Project A3

G. Kastirke, F. Ota, D. V. Rezvan, M. S. Schöffler, M. Weller, J. Rist, R. Boll, N.Anders, T. M. Baumann, S. Eckart, B. Erk, A. De Fanis, K. Fehre, A. Gatton, S. Grundmann, P. Grychtol, A. Hartung, M. Hofmann, M. Ilchen, C. Janke, M. Kircher, M. Kunitski, X. Li, T. Mazza, N. Melzer, J. Montano, V. Music, G. Nalin, Y. Ovcharenko, A. Pier, N. Rennhack, D. E. Rivas, R. Dörner, D. Rolles, A. Rudenko, P. Schmidt, J. Siebert, N. Strenger, D. Trabert, I. Vela-Perez, R. Wagner, T. Weber, J. B. Williams, P. Ziolkowski, L. Ph. H. Schmidt, A. Czasch, Y. Tamura, N. Hara, K. Yamazaki, K. Hatada, F. Trinter, M. Meyer, K. Ueda, Ph. V. Demekhin, and T. Jahnke

During the last decade, X-ray free-electron lasers (XFELs) have enabled the study of light–matter interaction under extreme conditions. Atoms which are subject to XFEL radiation are charged by a complex interplay of (several subsequent) photoionization events and electronic decay processes within a few femtoseconds. The interaction with molecules is even more intriguing, since intricate nuclear dynamics occur as the molecules start to dissociate during the charge-up process. Here, we demonstrate that by analyzing photoelectron angular emission distributions and kinetic energy release of charge states of ionic molecular fragments, we can obtain a detailed understanding of the charge-up and fragmentation dynamics. Our novel approach allows for gathering such information without the need of complex ab initio modeling. As an example, we provide a detailed view on the processes happening on a femtosecond time scale in oxygen molecules exposed to intense XFEL pulses.

Phys. Chem. Chem. Phys. 24, 27121 (2022)

Alexandra J. Feinberg et al.

Finding the lowest energy configuration of N unit charges on a sphere, known as Thomson's problem, is a long-standing query which has only been studied via numerical simulations. We present its physical realization using multiply charged He nanodroplets. The charge positions are determined by x-ray coherent diffractive imaging with Xe as a contrast agent. In neutral droplets, filaments resulting from Xe atoms condensing on quantum vortices are observed. Unique to charged droplets, however, Xe clusters that condense on charges are distributed on the surface in lattice-like structures, introducing He droplets as experimental model systems for the study of Thomson's problem.

Phys. Rev. Research 4, L022063 (2022)

X. Li, A. Rudenko, T. Mazza, A. Rörig, N. Anders, Th. M. Baumann, S. Eckart, B. Erk, A. De Fanis, K. Fehre, R. Dörner, L. Foucar, S. Grundmann, P. Grychtol, A. Hartung, M. Hofmann, M. Ilchen, Ch. Janke, G. Kastirke, M. Kircher, K. Kubicek, M. Kunitski, S. Meister, N. Melzer, J. Montano, V. Music, G. Nalin, Y. Ovcharenko, Ch. Passow, A. Pier, N. Rennhack, J. Rist, D. E. Rivas, I. Schlichting, L. Ph. H. Schmidt, Ph. Schmidt, M. S. Schöffler, J. Siebert, N. Strenger, D. Trabert, F. Trinter, I. Vela-Perez, R. Wagner, P. Walter, M. Weller, P. Ziolkowski, A. Czasch, M. Meyer, T. Jahnke, D. Rolles, and R. Boll

Extremely high charge states of atoms and molecules can be created when they are irradiated by intense x-ray pulses. At certain x-ray photon energies, electron ejection from atoms can be drastically enhanced by transient resonances created during the sequential ionization process. Here we report on the observation of such resonance effects in a molecule, CH₃I, and show the photon-energy-dependent shift of resonance-induced structures in ion charge state distributions. By comparing the ion charge state distribution of CH₃I with that from ionization of atomic xenon, molecule-specific features are observed, which can be attributed to ultrafast intramolecular charge rearrangement. In addition, we experimentally demonstrate that the charge-rearrangement-enhanced x-ray ionization of molecules, previously found with hard x rays, also plays a role in the soft x-ray regime.

