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|Title:||ALS-CXRO Seminar | Marcus Guehr (Stanford)|
|When:||05/30/2012 3:00 PM - 4:00 PM|
|Description:||ALS-CXRO Seminar Series|
Wed, May 30, 2012
Probing valence electrons with free electron lasers and high harmonic generation
Molecules are highly intertwined systems of interacting particles in which electrons screen the repulsive forces between the positively charged nuclei. The ultrafast community has devoted much effort to investigate how nuclei move on femtosecond timescales. We want to extend this frontier and address the question of why the nuclei move by studying electron dynamics with high temporal and spatial resolution. I will present experiments on electrons that utilize the femtosecond to attosecond temporal resolution and Angstrom spatial sensitivity available with strong field high harmonic generation (HHG) and x-ray free electron lasers.
In HHG, an electron tunnel ionizes from an atom or molecule at the crest of a strong IR laser field. The electron is subsequently accelerated into the ion under emission of extreme ultraviolet (XUV) light. We have established that the shape of an HHG spectrum is related to the valence electronic structure of atoms and molecules. Our experiments also show that the tunnel ionization creates a superposition of multiple ionic states and therefore an electronic wavepacket on the molecular ion.
In the second part of my talk the high spatial electronic sensitivity in soft x-ray (SXR) induced transitions will play a central role. We used the Linac Coherent Light Source (LCLS) and the Advanced Light Source (ALS) to study the ultrafast photoprotection mechanism of nucleobases. Although those bases are strong UV absorbers, they are not destroyed nor altered by the light due to fast electronic relaxation processes. The high spatial and temporal electronic sensitivity of soft x-ray pulses at LCLS can give us a precise answer on the photoprotection details.