J. Ullrich

Time-Resolved Measurement of Interatomic Coulombic Decay in Ne2

The lifetime of interatomic Coulombic decay (ICD) [L.u2009S. Cederbaum et al., Phys. Rev. Lett. 79, 4778 (1997)] in Ne2 is determined via an extreme ultraviolet pump-probe experiment at the Free-Electron Laser in Hamburg. The pump pulse creates a 2s inner-shell vacancy in one of the two Ne atoms, whereupon the ionized dimer undergoes ICD resulting in a repulsive Ne+(2p(-1))-Ne+(2p(-1)) state, which is probed with a second pulse, removing a further electron. The yield of coincident Ne+-Ne2+ pairs is recorded as a function of the pump-probe delay, allowing us to deduce the ICD lifetime of the Ne2(+)(2s(-1)) state to be (150±50)u2009u2009fs, in agreement with quantum calculations.

X-Ray Resonant Photoexcitation: Linewidths and Energies of Kα Transitions in Highly Charged Fe Ions

Photoabsorption by and fluorescence of the Kα transitions in highly charged iron ions are essential mechanisms for x-ray radiation transfer in astrophysical environments. We study photoabsorption due to the main Kα transitions in highly charged iron ions from heliumlike to fluorinelike (Fe24+ to Fe17+) using monochromatic x rays around 6.6 keV at the PETRA III synchrotron photon source. Natural linewidths were determined with hitherto unattained accuracy. The observed transitions are of particular interest for the understanding of photoexcited plasmas found in x-ray binary stars and active galactic nuclei.

AMO science at the FLASH and European XFEL free-electron laser facilities

Present performance and future development of the free-electron lasers (FELs) in Hamburg—FLASH for the extreme ultraviolet and the European XFEL for the soft and hard x-ray regimes—are presented. As an illustration of the unprecedented characteristics of these sources a few recent examples of experiments performed in the area of atomic, molecular and optical (AMO) physics are described. The results highlight in particular the available high photon intensities, the short pulse durations and the coherence of the FEL beam. Nonlinear processes involving for the first time inner-shell electrons, time-resolved experiments on the few femtosecond timescales, and imaging experiments on small particles have been the focus of these studies, demonstrating the unique potential of short-wavelength FELs and pointing to numerous exciting future opportunities.

Ultrafast dynamics in acetylene clocked in a femtosecond XUV stopwatch

Few-photon induced ultrafast dynamics in acetylene (C2H2) leading to several dissociation channels—deprotonation (H++C2H+ and H++C2H2+), symmetric break-up (CH++CH+) and isomerization (C++CH2+)-–were investigated employing the (XUV; extreme ultra-violet)-pump–(XUV; extreme ultra-violet)-probe scheme at the free-electron laser in Hamburg, combined with multi-hit coincidence detection. The kinetic energy releases and fragment-ion momentum distributions for various decay channels are presented. The C++CH2+ and H++C2H2+ channels reveal clear signatures of ultrafast molecular mechanisms, demonstrating potential applications of our pump-probe technique to complex systems in order to study a large variety of ultrafast phenomena in the XUV regime.

Ionization with low-frequency fields in the tunneling regime

Strong-field ionisation surprises with richness beyond current understanding despite decade long investigations. Ionisation with mid-IR light has promptly revealed unexpected kinetic energy structures that seem related to unanticipated quantum trajectories of the electrons. We measure first 3D momentum distributions in the deep tunneling regime (γ = 0.3) and observe surprising new electron dynamics of near-zero momentum electrons and extremely low momentum structures, below the eV, despite very high quiver energies of 95u2005eV. Such level of high-precision measurements at only 1u2005meV above the threshold, despite 5 orders higher ponderomotive energies, has now become possible with a specifically developed ultrafast mid-IR light source in combination with a reaction microscope, thereby permitting a new level of investigations into mid-IR recollision physics.

Inner-shell multiple ionization of polyatomic molecules with an intense x-ray free-electron laser studied by coincident ion momentum imaging.

The ionization and fragmentation of two selenium containing hydrocarbon molecules, methylselenol (CH3SeH) and ethylselenol (C2H5SeH), by intense (>1017xa0W cm−2) 5 fs x-ray pulses with photon energies of 1.7 and 2xa0keV has been studied by means of coincident ion momentum spectroscopy. Measuring charge states and ion kinetic energies, we find signatures of charge redistribution within the molecular environment. Furthermore, by analyzing fragment ion angular correlations, we can determine the laboratory-frame orientation of individual molecules and thus investigate the fragmentation dynamics in the molecular frame. This allows distinguishing protons originating from different molecular sites along with identifying the reaction channels that lead to their emission.

Using covariance mapping to investigate the dynamics of multi-photon ionization processes of Ne atoms exposed to X-FEL pulses

We report on a detailed investigation into the electron emission processes of Ne atoms exposed to intense femtosecond x-ray pulses, provided by the Linac Coherent Light Source Free Electron Laser (FEL) at Stanford. The covariance mapping technique is applied to analyse the data, and the capability of this approach to disentangle both linear and nonlinear correlation features which may be hidden on coincidence maps of the same data set is demonstrated. Different correction techniques which enable improvements on the quality of the spectral features extracted from the covariance maps are explored. Finally, a method for deriving characteristics of the x-ray FEL pulses based on covariance mapping in combination with model simulations is presented.

Extreme ultraviolet ionization of pure He nanodroplets: Mass-correlated photoelectron imaging, Penning ionization, and electron energy-loss spectra

The ionization dynamics of pure He nanodroplets irradiated by Extreme ultraviolet radiation is studied using Velocity-Map Imaging PhotoElectron-PhotoIon COincidence spectroscopy. We present photoelectron energy spectra and angular distributions measured in coincidence with the most abundant ions He(+), He2(+), and He3(+). Surprisingly, below the autoionization threshold of He droplets, we find indications for multiple excitation and subsequent ionization of the droplets by a Penning-like process. At high photon energies we observe inelastic collisions of photoelectrons with the surrounding He atoms in the droplets.

Decay Rate Measurement of the First Vibrationally Excited State of MgH+ in a Cryogenic Paul Trap

We present a method to measure the decay rate of the first excited vibrational state of polar molecular ions that are part of a Coulomb crystal in a cryogenic linear Paul trap. Specifically, we have monitored the decay of the |ν = 1, J = 1)(X) towards the |ν = 0, J = 0)(X) level in MgH+ by saturated laser excitation of the |ν = 0, J = 2)(X)-|ν = 1, J = 1)(X) transition followed by state selective resonance enhanced two-photon dissociation out of the |ν = 0, J=2)(X) level. The experimentally observed rate of 6.32(0.69) s(-1) is in excellent agreement with the theory value of 6.13(0.03) s(-1) (this Letter). The technique enables the determination of decay rates, and thus absorption strengths, with an accuracy at the few percent level.

Unusual under-threshold ionization of neon clusters studied by ion spectroscopy

We carried out time-of-flight mass spectrometry for neon clusters that were exposed to intense free electron laser pulses with the wavelength of 62 nm, which induce optical transition from the ground state (2s 2 2p 6 ) to an excited state (2s 2 2p 5 nl ) in the Ne atoms. In contrast to Ne + ions produced by two-photon absorption from isolated Ne atoms, the Ne + ion yield from Ne clusters shows a linear dependence on the laser intensity (I). We discuss the ionization mechanisms which give the linear behaviour with respect to I and expected features in the electron emission spectrum. (Some figures may appear in colour only in the online journal)