Phys. Rev. A 105, 053102 (2022)

R. Boll, J. M. Schäfer, B. Richard, K. Fehre, G. Kastirke, Z. Jurek, M. S. Schöffler, M. M. Abdullah, N. Anders, T. M. Baumann, S. Eckart, B. Erk, A. De Fanis, R. Dörner, S. Grundmann, P. Grychto, A. Hartung, M. Hofmann, M. Ilchen, L. Inhester, C. Janke, R. Jin, M. Kircher, K. Kubicek, M. Kunitski, X. Li, T. Mazza, S. Meister, N. Melzer, J. Montano, V. Music, G. Nalin, Y. Ovcharenko, C. Passow, A. Pier, N. Rennhack, J. Rist, D. E. Rivas, D. Rolles, I. Schlichting, L. Ph. H. Schmidt, P. Schmidt, J. Siebert, N. Strenger, D. Trabert, F. Trinter, I. Vela-Perez, R. Wagner, P. Walter, M. Weller, P. Ziolkowski, S.-K. Son, A. Rudenko, M. Meyer, R. Santra, and T. Jahnke

Following structural dynamics in real time is a fundamental goal towards a better understanding of chemical reactions. Recording snapshots of individual molecules with ultrashort exposure times is a key ingredient towards this goal, as atoms move on femtosecond (10⁻¹⁵ s) timescales. For condensed-phase samples, ultrafast, atomically resolved structure determination has been demonstrated using X-ray and electron diffraction. Pioneering experiments have also started addressing gaseous samples. However, they face the problem of low target densities, low scattering cross sections and random spatial orientation of the molecules. Therefore, obtaining images of entire, isolated molecules capturing all constituents, including hydrogen atoms, remains challenging. Here we demonstrate that intense femtosecond pulses from an X-ray free-electron laser trigger rapid and complete Coulomb explosions of 2-iodopyridine and 2-iodopyrazine molecules. We obtain intriguingly clear momentum images depicting ten or eleven atoms, including all the hydrogens, and thus overcome a so-far impregnable barrier for complete Coulomb explosion imaging—its limitation on molecules consisting of three to five atoms. In combination with state-of-the-art multi-coincidence techniques and elaborate theoretical modelling, this allows tracing ultrafast hydrogen emission and obtaining information on the result of intramolecular electron rearrangement. Our work represents an important step towards imaging femtosecond chemistry via Coulomb explosion.

Nature Physics 18, 423 (2022)

A. C. LaForge, S.-K. Son, D. Mishra, M. Ilchen, S. Duncanson, E. Eronen, E. Kukk, S. Wirok-Stoletow,, D. Kolbasova, P. Walter, R. Boll, A. De Fanis, M. Meyer, Y. Ovcharenko, D. E. Rivas, P. Schmidt, S. Usenko, R. Santra, and N. Berrah

Here, we report on the nonlinear ionization of argon atoms in the short wavelength regime using ultraintense x rays from the European XFEL. After sequential multiphoton ionization, high charge states are obtained. For photon energies that are insufficient to directly ionize a 1s electron, a different mechanism is required to obtain ionization to Ar¹⁷⁺. We propose this occurs through a two-color process where the second harmonic of the FEL pulse resonantly excites the system via a 1s→2p transition followed by ionization by the fundamental FEL pulse, which is a type of x-ray resonance-enhanced multiphoton ionization (REMPI). This resonant phenomenon occurs not only for Ar¹⁶⁺, but also through lower charge states, where multiple ionization competes with decay lifetimes, making x-ray REMPI distinctive from conventional REMPI. With the aid of state-of-the-art theoretical calculations, we explain the effects of x-ray REMPI on the relevant ion yields and spectral profile.

