Session J7: Invited Session: AMO Physics with Free Electron Lasers

Imaging charge and energy transfer in molecules using free-electron lasers

Charge and energy transfer reactions drive numerous important processes in physics, chemistry and biology, with applications ranging from X-ray astrophysics to artificial photosynthesis and molecular electronics. Experimentally, the central goal in studies of transfer phenomena is to trace the spatial localization of charge at a given time. Because of their element and site sensitivity, ultrafast X-rays provide a promising tool to address this goal. In this talk I will discuss several experiments where free-electron lasers were employed to study charge and energy transfer dynamics in fragmenting molecules. In a first example, we used intense, 70 femtosecond 1.5 keV pulses from the Linac Coherent Light Source (LCLS) to study distance dependence of electron transfer in laser-dissociated methyl iodide molecules. Inducing well-localized positive charge on the heavy iodine atom, we observe signature of electron transition from the separated methyl group up to the distances of 35 atomic units. In a complementary experiment, we studied charge exchange between two partners in a dissociating molecular iodine employing a pump-probe arrangement with two identical 90 eV pulses from the Free-Electron LASer in Hamburg (FLASH). In both cases, the effective spatial range of the electron transfer can be reasonably described by a classical over-the-barrier model developed for ion-atom collisions. Finally, I will discuss a time-resolved measurement on non-local relaxation mechanism based on a long-range energy transfer, the so-called interatomic Coulombic decay.   

Ultrafast nonadiabatic processes in photoionized molecular systems probed by time-resolved core-level spectroscopy

There is a fundamental interest in understanding the coupled nuclear and electronic dynamics associated to charge transfer processes in complex molecules and materials, which is often mediated by electron, electron hole or proton motion. For example, the response of the light hydrogen atoms to a newly created electron hole in a water cluster can be as fast as sub 5 fs [PRL {\bf 110}, 038302 (2013)] and involve strong nuclear-electronic couplings. With dramatic improvements in the techniques to generate extreme ultraviolet (XUV) and x-ray femtosecond pulses, it becomes now possible to trigger and probe these kinds of processes in real time. Here, we study the dynamics of an electron hole created by photoionization in the valence shell of protonated water clusters H$^+$(H$_2$O)$_n$ and its correlated motion with protons in the hydrogen bond network due to electrostatic interactions. We will discuss from a theory perspective how key aspects of the electron hole dynamics can be mapped out to core-level transient x-ray absorption spectra with femtosecond resolution.   

X-ray FEL induced multiphton ionization and molecular dissociation

X-ray Free electron lasers (FELs) enable multiphoton absorption at the core levels which is not possible with conventional light sources. Multiphoton ionization and the subsequent core-hole states relaxation lead to dramatic dynamics of the molecules. We present our experimental as well as theoretical results on multiphoton ionization and molecular fragmentation dynamics with the Linac Coherent Light Source (LCLS) at SLAC National Laboratory. We investigated simple diatomic system, N2 molecules, where we used multiphoton ionization as an internal clock for imaging the dynamics in time and the internuclear separation domain [1]. We observed the modification of the ionization dynamic by varying the x-ray beam parameters and the effect of the spatial distribution on the ionization. We also investigated a complex system, C60, where we developed a full model to simulate the multiphoton ionization that results in various molecular ions and atomic carbon ions up to charge 6+. The calculation agrees well with our experimental results in ion kinetic energy distribution and charge state distribution. Moreover, our model provides further insights into the photoionization and dissociation dynamics as a function of time and molecular size.\\[4pt] This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. Thank T. Osipov, B. Murphy, Z. Jurek, S.-K. Son, R. Santra, and N. Berrah, M. Hoener, O. Gessner, F. Tarantelli, S.T. Pratt, O. Kornilov, C. Buth, M. G\"uehr, E. Kanter, C. Bostedt, J. D. Bozek, P. H. Bucksbaum, M. Chen, R. Coffee, J. Cryan, L. DiMauro, M. Glownia, E. Kukk, S.R. Leone, L. Avaldi, P. Bolognesi, J. Eland, J. Farrell, R. Feifel, L. Frasinski, D.T. Ha, K. Hoffmann, B. McFarland, C. Miron, M. Mucke, R. Squibb, K. Ueda for their contributions to this work.\\[4pt] [1] \textit{Multiphoton ionization as a clock to reveal molecular dynamics with intense short X-FEL pulses}. L. Fang, T. Osipov, B. Murphy, F. Tarantelli, E. Kukk, J.P. Cryan, M. Glownia, P.H. Bucksbaum, R.N. Coffee, M. Chen, C. Buth and N. Berrah, Phys. Rev. Lett. \textbf{109}, 263001 (2012)   

SACLA: new opportunities for atomic, molecular, and cluster science with XFEL

Angstrom Compact free electron LAser (SACLA), started user operation in Japan [1,2]. We set up the program to investigate the dynamical behavior of heavy atoms as an isolated atom, in the molecule, and in the cluster, with SACLA. At 5.5 keV, with the fluence of 50 $\mu $J/$\mu $m$^{2}$, we could identify that Xe$^{n+}$ with $n$ up to 26 is produced, evidencing the occurrence of deep inner-shell ionization and sequential electronic decay cycles repeated multiple times in the heavy atom within the XFEL pulse of $\sim$ 10 fs [3]. Reducing the photon energy to 5 keV, with the fluence of 50 $\mu $J/$\mu $m$^{2}$, we could identify the occurrence of resonance-enabled x-ray multiple ionization [4]. The results for momentum-resolved multiple ion coincidence study on iodine-contained organic molecules illustrates that the charges are produced in the iodine site by the deep inner-shell ionization and sequential electronic decay cycles and spreads over the entire molecule within the FEL pulse duration (\textless 10 fs), leading to Coulomb explosion. The light atoms, however, move significantly, a few hundreds pm for hydrogen atoms, before completing the ionization-decay cycles within the pulse duration of 10 fs. The results for electron spectroscopy on argon and xenon clusters illustrate that nanoplasma are formed by the XFEL pulse and continuous thermal emission from the plasma occurs in the time scale of ps. \\[4pt] [1] T. Ishikawa \textit{et al.}, Nature Photon. \textbf{6}, 540 (2012).\\[0pt] [2] M. Yabashi, H. Tanaka, T. Tanaka, H. Tomizawa, T. Togashi, M. Nagasono, T. Ishikawa, J. R. Harries, Y. Hikosaka, A. Hishikawa, K. Nagaya, N. Saito, E. Shigemasa, K. Yamanouchi, and K. Ueda, J. Phys. B: At. Mol. Opt. Phys. \textbf{46}, 164001 (2013).\\[0pt] [3] H. Fukuzawa, S.-K. Son, K. Motomura \textit{et al.}, Phys. Rev. Lett. \textbf{110}, 173005 (2103).\\[0pt] [4] K. Motomura, H. Fukuzawa, S.-K. Son \textit{et al.}, J. Phys. B: At. Mol. Opt. Phys. \textbf{46, }164024 (2013).