Session B7: Invited Session: Ultrafast X-ray Imaging of Electronic and Structural Dynamics

Partial covariance mapping techniques at FELs

The development of free-electron lasers (FELs) is driven by the desire to access the structure and chemical dynamics of biomolecules with atomic resolution. Short, intense FEL pulses have the potential to record x-ray diffraction images before the molecular structure is destroyed by radiation damage [1]. However, even during the shortest, few-femtosecond pulses currently available, there are some significant changes induced by massive ionisation and onset of Coulomb explosion. To interpret the diffraction images it is vital to gain insight into the electronic and nuclear dynamics during multiple core and valence ionisations that compete with Auger cascades. This paper focuses on a technique that is capable to probe these processes. The covariance mapping technique [2] is well suited to the high intensity and low repetition rate of FEL pulses. While the multitude of charges ejected at each pulse overwhelm conventional coincidence methods [3], an improved technique of \textit{partial} covariance mapping can cope with hundreds of photoelectrons [4] or photoions [5] detected at each FEL shot. The technique, however, often reveals spurious, uninteresting correlations that spoil the maps. This work will discuss the strengths and limitations of various forms of covariance mapping techniques. Quantitative information extracted from the maps will be linked to theoretical modelling of ionisation and fragmentation paths. Special attention will be given to critical experimental parameters, such as counting rate, FEL intensity fluctuations, vacuum impurities or detector efficiency and nonlinearities. Methods of assessing and optimising signal-to-noise ratio will be described. Emphasis will be put on possible future developments such as multidimensional covariance mapping, compensation for various experimental instabilities and improvements in the detector response. \\[4pt] [1] R. Neutze, R. Wouts, D. van der Spoel, E. Weckert and J. Hajdu, \textit{Nature (London)}, 2000, \textbf{406}, 752.\\[0pt] [2] L. J. Frasinski, K. Codling and P. A. Hatherly, \textit{Science}, 1989, \textbf{246}, 1029.\\[0pt] [3] J. H. D. Eland, \textit{Adv. Chem. Phys.}, 2009, \textbf{141}, 103.\\[0pt] [4] L. J. Frasinski, \textit{et al.}, \textit{Phys. Rev. Lett.}, 2013, \textbf{111}, 073002.\\[0pt] [5] O. Kornilov, \textit{et al.}, \textit{J. Phys. B: At. Mol. Opt. Phys.}, 2013, \textbf{46}, 164028.   

Imaging of Quantum Vortices in Superfluid Helium Droplets

Quantum rotation in single, isolated superfluid He nanodroplets is studied via x-ray diffraction imaging. The images indicate large centrifugal shape deformations of the droplets, providing a direct measure of the angular velocity. The droplets have axisymmetric shapes that persist to unusually high angular velocities well beyond the limits of classical liquid rotors. Regular vortex arrays formed within the rotating droplets are observed and characterized through their specific Bragg patterns. The droplets have high density of vortices giving access to unexplored regime of ultimate quantum vorticity.   

Transient electron density maps of ionic materials from femtosecond x-ray powder diffraction

X-ray diffraction represents a key method for spatially resolving electron distributions in crystalline materials. So far, electron density maps have been derived from stationary diffraction patterns, providing detailed insight into the equilibrium charge distribution and crystal geometry. Functional processes in condensed matter are frequently connected with nonequilibrium excitations resulting in atomic motions and charge relocations on ultrashort time scales. Transient structures are resolved in space and time by novel x-ray diffraction methods with a femtosecond time resolution, giving access to the driving mechanisms and underlying interactions [1]. In this talk, new results are presented on transient electron distributions of ionic materials mapped with the help of femtosecond x-ray powder diffraction. Experiments are based on a pump-probe approach in which an optical pulse initiates structural dynamics and a hard x-ray pulse from a synchronized laser-driven plasma source is diffracted from the excited powder sample. Such measurements reveal the interplay of lattice and charge motions in the photoexcited prototype material KDP (KH$_{2}$PO$_{4})$ which occur on distinctly different length scales [2]. As a second topic, electron relocations induced by strong external optical fields will be discussed [3,4]. This interaction mechanism allows for generating coherent superpositions of valence and conduction band quantum states and inducing fully reversible charge dynamics. While the materials LiBH$_{4}$ and NaBH$_{4}$ display electron relocations from the (BH$_{4})^{-}$ ions to the neighboring Li$^{+}$ and Na$^{+}$ ions, LiH exhibits an electron transfer from Li to H. The latter is a manifestation of electron correlations and in agreement with theoretical calculations.\\[4pt] [1] T. Elsaesser, M. Woerner, J. Chem. Phys. 140, 020901 (2014)\\[0pt] [2] F. Zamponi et al., Proc. Nat. Acad. Sci. USA 109, 5207 (2012)\\[0pt] [3] J. Stingl et al., Phys. Rev. Lett. 109, 147402 (2012)\\[0pt] [4] V. Juv\'{e} et al., Phys. Rev. Lett. 111, 217401 (2013)   

Observation of ultrafast charge migration in an amino acid

Electron transfer within a single molecule is the fundamental step of many biological processes and chemical reactions. It plays a crucial role in catalysis, DNA damage, photosynthesis and photovoltaics. The investigation of this process has been the subject of considerable research effort [1]. Electron transfer driven by solely electronic correlations is well known as ``charge migration'' and it occurs in a few femtoseconds. In this work we present the first observation of ultrafast charge migration in the amino acid phenylalanine using XUV attosecond pulses. Neutral molecules were produced in gas phase by heating a thin metallic foil with a CW laser. Phenylalanine molecules were irradiated by a 250-as pump pulse with photon energy in the range 16-35 eV, followed by a 4-fs VIS/NIR probe pulse. The produced parent and fragment ions were then extracted into a linear TOF device for mass analysis. The main contributions in the mass spectrum correspond to the parent ion M$+$ (165 Da), the immonium ion (M-COOH $=$ 120), the backbone of the amino acid (M-R $=$ 74) and the phenyl groups (R $=$ 91, R $+$ H $=$ 92). A small peak at 60 can be assigned to the doubly charged immonium ion [2]. Pump-probe measurements evidenced an exponential decay of the yield of fragment 60 with a time constant of 30 fs. This ultrashort time constant suggests that the dication dynamics is initiated by ionization of an inner-valence electron. By increasing the temporal resolution of the measurement we were able to observe a clear modulation of the yield with a periodicity of a few femtoseconds. This ultrafast dynamics can only be associated with purely electronic processes, thus constituting a clear experimental evidence of charge migration in biomolecules. \\[4pt] [1] O. Bixner et al., J. Chem. Phys. 136, 204503 (2012).\\[0pt] [2] L. Belshaw et al., J. Phys. Chem. Lett. 3, 3751 (2012).