Phys. Rev. Lett. 127, 213202 (2021)

L. Varvarezos, J.C. Costello, C. Long, A.J. Achner, R. Wagner, M. Meyer and P. Grychtol

The effect of laser wavelength on the dissociation mechanisms in methane is examined over a broad range of intensities for both 800 nm and 400 nm laser fields. It is found that, at lower laser intensities, the dissociation pathways identified with the aid of theoretical calculations for the methane cation can account for most of the experimental findings, including the differences observed for irradiation by 800 nm and the 400 nm fields. As the laser intensity increases, the significance of the Coulomb explosion mechanism, along with the contribution of the rescattering process and the concomitant dissociation pathways, is highlighted.

Chem. Phys. Lett. 775, 138687 (2021)

T. Mazza, M. Ilchen, M. D. Kiselev, E. V. Gryzlova, T. M. Baumann, R. Boll, A. De Fanis, P. Grychtol, J. Montaño, V. Music, Y. Ovcharenko, N. Rennhack, D. E. Rivas, Ph. Schmidt, R. Wagner, P. Ziolkowski, N. Berrah, B. Erk, P. Johnsson, C. Küstner-Wetekam, L. Marder, M. Martins, C. Ott, S. Pathak, T. Pfeifer, D. Rolles, O. Zatsarinny, A. N. Grum-Grzhimailo, and M. Meyer

The nature of transient electronic states created by photoabsorption critically determines the dynamics of the subsequently evolving system. Here, we investigate K-shell photoionized atomic neon by absorbing a second photon within the Auger-decay lifetime of 2.4 fs using the European XFEL, a unique high-repetition-rate, wavelength-tunable x-ray free-electron laser. By high-resolution electron spectroscopy, we map out the transient Rydberg resonances unraveling the details of the subsequent decay of the hollow atom. So far, ultra-short-lived electronic transients, which are often inaccessible by experiments, were mainly inferred from theory but are now addressed by nonlinear x-ray absorption. The successful characterization of these resonances with femtosecond lifetimes provides the basis for a novel class of site-specific, nonlinear, and time-resolved studies with strong impact for a wide range of topics in physics and chemistry.

Phys. Rev. X 10, 041056 (2020)

S. Usenko et al.

Wave packet interferometry provides benchmark information on light-induced electronic quantum states by monitoring their relative amplitudes and phases during coherent excitation, propagation, and decay. The relative phase control of soft x-ray pulse replicas on the single-digit attosecond timescale achieved in our experiments makes this method a powerful tool to probe ultrafast quantum phenomena such as the excitation of Auger shake-up states with sub-cycle precision. In this contribution we present first results obtained for different Auger decay channels upon generating L-shell vacancies in argon atoms using Michelson-type all-reflective interferometric autocorrelation at a central free-electron laser photon energy of 274.7 eV.

J. Phys. B 53, 244008 (2020)

Sadia Bari, Ludger Inhester, Kaja Schubert, Karolin Mertens, Jan O. Schunck, Simon Dörner, Sascha Deinert, Schwob Lucas, Stefan Schippers, Alfred Müller, Stephan Klumpp and Michael Martins

On yields following X-ray absorption of the cationic series NH_y⁺(y= 0–3) were measured to identify the characteristic absorption resonances in the energy range of the atomic nitrogen K-edge. Significant changes in the position of the absorption resonances were observed depending on the number of hydrogen atoms bound to the central nitrogen atom. Configuration interaction (CI) calculations were performed to obtain line assignments in the frame of molecular group theory. To validate the calculations, our assignment for the atomic cation N⁺, measured as a reference, was compared with published theoretical and experimental data.

PCCP accepted (2019)

A. Müller, M. Martins, A. L. D. Kilcoyne, R. A. Phaneuf, J. Hellhund, A. _Borovik, Jr., K. Holste, S. Bari, T. Buhr, S. Klumpp, A. Perry-Sassmannshausen, S. Reinwardt, S. Ricz, K. Schubert, and S. Schippers

