Watching Chemistry in Action: Probing Ultrafast Chemical Dynamics by Time-Resolved Photoelectron Diffraction
CFM Seminars
- Speaker
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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).