Armin Azima

Single-shot timing measurement of extreme-ultraviolet free-electron laser pulses

Arrival time fluctuations of extreme-ultraviolet (EUV) pulses from the free-electron laser in Hamburg (FLASH) are measured single-pulse resolved at the experimental end-station. To this end, they are non-collinearly superimposed in space and time with visible femtosecond laser pulses on a GaAs substrate. The EUV irradiation induces changes of the reflectivity for the visible pulse. The temporal delay between the two light pulses is directly encoded in the spatial position of the reflectivity change which is captured with a CCD camera. For each single shot, the relative EUV/visible arrival-time can be measured with about 40 fs rms accuracy. The method constitutes a novel route for an improvement of future pump–probe experiments at short-wavelength free-electron lasers (FELs) by a pulse-wise correction with simultaneously measured arrival times of individual EUV pulses.

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

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.

Time-resolved pump-probe experiments beyond the jitter limitations at FLASH

Using a noninvasive, electro-optically based electron bunch arrival time measurement at FLASH (free electron laser in Hamburg) the temporal resolution of two-color pump-probe experiments has been significantly improved. The system determines the relative arrival time of the extended ultraviolet pulse of FLASH and an amplified Ti:sapphire femtosecond-laser pulse at the interaction region better than 90 fs rms. In a benchmarking pump-probe experiment using two-color above threshold ionization of noble gases, an enhancement in the timing resolution by a factor of 4 compared to the uncorrected data is obtained.

Time-resolved ion spectrometry on xenon with the jitter-compensated soft x-ray pulses of a free-electron laser

Atomic inner-shell relaxation dynamics were measured at the free-electron laser in Hamburg, FLASH, delivering 92 eV pulses. The decay of 4d core holes created in xenon was followed by detection of ion charge states after illumination with delayed 400 nm laser pulses. A timing jitter of the order of several hundred femtoseconds between laser- and accelerator-pulses was compensated for by a simultaneous delay measurement in a single-shot x-ray/laser cross-correlator. After sorting of the tagged spectra according to the measured delays, a temporal resolution equivalent to the pulse duration of the optical laser could be established. While results on ion charge states up to Xe4+ are compatible with a previous study using a high-harmonic soft x-ray source, a new relaxation pathway is opened by the nonlinear excitation of xenon atoms in the intense free-electron laser light field, leading to the formation of Xe5+.

Electron wave packet sampling with laser-generated extreme ultraviolet and terahertz fields

We report on transferring the concept of light-field streaking with intense terahertz fields from free-electron lasers to the laboratory scale. Utilizing a commercial laser system, synchronized 300 μm terahertz and 13 nm extreme ultraviolet pulses are generated by optical rectification and high harmonic generation, respectively. The terahertz fields are sufficiently strong to support electron wave packet sampling with a few fs resolution. The capability of this approach is demonstrated by measuring the duration of electron pulses formed by direct photoemission from a neon gas target.

Ultrafast photofragmentation dynamics of molecular iodine driven with timed XUV and near-infrared light pulses.

Photofragmentation dynamics of molecular iodine was studied as a response to the joint illumination with femtosecond 800 nm near-infrared and 13 nm extreme ultraviolet (XUV) pulses delivered by the free-electron laser facility FLASH. The interaction of the molecular target with two light pulses of different wavelengths but comparable pulse energy elucidates a complex intertwined electronic and nuclear dynamics. To follow distinct pathways out of a multitude of reaction channels, the recoil of created ionic fragments is analyzed. The delayed XUV pulse provides a way of following molecular photodissociation of I(2) with a characteristic time-constant of (55 ± 10) fs after the laser-induced formation of antibonding states. A preceding XUV pulse, on the other hand, preferably creates a 4d(-1) inner-shell vacancy followed by the fast Auger cascade with a revealed characteristic time constant τ(A2)=(23±11) fs for the second Auger decay transition. Some fraction of molecular cationic states undergoes subsequent Coulomb explosion, and the evolution of the launched molecular wave packet on the repulsive Coulomb potential was accessed by the laser-induced postionization. A further unexpected photofragmentation channel, which relies on the collective action of XUV and laser fields, is attributed to a laser-promoted charge transfer transition in the exploding molecule.

Michelson-type all-reflective interferometric autocorrelation in the VUV regime

We demonstrate second-order interferometric autocorrelation of a pulse in the vacuum-ultraviolet (VUV) spectral range using an optical arrangement equivalent to a Michelson interferometer. In an all-reflective design, wavefront splitting is realized with two moveable interdigitated reflective gratings forming a diffraction pattern with well separated orders and an intensity distribution depending on the precisely adjustable path-length difference. An imaging timeof-flight spectrometer is able to spatially select ions created by nonlinear twophoton absorption in the focus of the zeroth diffraction order. This arrangement is used to demonstrate interferometric autocorrelation in krypton with femtosecond VUV pulses at 160nm wavelength. In addition to the pulse duration, which is already accessible with non-collinear intensity autocorrelation, the full interferometric contrast of the presented approach enables us to extract also information on temporal phases.

Femtosecond dynamics of correlated many-body states in C60 fullerenes

Fullerene complexes may play a key role in the design of future molecular electronics and nanostructured devices with potential applications in light harvesting using organic solar cells. Charge and energy flow in these systems is mediated by many-body effects. We studied the structure and dynamics of laser-induced multi-electron excitations in isolated C60 by two-photon photoionization as a function of excitation wavelength using a tunable fs UV laser and developed a corresponding theoretical framework on the basis of ab initio calculations. The measured resonance line width gives direct information on the excited state lifetime. From the spectral deconvolution we derive a lower limit for purely electronic relaxation on the order of fs. Energy dissipation towards nuclear degrees of freedom is studied with time-resolved techniques. The evaluation of the nonlinear autocorrelation trace gives a characteristic time constant of fs for the exponential decay. In line with the experiment, the observed transient dynamics is explained theoretically by nonadiabatic (vibronic) couplings involving the correlated electronic, the nuclear degrees of freedom (accounting for the Herzberg–Teller coupling), and their interplay.

Atomic photoionization in weak and strong two-color radiation fields

Two-color photoionization processes in rare gases have been studied using the combination of XUV pulses from the Free Electron Laser in Hamburg (FLASH) and intense femtosecond pulses from an external synchronized near infrared laser. In the low field regime of the NIR dressing laser ( 1111 W/cm2), multi-photon processes are dominant and theoretical descriptions beyond time-dependent second order perturbation formalism have to be applied.