M. Hoener

Femtosecond diffractive imaging with a soft-X-ray free-electron laser

Theory predicts1,2,3,4 that, with an ultrashort and extremely bright coherent X-ray pulse, a single diffraction pattern may be recorded from a large macromolecule, a virus or a cell before the sample explodes and turns into a plasma. Here we report the first experimental demonstration of this principle using the FLASH soft-X-ray free-electron laser. An intense 25 fs, 4×1013 W cm−2 pulse, containing 1012 photons at 32 nm wavelength, produced a coherent diffraction pattern from a nanostructured non-periodic object, before destroying it at 60,000 K. A novel X-ray camera assured single-photon detection sensitivity by filtering out parasitic scattering and plasma radiation. The reconstructed image, obtained directly from the coherent pattern by phase retrieval through oversampling5,6,7,8,9, shows no measurable damage, and is reconstructed at the diffraction-limited resolution. A three-dimensional data set may be assembled from such images when copies of a reproducible sample are exposed to the beam one by one10.

Double Core-Hole Production in N2: Beating the Auger Clock

We investigate the creation of double K-shell holes in N2 molecules via sequential absorption of two photons on a time scale shorter than the core-hole lifetime by using intense x-ray pulses from the Linac Coherent Light Source free electron laser. The production and decay of these states is characterized by photoelectron spectroscopy and Auger electron spectroscopy. In molecules, two types of double core holes are expected, the first with two core holes on the same N atom, and the second with one core hole on each N atom. We report the first direct observations of the former type of core hole in a molecule, in good agreement with theory, and provide an experimental upper bound for the relative contribution of the latter type.

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.

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

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 energy. In this topical review we illustrate with specific examples the gamut of apposite experiments in atomic, molecular physics currently underway at the SCSS Test Accelerator (Japan), FLASH (Hamburg) and LCLS (Stanford).

Experimental determination of the ionization potentials of the first five members of the nanodiamond series

The ionization potentials of size- and isomer-selected diamondoids (nanodiamond containing one to five crystal cages) have been measured by means of total-ion-yield spectroscopy. We find a monotonic decrease of the ionization potential with increasing diamondoid size. This experimental result is compared to recent theoretical predictions and comparable investigations on related carbon clusters, the fullerenes, which show isomer effects to be stronger than size dependence.

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.

Ultrafast absorption of intense x rays by nitrogen molecules

We devise a theoretical description for the response of nitrogen molecules (N(2)) to ultrashort and intense x rays from the free electron laser Linac Coherent Light Source (LCLS). We set out from a rate-equation description for the x-ray absorption by a nitrogen atom. The equations are formulated using all one-x-ray-photon absorption cross sections and the Auger and radiative decay widths of multiply-ionized nitrogen atoms. Cross sections are obtained with a one-electron theory and decay widths are determined from ab initio computations using the Dirac-Hartree-Slater (DHS) method. We also calculate all binding and transition energies of nitrogen atoms in all charge states with the DHS method as the difference of two self-consistent field (SCF) calculations (ΔSCF method). To describe the interaction with N(2), a detailed investigation of intense x-ray-induced ionization and molecular fragmentation are carried out. As a figure of merit, we calculate ion yields and the average charge state measured in recent experiments at the LCLS. We use a series of phenomenological models of increasing sophistication to unravel the mechanisms of the interaction of x rays with N(2): a single atom, a symmetric-sharing model, and a fragmentation-matrix model are developed. The role of the formation and decay of single and double core holes, the metastable states of N(2)(2+), and molecular fragmentation are explained.

Short-pulse Laser Induced Transient Structure Formation and Ablation Studied with Time-resolved Coherent XUV-scattering

The structural dynamics of short‐pulse laser irradiated surfaces and nano‐structures has been studied with nm spatial and ultrafast temporal resolution by means of single‐shot coherent XUV‐scattering techniques. The experiments allowed us to time‐resolve the formation of laser‐induced periodic surface structures, and to follow the expansion and disintegration of nano‐objects during laser ablation.

X-Ray FEL Induced Double Core-Hole and High Charge State Production

We report novel results on multiple photoionization of the N2 molecule by femtosecond x-ray pulses from the Linac Coherent Light Source free electron laser. Ion spectroscopy reveals the ionization and dissociation dynamics, including a molecular mechanism of frustrated absorption that suppresses the formation of high charge states at short pulse durations. Auger and photo-electron analysis of double core hole production (through absorption of two photons on the time scale shorter than the Auger decay) agrees well with theory for the case of both K-shell holes on the same N atom, and gives an experimental upper bound for the relative contribution in case of one core hole on each N atom.

Photo-multidetachment and Fragmentation of C60 Anions

Absolute single-photon multi-electron ionization and fragmentation cross sections for C−60 ions are measured for the first time. It is determined that the excess electron in the anion causes a significant increase in cross section and an overall energy scaling, but otherwise has little effect on the observed structure. Appearance energies (apparent thresholds) of the fragmentation products are also observed to be significantly lower for the anion, as compared to the neutral molecule.