Photoprocesses of the endohedral fullerene ions S_c3N@C₊₈₀ and S_c3N@C₈₀− in the gas phase have been investigated in the photon energy ranges 30–50 eV and 280–420 eV. Single and double ionization as well as single ionization accompanied by the release of a C₂ dimer were observed as a function of the photon energy for the positive parent ion and double detachment was measured for the negative parent ion. The emphasis of the experiments was on the specific effects of the encapsulated trimetallic nitride cluster S_c3N on the observed reactions. Clear evidence of photoexcitation near the Sc L edge is obtained with the dominating contributions visible in the one- and two-electron-removal channels. K-vacancy production in the encapsulated central nitrogen atom is seen in the single ionization of S_c3N@C₊₈₀ but is much less pronounced in the photoionization-with-fragmentation channel. Comparison of the cross sections near the carbon K edge with the corresponding channels measured previously in the photoionization of L_u3N@C₊₈₀ reveal strong similarities. Previously predicted sharp resonance features in the ionization of S_c3N@C+80 ions below the Sc M edge are not confirmed. The experiments are accompanied by quantum-chemistry calculations in the Hartree-Fock approximation and by model calculations employing density functional theory (DFT).

Phys. Rev. A 99, 063401 (2019)

Stefan Schippers, Ticia Buhr, Alexander Borovik Jr., Kristof Holste, Alexander Perry-Sassmannshausen, Karolin Mertens, Simon Reinwardt, Michael Martins, Stephan Klumpp, Kaja Schubert, Sadia Bari, Randolf Beerwerth, Stephan Fritzsche, Sandor Ricz, Jonas Hellhund, and Alfred Müller

The Photon‐Ion Spectrometer at PETRA III—in short, PIPE—is a permanently installed user facility at the "Variable Polarization XUV Beamline" P04 of the synchrotron light source PETRA III operated by DESY in Hamburg, Germany. The careful design of the PIPE ion‐optics in combination with the record‐high photon flux at P04 has lead to a breakthrough in experimental studies of photon interactions with ionized small quantum systems. This short review provides an overview over the published scientific results from photon‐ion merged‐beams experiments at PIPE that were obtained since the start of P04 operations in 2013. The topics covered comprise photoionization of ions of astrophysical relevance, quantitative studies of multi‐electron processes upon inner‐shell photoexcitation and photoionization of negative and positive atomic ions, precision spectroscopy of photoionization resonances, photoionization and photofragmentation of molecular ions, and of endohedral fullerene ions.

X-Ray Spectrometry, 1-10 (2019). 10.1002/xrs.3035

M. Hollstein, K. Mertens, S. Klumpp, N. Gerken, S. Palutke, I. Baev, G. Brenner, S. Dziarzhytski, M. Meyer, W. Wurth, D. Pfannkuche1, M. Martins

We present studies on intra-molecular charge redistribution in iodine containing molecules upon iodine-4d photoionization. For this, we employed an XUV-pump-XUV-probe scheme based on time-delayed femtosecond pulses delivered by the free-electron laser at DESY in Hamburg (FLASH). The experimental results show delay dependent and molecule-specific iodine charge state distributions that arise upon multiple iodine-4d photoionization. Using the example of CH₃I and CH₂I₂, we compare the delay-dependent yields of I³⁺. We model the involved processes using advanced ab initio electronic structure calculations which include electron correlations combined with a classical model of the nuclear motion. The qualitative agreement of our model with the experimental results allows us to relate the observed, strongly molecule-specific efficiencies of the intra-molecular charge rearrangement not only to molecule-specific fragmentation timescales but also to molecule-specific electronic structure and molecular environment.

New J.Phys 21, 033017 (2019)

Stephan Klumpp, Alexander A. Guda, Kaja Schubert, Karolin Mertens, Jonas Hellhund, Alfred Müller, Stefan Schippers, Sadia Bari, and Michael Martins

Yields of atomic iodine I^q+ (q≥2) fragments resulting from photoexcitation and photoionization of the target ions IH+ and I+ have been measured in the photon-energy range 610–680 eV, which comprises the thresholds for iodine 3d ionization. The measured ion-yield spectra show two strong and broad resonance features due to the excitation of the 3d_3/2,5/2 electrons into ɛf states rather similar for both parent ions. In the 3d pre-edge range, excitations into (npπ)-like orbitals and into an additional σ^∗ orbital are found for IH+, which have been identified by comparison of the atomic I+ and molecular IH+ data and with the help of (time-dependent) density functional theory (DFT) and atomic Hartree-Fock calculations. The (5p π) orbital is almost atomlike, whereas all other resonances of the IH+ primary ion show a more pronounced molecular character, which is deduced from the chemical shifts of the resonances and the theoretical analysis.

