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- ItemExperimental strategies for optical pump - Soft x-ray probe experiments at the LCLS(Bristol : Institute of Physics Publishing, 2014) McFarland, B.K.; Berrah, N.; Bostedt, C.; Bozek, J.; Bucksbaum, P.H.; Castagna, J.C.; Coffee, R.N.; Cryan, J.P.; Fang, L.; Farrell, J.P.; Feifel, R.; Gaffney, K.J.; Glownia, J.M.; Martinez, T.J.; Miyabe, S.; Mucke, M.; Murphy, B.; Natan, A.; Osipov, T.; Petrovic, V.S.; Schorb, S.; Schultz, T.; Spector, L.S.; Swiggers, M.; Tarantelli, F.; Tenney, I.; Wang, S.; White, J.L.; White, W.; Gühr, M.Free electron laser (FEL) based x-ray sources show great promise for use in ultrafast molecular studies due to the short pulse durations and site/element sensitivity in this spectral range. However, the self amplified spontaneous emission (SASE) process mostly used in FELs is intrinsically noisy resulting in highly fluctuating beam parameters. Additionally timing synchronization of optical and FEL sources adds delay jitter in pump-probe experiments. We show how we mitigate the effects of source noise for the case of ultrafast molecular spectroscopy of the nucleobase thymine. Using binning and resorting techniques allows us to increase time and spectral resolution. In addition, choosing observables independent of noisy beam parameters enhances the signal fidelity.
- ItemOptimization of quantum trajectories driven by strong-field waveforms(College Park : American Institute of Physics Inc., 2014) Haessler, S.; Balciunas, T.; Fan, G.; Andriukaitis, G.; Pugžlys, A.; Baltuška, A.; Witting, T.; Squibb, R.; Zaïr, A.; Tisch, J.W.G.; Marangos; Chipperfield, L.E.Quasifree field-driven electron trajectories are a key element of strong-field dynamics. Upon recollision with the parent ion, the energy transferred from the field to the electron may be released as attosecondduration extreme ultaviolet emission in the process of high-harmonic generation. The conventional sinusoidal driver fields set limitations on the maximum value of this energy transfer and the efficient return of the launched electron trajectories. It has been predicted that these limits can be significantly exceeded by an appropriately ramped-up cycle shape [L. E. Chipperfield et al., Phys. Rev. Lett. 102, 063003 (2009)]. Here, we present an experimental realization of similar cycle-shaped waveforms and demonstrate control of the high-harmonic generation process on the single-atom quantum level via attosecond steering of the electron trajectories.With our improved optical cycles, we boost the field ionization launching the electron trajectories, increase the subsequent field-to-electron energy transfer, and reduce the trajectory duration. We demonstrate, in realistic experimental conditions, 2 orders of magnitude enhancement of the generated extreme ultraviolet flux together with an increased spectral extension. This application, which is only one example of what can be achieved with cycle-shaped high-field light waves, has significant implications for attosecond spectroscopy and molecular self-probing.
- ItemBite-outs and other depletions of mesospheric electrons(Amsterdam [u.a.] : Elsevier, 2011) Friedrich, M.; Rapp, M.; Plane, J.M.C.; Torkar, K.M.The ionised mesosphere is less understood than other parts of the ionosphere because of the challenges of making appropriate measurements in this complex region. We use rocket borne in situ measurements of absolute electron density by the Faraday rotation technique and accompanying DC-probe measurements to study the effect of particles on the D-region charge balance. Several examples of electron bite-outs, their actual depth as well as simultaneous observations of positive ions are presented. For a better understanding of the various dependencies we use the ratio Β/αi (attachment rate over ion-ion recombination coefficient), derived from the electron and ion density profiles by applying a simplified ion-chemical scheme, and correlate this term with solar zenith angle and moon brightness. The probable causes are different for day and night; recent in situ measurements support existing hypotheses for daytime cases, but also reveal behaviour at night hitherto not reported in the literature. Within the large range of Β/αi values obtained from the analysis of 28 high latitude night flights one finds that the intensity of scattered sunlight after sunset, and even moonlight, apparently can photodetach electrons from meteoric smoke particles (MSP) and molecular anions. The large range of values itself can best be explained by the variability of the MSPs and by occasionally occurring atomic oxygen impacting on the negative ion chemistry in the night-time mesosphere under disturbed conditions.
