Watching Chemistry in Action: Probing Ultrafast Chemical Dynamics by Time-Resolved Photoelectron Diffraction

CFM Seminars

Speaker

Daniel Rolles, Max Planck Advanced Study Group, Center for Free-Electron Laser Science (CFEL), Hamburg, Germany

When

2012/06/13
14:00

Place

Auditorium of the Centro de Fisica de Materiales, Paseo Manuel de Lardizabal 5, Donostia-San Sebastián

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**Watching Chemistry in Action:** **Probing Ultrafast Chemical Dynamics** **by Time-Resolved Photoelectron Diffraction** Daniel Rolles _Max Planck Advanced Study Group, Center for Free-Electron Laser Science (CFEL), Hamburg, Germany_ Taking a movie of a chemical reaction with atomic resolution? Watching the making and breaking of chemical bonds in real time? The successes of the world's first VUV and X-Ray Free-Electron Lasers (FELs) FLASH at DESY in Hamburg and LCLS at SLAC in Stanford together with the continuing technical advances in the creation of (sub-) femtosecond VUV pulses by high harmonic generation (HHG) have turned the once lofty vision of "recording a molecular movi" with femtosecond temporal and atomic scale structural resolution into a realistic scenario. By means of femtosecond pump-probe experiments with intense and short-pulse VUV and X-ray FEL radiation, our goal is to establish and further develop the experimental techniques capable of imaging photochemical reactions in gas- phase molecules in order to study exemplary reactions of chemical relevance with the aim to clarifying their pathways. I will present results from exemplary photoelectron diffraction and Coulomb explosion imaging studies on adiabatically laser-aligned [1] and mixed-field oriented carbonyl sulfide, dibromobenzene, and p-fluorophenylacetylene molecules recorded after inner-shell photoionization. The experiments were performed with a double-sided velocity map imaging (VMI) spectrometer installed in the CFEL-ASG MultiPurpose (CAMP) chamber [2], which allowed recording electron and ion distributions at the same time. Through comparison with multiple scattering calculations [3], the measured photoelectron diffraction images can be related to the geometric structure of the molecules. In a second step, time-resolved photoelectron diffraction patterns of laser- aligned molecules were recorded by dissociating the molecules with a femtosecond infrared (IR) laser pulse prior to the FEL ionization and varying the delay between IR pump and FEL probe pulse. Time-dependent changes are observed in both electron and ion distributions. [1] H. Stapelfeldt, T. Seideman, _Aligning molecules with strong laser pulses_ , Rev. Mod. Phys. **75** , 543-557 (2003). [2] L. Strüder _et al_., _Large-format, high-speed, X-ray pnCCDs combined with electron and ion imaging spectrometers in a multipurpose chamber for experiments at 4th generation light sources_ , Nucl. Instr. and Meth. in Phys. Res. A **614** , 483-496 (2010). [3] F. J. Garcia de Abajo, M. A. van Hove, C. S. Fadley, _Multiple scattering of electrons in solids and molecules: A cluster-model approach_ , Phys. Rev. B **63** , 075404 (2001).