Phys.Rev.A 97, 033401 (2018)
doi:10.1103/PhysRevA.97.033401

A. Müller, E. Lindroth, S. Bari, A. Borovik, Jr., P.-M. Hillenbrand, K. Holste, P. Indelicato, A. L. D. Kilcoyne, S. Klumpp, M. Martins, J. Viefhaus, P. Wilhelm, and S. Schippers

In a joint experimental and theoretical endeavor, photoionization of metastable C⁴⁺(1s2s³S₁) ions via intermediate levels with hollow, double-K-vacancy configurations 2s2p, 2s3p, 2p3s, 2p3d, 2s4p, 2p4s, and 2p4d has been investigated. High-resolution photon-ion merged-beams measurements were carried out with the resolving power reaching up to 25 800 which is sufficient to separate the leading fine-structure components of the 2s2p³P term. Many-body perturbation theory was employed to determine level-to-level cross sections for K-shell excitation with subsequent autoionization. The resonance energies were calculated with inclusion of electron correlation and radiative contributions. Their uncertainties are estimated to be below ±1 meV. Detailed balance confirms the present photoionization cross-section results by comparison with previous dielectronic-recombination measurements. The high accuracy of the theoretical transition energies together with the present experimental results qualify photoabsorption resonances in heliumlike ions as new, greatly improved energy-reference standards at synchrotron radiation facilities.

Phys. Rev. A 98, 033416 (2018)

A. Müller, A. Borovik, Jr., S. Bari, T. Buhr, K. Holste, M. Martins, A. Perry-Saßmannshausen, R. A. Phaneuf, S. Reinwardt, S. Ricz, K. Schubert, and S. Schippers

Double and triple detachment of the F⁻(1s²2s²2p⁶) negative ion by a single photon have been investigated in the photon energy range 660 to 1000 eV. The experimental data provide unambiguous evidence for the dominant role of direct photodouble detachment with a subsequent single-Auger process in the reaction channel leading to F²+ product ions. Absolute cross sections were determined for the direct removal of a (1s+2p) pair of electrons from F⁻ by the absorption of a single photon.

Phys.Rev.Lett. 120 133202(2018)

L. Müller, G. Hartmann, S. Schleitzer, M. H. Berntsen, M. Walther, R. Rysov, W. Roseker, F. Scholz, J. Seltmann, L. Glaser, J. Viefhaus, K. Mertens, K. Bagschik, R. Frömter, A. De Fanis, I. Shevchuk, K. Medjanik, G. Öhrwall, H. P. Oepen, M. Martins, M. Meyer, and G. Grübel

A transmission polarizer for producing elliptically polarized soft X-ray radiation from linearly polarized light is presented. The setup is intended for use at synchrotron and free-electron laser beamlines that do not directly offer circularly polarized light for, e.g., X-ray magnetic circular dichroism (XMCD) measurements or holographic imaging. Here, we investigate the degree of ellipticity upon transmission of linearly polarized radiation through a cobalt thin film. The experiment was performed at a photon energy resonant to the Co L₃-edge, i.e., 778 eV, and the polarization of the transmitted radiation was determined using a polarization analyzer that measures the directional dependence of photo electrons emitted from a gas target. Elliptically polarized radiation can be created at any absorption edge showing the XMCD effect by using the respective magnetic element.