- ItemInterference structure of above-threshold ionization versus above-threshold detachment(Bristol : IOP, 2012) Korneev, Ph.A.; Popruzhenko, S.V.; Goreslavski, S.P.; Becker, W.; Paulus, G.G.; Fetić, B.; Milošević, D.B.Laser-induced electron detachment or ionization of atoms and negative ions is considered. In the context of the saddle-point evaluation of the strong-field approximation (SFA), the velocity maps of the direct electrons (those that do not undergo rescattering) exhibit a characteristic structure due to the constructive and destructive interference of electrons liberated from their parent atoms/ions within certain windows of time. This structure is defined by the above-threshold ionization rings at fixed electron energy and by two sets of curves in momentum space on which destructive interference occurs. The spectra obtained with the SFA are compared with those obtained by numerical solution of the time-dependent Schrödinger equation. For detachment, the agreement is excellent. For ionization, the effect of the Coulomb field is most pronounced for electrons emitted in a direction close to laser polarization, while for nearperpendicular emission the qualitative appearance of the spectrum is unaffected.
- ItemIn situ single-shot diffractive fluence mapping for X-ray free-electron laser pulses([London] : Nature Publishing Group UK, 2018) Schneider, Michael; Günther, Christian M.; Pfau, Bastian; Capotondi, Flavio; Manfredda, Michele; Zangrando, Marco; Mahne, Nicola; Raimondi, Lorenzo; Pedersoli, Emanuele; Naumenko, Denys; Eisebitt, StefanFree-electron lasers (FELs) in the extreme ultraviolet (XUV) and X-ray regime opened up the possibility for experiments at high power densities, in particular allowing for fluence-dependent absorption and scattering experiments to reveal non-linear light-matter interactions at ever shorter wavelengths. Findings of such non-linear effects are met with tremendous interest, but prove difficult to understand and model due to the inherent shot-to-shot fluctuations in photon intensity and the often structured, non-Gaussian spatial intensity profile of a focused FEL beam. Presently, the focused beam is characterized and optimized separately from the actual experiment. Here, we present the simultaneous measurement of XUV diffraction signals from solid samples in tandem with the corresponding single-shot spatial fluence distribution on the actual sample. Our in situ characterization scheme enables direct monitoring of the sample illumination, providing a basis to optimize and quantitatively understand FEL experiments.
- ItemCorrelated electronic decay following intense near-infrared ionization of clusters(Bristol : IOP Publ., 2015) Schütte, Bernd; Arbeiter, Mathias; Fennel, Thomas; Jabbari, Ghazal; Kuleff, Alexander I.; Vrakking, Marc J. J.; Rouzée, ArnaudWe report on a novel correlated electronic decay process following extensive Rydberg atom formation in clusters ionized by intense near-infrared fields. A peak close to the atomic ionization potential is found in the electron kinetic energy spectrum. This new contribution is attributed to an energy transfer between two electrons, where one electron decays from a Rydberg state to the ground state and transfers its excess energy to a weakly bound cluster electron in the environment that can escape from the cluster. The process is a result of nanoplasma formation and is therefore expected to be important, whenever intense laser pulses interact with nanometer-sized particles.
- ItemInteratomic Coulombic Decay Processes after Multiple Valence Excitations in Ne Clusters(Bristol : IOP Publ., 2015) Iablonskyi, D.; Nagaya, K.; Fukuzawa, H.; Motomura, K.; Kumagai, Y.; Mondal, S.; Tachibana, T.; Takanashi, T.; Nishiyama, T.; Matsunami, K.; Johnsson, P.; Piseri, P.; Sansone, G.; Dubrouil, A.; Reduzzi, M.; Carpeggiani, P.; Vozzi, C.; Devetta, M.; Negro, M.; Faccialà, D.; Calegari, F.; Trabattoni, A.; Castrovilli, M.; Ovcharenko, Y.; Möller, T.; Mudrich, M.; Stienkemeier, F.; Coreno, M.; Alagia, M.; Schütte, B.; Berrah, N.; Callegari, C.; Plekan, O.; Finetti, P.; Spezzani, C.; Ferrari, E.; Allaria, E.; Penco, G.; Serpico, C.; De Ninno, G.; Diviacco, B.; Di Mitri, S.; Giannessi, L.; Prince, K..; Yao, M.; Ueda, K.We present a comprehensive analysis of autoionization processes in Ne clusters (~5000 atoms) after multiple valence excitations by free electron laser radiation. The evolution from 2-body interatomic Coulombic decay (ICD) to 3-body ICD is demonstrated when changing from surface to bulk Frenkel exciton excitation. Super Coster-Kronig type 2-body ICD is observed at Wannier exciton which quenches the main ICD channel.
