and 4 microns. Numerical solution of the time-dependent Schrodinger equation as well as Strong Field approximation calculations are reported. Photoelectron and high harmonic spectra are discussed. Preliminary experimental results are compared to the theoretical predictions.

Femtosecond profiling of shaped x-ray pulses

Arbitrary manipulation of the temporal and spectral properties of x-ray pulses at free-electron lasers would revolutionize many experimental applications. At the Linac Coherent Light Source at Stanford National Accelerator Laboratory, the momentum phase-space of the free-electron laser driving electron bunch can be tuned to emit a pair of x-ray pulses with independently variable photon energy and femtosecond delay. However, while accelerator parameters can easily be adjusted to tune the electron bunch phase-space, the final impact of these actuators on the x-ray pulse cannot be predicted with sufficient precision. Furthermore, shot-to-shot instabilities that distort the pulse shape unpredictably cannot be fully suppressed. Therefore, the ability to directly characterize the x-rays is essential to ensure precise and consistent control. In this work, we have generated x-ray pulse pairs via electron bunch shaping and characterized them on a single-shot basis with femtosecond resolution through time-resolved photoelectron streaking spectroscopy. This achievement completes an important step toward future x-ray pulse shaping techniques.