How does intense X-ray radiation interact with molecules? Can one follow (and eventually control) the ultrafast rearrangement of electronic structure and the position of the nuclei during chemical reactions in real time? What is the mechanism and time scale of X-ray induced radiation damage?
These fundamental questions are at the heart of the remarkable effort of the international scientific community in the development and application of a new generation of light sources – free electron lasers (FEL) – delivering extremely intense, coherent short pulses of XUV/X-ray radiation.
The FEL sources available for users worldwide provide femtosecond (1 fs=10-15 s) pulses with up to 1013 photons per pulse and cover the photon energy range from 20 eV to 10 keV. A unique combination of short pulse duration with extremely high peak brilliance up to nine orders of magnitude higher than that achieved at the third-generation synchrotrons, offers unprecedented scientific possibilities in many areas of physics, chemistry, material and life sciences. First of all, high intensity FEL sources allow for novel fundamental studies of non-linear processes in the XUV/X-ray domain, characterized by the dynamical evolution of the target during interaction with the radiation pulse due to absorption of multiple photons within a single pulse. Understanding of these basic phenomena is essential for further studies of dynamic processes in complex systems. Second, ultrafast XUV FEL pulses allow extending the powerful techniques of femtochemistry towards time-resolved spectroscopy of molecular inner-valence shells revealing the coupled motion of electrons and nuclei during chemical reactions. In particular, time-resolved formation of intermediate structures and isomerization processes can be illuminated. Finally, an ultrafast exposure of a molecular system to an intense X-ray pulse provides an opportunity to obtain crucial information on the unperturbed molecular structure through X-ray diffraction or photoelectron diffraction imaging before any changes occur to the atomic positions or to the electronic structure of the object. Essential for these techniques is the understanding of the mechanisms of the X-ray induced radiation damage to the system under investigation.
Our group has a long-standing collaboration with Max Planck Advanced Study Group (Center for FEL Science, Hamburg) and the expert teams of A. Rudenko (J. R. Macdonald Laboratory, Manhattan, USA) and R. Moshammer (Max-Planck-Institut für Kernphysik, Heidelberg) in the area of application of XFEL sources for studies on atomic and molecular systems. In the framework of these collaborations the members of our group participated in a series of experiments at FLASH (Free electron LASer in Hamburg) and LCLS (Linac Coherent Light Source, Stanford) XFELs. The highlights of the recent results are presented below.
1) The experiments performed at LCLS have shown an unprecedentedly high degree of ionization of xenon atoms by 1.5 keV free-electron laser pulses to charge states with ionization energies far exceeding the photon energy. Comparing ion charge-state distributions and fluorescence spectra with state-of-the-art calculations, we find that these surprisingly high charge states are created via excitation of transient resonances in highly charged ions, and predict resonance enhanced absorption to be a general phenomenon in the interaction of intense X-rays with systems containing high-Z constituents. [Rudek et al. Nature Photonics 6, 857-864 (2012), Rudek et al. Phys. Rev. A 87, 023413 (2013)].
2) The Interatomic Coulombic Decay (ICD) mechanism of energy transfer between loosely bound atoms through Coulomb interaction of active electrons was theoretically predicted by L. Cederbaum [Cederbaum et al., Phys. Rev. Lett. 79, 4778 (1997)] and remains a hot topic of experimental and theoretical investigations. A direct measurement of the ICD lifetime in Neon dimers has been performed at FLASH via a pump-probe experiment in the extreme ultraviolet wavelength regime. The pump pulse creates a 2s inner-shell vacancy in one of the two Ne atoms, whereupon the ionized dimer undergoes ICD resulting in a repulsive Ne+(2p−1)−Ne+(2p−1) state, which is probed with a second pulse, removing a further electron. The yield of coincident Ne+ - Ne2+ pairs is recorded as a function of the pump-probe delay, which allows deducing the ICD lifetime of the Ne2+(2s-1) state to be (150±50) fs, in agreement with quantum calculations [Schnorr et al Phys. Rev. Lett. 111 093402 (2013)].
3) Polyatomic organic molecules bridge the gap between our understanding of the single-atom response to intense, ultrafast X-ray radiation and the behavior of larger nano-scale systems. Our experiments on methylselenol and ethylselenol molecules performed at LCLS show that embedding a heavy (Se) atom into an organic molecule strongly enhances degree of molecular ionization, and creates a “hot spot” of a localized high positive charge. This charge is subsequently shared with the neighboring atoms and is eventually distributed over the molecule, affecting the molecular fragmentation dynamics. Understanding these charge rearrangement processes and, in particular, the complex electronic and nuclear dynamics induced by multiple ionization, is crucial for coherent X-ray diffraction imaging schemes based on the so-called “diffract-before-destroy” concept. Furthermore, we have demonstrated that coincident Coulomb explosion imaging allows determining the laboratory-frame orientation of individual molecules also in the case of mutliphoton induced fragmentation, thus enabling investigation of fragmentation dynamics in the molecular frame. [Erk et al. Phys. Rev. Lett. 110, 053003 (2013), Erk et al. J. Phys. B 46 164031 (2013)
4) The most recently published results obtained at LCLS concern time-resolved investigations of ultrafast charge rearrangement and local radiation damage in dissociating molecules. Exploiting an infrared pump laser pulse for triggering the dissociation process, a localized multiple XFEL ionization of a heavy atom embedded in a molecular environment can be probed as a function of the internuclear distance increasing with the IR-pump/XFEL-probe delay. We explored the influence of molecular environment on multiple ionization of the iodine atom in a dissociating CH3I molecule. We observe considerable enhancement of the production of the highest charge states of iodine at very large internuclear distances, essentially originating from the ionization of the atomic iodine. For shorter distances the iodine charge state distribution shifts towards lower charge states reflecting electron transfer from the methyl group to the iodine atom. This result clearly shows that an interatomic charge redistribution plays an important role in the inner-shell molecular ionization for the CH3I molecule ([Erk et al. Science 345, 288 (2014)]