- ItemTerahertz emission from lithium doped silicon under continuous wave interband optical excitation(Bristol : IOP Publ., 2015) Andrianov, A.V.; Zakhar'in, A.O.; Zhukavin, R.K.; Shastin, V.N.; Abrosimov, N.V.We report on experimental observation and study of terahertz emission from lithium doped silicon crystals under continuous wave band-to-band optical excitation. It is shown that radiative transitions of electrons from 2P excited states of lithium donor to the 1S(A1) donor ground state prevail in the emission spectrum. The terahertz emission occurs due to capture of nonequilibrium electrons to charged donors, which in turn are generated in the crystal as a result of impurity assisted electron-hole recombination. Besides the intracentre radiative transitions the terahertz emission spectrum exhibits also features at about 12.7 and 15.27 meV, which could be related to intraexciton transitions and transitions from the continuum to the free exciton ground state.
- ItemThe interplay between spin densities and magnetic superexchange interactions: Case studies of monoand trinuclear bis(oxamato)-type complexes(Frankfurt am Main : Beilstein-Institut zur Förderung der Chemischen Wissenschaften, 2017) Aliabadi, A.; Büchner, B.; Kataev, V.; Rüffer, T.For future molecular spintronic applications the possibility to modify and tailor the magnetic properties of transition-metal complexes is very promising. One of such possibilities is given by the countless derivatization offered by carbon chemistry. They allow for altering chemical structures and, in doing so, to tune magnetic properties of molecular spin-carrying compounds. With emphasis on the interplay of the spin density distribution of mononuclear and magnetic superexchange couplings of trinuclear bis(oxamato)- type complexes we review on efforts on such magneto-structural correlations.
- ItemThree-Dimensional Shapes of Spinning Helium Nanodroplets(College Park, Md. : APS, 2018) Langbehn, Bruno; Sander, Katharina; Ovcharenko, Yevheniy; Peltz, Christian; Clark, Andrew; Coreno, Marcello; Cucini, Riccardo; Drabbels, Marcel; Finetti, Paola; Di Fraia, Michele; Giannessi, Luca; Grazioli, Cesare; Iablonskyi, Denys; LaForge, Aaron C.; Nishiyama, Toshiyuki; Oliver Álvarez de Lara, Verónica; Piseri, Paolo; Plekan, Oksana; Ueda, Kiyoshi; Zimmermann, Julian; Prince, Kevin C.; Stienkemeier, Frank; Callegari, Carlo; Fennel, Thomas; Rupp, Daniela; Möller, ThomasA significant fraction of superfluid helium nanodroplets produced in a free-jet expansion has been observed to gain high angular momentum resulting in large centrifugal deformation. We measured single-shot diffraction patterns of individual rotating helium nanodroplets up to large scattering angles using intense extreme ultraviolet light pulses from the FERMI free-electron laser. Distinct asymmetric features in the wide-angle diffraction patterns enable the unique and systematic identification of the three-dimensional droplet shapes. The analysis of a large data set allows us to follow the evolution from axisymmetric oblate to triaxial prolate and two-lobed droplets. We find that the shapes of spinning superfluid helium droplets exhibit the same stages as classical rotating droplets while the previously reported metastable, oblate shapes of quantum droplets are not observed. Our three-dimensional analysis represents a valuable landmark for clarifying the interrelation between morphology and superfluidity on the nanometer scale.