Rev.Sci.Instrum. 89, 036103 (2018)

W. Helml, I. Grguras, P. N. Juranic, S. Düsterer, T. Mazza, A. R. Maier, N. Hartmann, M. Ilchen, G. Hartmann, L. Patthey, C. Callegari, J. T. Costello, M. Meyer, R. N. Coffee, A. L. Cavalieri, R. Kienberger

For the investigation of processes happening on the time scale of the motion of bound electrons, well-controlled X-ray pulses with durations in the few-femtosecond and even sub-femtosecond range are a necessary prerequisite. Novel free-electron lasers sources provide these ultrashort, high-brightness X-ray pulses, but their unique aspects open up concomitant challenges for their characterization on a suitable time scale. In this review paper we describe progress and results of recent work on ultrafast pulse characterization at soft and hard X-ray free-electron lasers. We report on different approaches to laser-assisted time-domain measurements, with specific focus on single-shot characterization of ultrashort X-ray pulses from self-amplified spontaneous emission-based and seeded free-electron lasers. The method relying on the sideband measurement of X-ray electron ionization in the presence of a dressing optical laser field is described first. When the X-ray pulse duration is shorter than half the oscillation period of the streaking field, few-femtosecond characterization becomes feasible via linear streaking spectroscopy. Finally, using terahertz fields alleviates the issue of arrival time jitter between streaking laser and X-ray pulse, but compromises the achievable temporal resolution. Possible solutions to these remaining challenges for single-shot, full time–energy characterization of X-ray free-electron laser pulses are proposed in the outlook at the end of the review.

Appl. Sci. 7, 915 (2017)

M. Waitz, R.Y. Bello, D. Metz, J. Lower, F. Trinter, C. Schober, M. Keiling, U. Lenz, M. Pitzer, K. Mertens, M. Martins, J. Viefhaus, S. Klumpp, T. Weber, L.Ph.H. Schmidt, J.B. Williams, M.S. Schöffler, V.V. Serov, A.S. Kheifets, L. Argenti, A. Palacios, F. Martı́n, T. Jahnke, and R. Dörner

The toolbox for imaging molecules is well-equipped today. Some techniques visualize the geometrical structure, others the electron density or electron orbitals. Molecules are many-body systems for which the correlation between the constituents is decisive and the spatial and the momentum distribution of one electron depends on those of the other electrons and the nuclei. Such correlations have escaped direct observation by imaging techniques so far. Here, we implement an imaging scheme which visualizes correlations between electrons by coincident detection of the reaction fragments after high energy photofragmentation. With this technique, we examine the H₂ two-electron wave function in which electron–electron correlation beyond the mean-field level is prominent. We visualize the dependence of the wave function on the internuclear distance. High energy photoelectrons are shown to be a powerful tool for molecular imaging. Our study paves the way for future time resolved correlation imaging at FELs and laser based X-ray sources.

Nat.Comm. 8 2266 (2017)

S. Klumpp, N. Gerken, K. Mertens, M. Richter, B. Sonntag, A. A. Sorokin, M. Braune, K. Tiedtke, P. Zimmermann and M. Martins

The multiple ionisation of atomic Mn, excited at (photon energy: 52.1 eV) and above (photon energy: 61.1 eV) the discrete giant 3p-3d resonance, was studied using high irradiation free-electron-laser soft x-ray pulses from the BL2 beamline of FLASH, DESY, Hamburg. In particular, the impact of the giant resonance on the ionisation mechanism was investigated. Ion mass-over-charge spectra were obtained by means of ion time-of-flight spectrometry. For the two photon energies, the yield of the different ionic charge states Mnq+ (q = 0–7) was determined as a function of the irradiance of the soft x-ray pulses. The maximum charge state observed was Mn6+ for resonant excitation at 52.1 eV and Mn7+ for non-resonant excitation at 61.1 eV at a maximum irradiation of 3 x 10 hoch 13 Wcmhoch -2 .

New J. Phys. 19, 043002 (2017)

M. Ilchen, T. Mazza, E. T. Karamatskos, D. Markellos, S. Bakhtiarzadeh, A. J. Rafipoor, T. J. Kelly, N. Walsh, J. T. Costello, P. O’Keeffe, N. Gerken, M. Martins, P. Lambropoulos, M. Meyer
In a combined experimental and theoretical study we have investigated the ionization of atomic argon upon irradiation with intense soft-x-ray pulses of 105 eV photon energy from the free-electron laser FLASH. The measured ion yields show charge states up to Ar7+. The comparison with the theoretical study of the underlying photoionization dynamics highlights the importance of excited states in general and of processes governed by electron correlation in particular, namely, ionization with excitation and shake-off, processes usually inaccessible by measurements of ionic yields only. The Ar7+ yield shows a clear deviation from the predictions of the commonly used model of sequential ionization via single-electron processes and the observed signal can only be explained by taking into account the full multiplet structure of the involved configurations and by inclusion of two-electron processes. The competing process of two-photon ionization from the ground state of Ar6+ is calculated to be orders of magnitude smaller.
Phys. Rev. A 94, 013413 (2016)

