Researchers use single-cycle THz pulses from an optical laser to extend streaking techniques of attosecond metrology to measure the temporal profile and arrival time of individual FEL pulses with ∼5 fs precision.

Ultrafast X-ray pulse characterization at free-electron lasers

Free-electron lasers are fourth-generation light sources that deliver extremely intense (>1012 photons per pulse), ultrashort (∼10−14 s = 10 fs) light pulses at up to kilohertz repetition rates with unprecedented coherence properties and span a broad wavelength regime from soft (∼10 eV) to hard X-ray energies (∼15 keV). They thus enable a whole suite of novel experiments in molecular physics and chemistry: Inspecting radiation-induced reactions in cold molecular ions provides unprecedented insight into the photochemistry of interstellar clouds and upper planetary atmospheres; double core-hole photoelectron spectroscopy offers enhanced sensitivity for chemical analysis; the dynamics of highly excited molecular states, pumped by vacuum ultraviolet pulses, can be inspected; and vacuum ultraviolet or X-ray probe pulses generally hold the promise to trace chemical reactions along an entire reaction coordinate with atomic spatial and temporal resolution. This review intends to provide a first overview on upcomi...

Free-Electron Lasers: New Avenues in Molecular Physics and Photochemistry

For a fundamental understanding of ultrafast dynamics in chemistry, biology and materials science it has been a long-standing dream to record a molecular movie in which both the atomic trajectories and the chemical states of every atom in matter are followed in real time1. Free-electron lasers provide this perspective as they deliver brilliant femtosecond X-ray pulses spanning a wide photon energy range, which is necessary to gather element-specific and chemical-state-selective information with femtosecond time resolution. The key challenge lies in synchronizing the free-electron lasers with separate optical lasers. We exploit the peak brilliance of the free-electron laser in Hamburg2,3 (FLASH) and establish X-ray- pulse-induced transient changes of the optical reflectivity in GaAs as a powerful tool for X-ray/optical cross-correlation. This constitutes a breakthrough in the path towards recording a molecular movie and—equally importantly—opens up the field of femtosecond X-ray-induced dynamics, only accessible with high-brilliance X-ray free-electron lasers.

/pdf/a-femtosecond-x-ray-optical-cross-correlator-3y0neiptmk.pdf

A femtosecond X-ray/optical cross-correlator

We present a new technique for measuring the relative delay between a soft x-ray FEL pulse and an optical laser that indicates a sub 25 fs RMS measurement error. An ultra-short x-ray pulse photo-ionizes a semiconductor (Si3N4) membrane and changes the optical transmission. An optical continuum pulse with a temporally chirped bandwidth spanning 630 nm – 710 nm interacts with the membrane such that the timing of the x-ray pulse can be determined from the onset of the spectral modulation of the transmitted optical pulse. This experiment demonstrates a nearly in situ single-shot measurement of the x-ray pulse arrival time relative to the ultra-short optical pulse.

Spectral encoding of x-ray/optical relative delay

The advent of free electron laser (FEL) facilities capable of delivering high intensity pulses in the extreme-UV to X-ray spectral range has opened up a wide vista of opportunities to study and control light matter interactions in hitherto unexplored parameter regimes. In particular, current short wavelength FELs can uniquely drive non-linear processes mediated by inner shell electrons and in fields where the photon energy can be as high as 10 keV and so the corresponding optical period reaches below one attosecond. Combined with ultrafast optical lasers, or simply employing wavefront division, pump probe experiments can be performed with femtosecond time resolution. As single photon ionization of atoms and molecules is by now very well understood, they provide the ideal targets for early experiments by which not only FELs can be characterised and benchmarked but can also be the natural departure point in the hunt for non-linear behaviour of atomistic systems bathed in laser fields of ultrahigh photon ene...

/pdf/non-linear-processes-in-the-interaction-of-atoms-and-263v42422y.pdf

Non-linear processes in the interaction of atoms and molecules with intense EUV and X-ray fields from SASE free electron lasers (FELs)

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 120fs 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 250fs 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.

/pdf/single-shot-characterization-of-independent-femtosecond-psjb084lsw.pdf

Single-shot characterization of independent femtosecond extreme ultraviolet free electron and infrared laser pulses

We have obtained density and temperature informations on an expanding KrF laser-ablated magnesium plume via optical Thomson scatter with a frequency doubled Nd:YAG laser. The electron temperature was found to decay with the expected Te∝t−1 dependence. However, we have found the electron density to have a time dependence ne∝t−4.95 which can be explained by strong recombination processes. We also observed atomic Raman satellites originating from transitions between the different angular momentum levels of the metastable P03 term in MgI.

J. Pedregosa Gutierrez

Papers

Single-shot characterization of independent femtosecond extreme ultraviolet free electron and infrared laser pulses

Optical Thomson scatter from laser-ablated plumes