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Univ. Paris-Saclay
Bonding, structure and ultrafast dynamics with x-ray spectroscopy
Philippe WERNET, BESSY, Berlin, Germany
Mardi 13/11/2007, 14:00
NIMBE Bât 522, p 138, CEA-Saclay
The bonding between atoms determines the structure in matter and vice versa. Both bonding and structure change at the atomic level and on ultrafast time scales during chemical reactions and phase transitions. Our aim is to elucidate bonding and structure of matter with x-ray spectroscopy and to follow how they change on the pico- to femtosecond (fs) time scales. X-ray spectroscopy provides a link between electronic and geometric structures and recent advances in short-pulse x-ray sources give unprecedented access to the ultrafast dynamics with x-ray spectroscopy. Selected topics from our ongoing investigations on bonding and structure in liquid water and ice from synchrotron radiation x-ray absorption spectroscopy and x-ray Raman scattering will be presented. Unexpected results on the local structure of liquid water are contrasted to textbook knowledge. They are used to motivate x-ray spectroscopy as a method to study bonding and structure in molecular systems. Recent results from investigations of the electronic structure of Br2 molecules in the gas phase during dissociation will be discussed. Here we used high-order harmonic generation of femtosecond laser pulses in rare gases as a source of fs vacuum ultra violet (VUV) pulses for time-resolved photoelectron spectroscopy. Laser pulses with a wavelength of 400 nm are used to excite the molecules to a dissociative state. The delayed VUV (53 nm wavelength) probe pulses then ionize the molecules allowing us to measure photoelectron spectra with an overall time resolution of 140 fs. Changes of the valence band spectrum indicate ultrafast breaking of the chemical bonds and rearrangements of the valence electronic structure are monitored all the way from the excited molecules into separate atoms.


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