K. Mertens, N. Gerken, S. Klumpp, M. Braune and M. Martins

The fragmentation pattern of the iodine-containing molecules CH2I2 and CH3I following a strong multiphoton excitation in the vicinity of the iodine 4d giant resonance regime is studied using soft X-ray free electron laser radiation. A strong difference of the charge distribution and the kinetic energy release (KER) for the two molecules is found. The effects can be attributed to charge rearrangement processes induced by the photoexcitation. The difference in the observed distribution for higher charge states of iodine and carbon fragments is consistent with an over-the-barrier model for the charge rearrangement in the dissociating molecules. The KER for singly ionised carbon fragments indicates an ultrafast charge rearrangement before the dissociation starts.

J.Mod.Opt.,DOI 10.1080/09500340.2015.1046521 (2015)

T. Mazza, A. Karamatskou, M. Ilchen, S. Bakhtiarzadeh, A. J. Rafipoor, P. O’Keeffe, T. J. Kelly, N. Walsh, J. T. Costello, M. Meyer, R. Santra
Collective behaviour is a characteristic feature in many-body systems, important for developments in fields such as magnetism, superconductivity, photonics and electronics. Recently, there has been increasing interest in the optically nonlinear response of collective excitations. Here we demonstrate how the nonlinear interaction of a many-body system with intense XUV radiation can be used as an effective probe for characterizing otherwise unresolved features of its collective response. Resonant photoionization of atomic xenon was chosen as a case study. The excellent agreement between experiment and theory strongly supports the prediction that two distinct poles underlie the giant dipole resonance. Our results pave the way towards a deeper understanding of collective behaviour in atoms, molecules and solid-state systems using nonlinear spectroscopic techniques enabled by modern short-wavelength light sources.
Nature Communications 6, Article number: 6799 (2015)

M. Zitnik, A. Mihelic, K. Bucar, M. Kavcic, J.-E. Rubensson, M. Svanquist, J. Söderström, R. Feifel, C. Sathe, Y. Ovcharenko, V. Lyamayev, T. Mazza, M. Meyer, M. Simon, L. Journel, J. Lüning, O. Plekan, M. Coreno, M. Devetta, M. Di Fraia, et. al
Seeded free electron lasers theoretically have the intensity, tunability, and resolution required for multiphoton spectroscopy of atomic and molecular species. Using the seeded free electron laser FERMI and a novel detection scheme, we have revealed the two-photon excitation spectra of dipole-forbidden doubly excited states in helium. The spectral profiles of the lowest (−1,0)+1 Se1 and (0,1)0 De1 resonances display energy shifts in the meV range that depend on the pulse intensity. The results are explained by an effective two-level model based on calculated Rabi frequencies and decay rates.
Phys. Rev. Lett. 113, 193201 (2014)

N. Gerken, S. Klumpp, A. A. Sorokin, K. Tiedtke, M. Richter, V. Bürk, K. Mertens, P. Jurani, M. Martins
The time-dependent multiphoton ionization of xenon atoms is studied with femtosecond pulses in the excitation range of the 4d giant resonance at the photon energy of 93 eV. Benefiting from a new operation mode of the free electron laser FLASH, the measurements are performed with varying pulse durations. A strong dependence of the ion charge distribution on the pulse duration allows the different multiphoton mechanisms behind the multiple photoionization of xenon to be disentangled up to a charge state of Xe10+. The results up to Xe8+ are well explained by sequences of single photon, multiphoton, and Auger processes, but higher charge state generation suggests the need for collective electron multiphoton excitations.
Phys. Rev. Lett. 112, 213002 (2014)