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Charge migration in amino acids: observing pure electron dynamics induced by XUV attosecond pulses

Date Do, 17.12.2015 - Do, 17.12.2015
Time 16.30
Speaker Dr. Francesca Calegari, Institute for Photonics and Nanotechnologies, IFN-CNR, Milano, Italy
Location EPF Lausanne, CH G1 495
Program Dynamical processes in molecules occur on an ultrafast temporal scale, ranging from picoseconds (1ps=10-12 s) to femtoseconds (1fs =10-15 s) when concerning with a structural change, down to attoseconds (1as = 10-18 s) when dealing with electrons. Electron dynamics plays a very important role in bond-formation and bond-breakage, thus determining the final chemical reactivity of the molecule. Recently, theoretical studies have pointed out that after sudden ionization of a large molecule very efficient “charge migration” can occur along the molecular backbone on a temporal scale ranging from few femtoseconds down to tens of attoseconds.
In this talk I will report on clear experimental evidence of charge migration in aromatic amino acids, induced by attosecond ionization. In our experiments, charge migration was measured by using a two-color, pump-probe technique. Charge dynamics was initiated by isolated sub-300-as pulses, with photon energies in the spectral range between 17 eV and 35 eV and probed by 4-fs, waveform-controlled near infrared (NIR) pulses, with central wavelength of 720 nm. A clean plume of neutral amino acids (phenylalanine and tryptophan) was generated by evaporation from a thin metallic foil heated by a CW diode laser. The ions produced by the interaction of the molecules with pump and probe pulses were then collected by a linear time-of-flight device for mass analysis.
We have measured the evolution of the yield of the doubly charged immonium ion (m/z = 60 for phenylalanine and m/z=79.5 for tryptophan) as a function of the delay between the attosecond pump pulse and the NIR probe pulse. We were able to identify the presence of fast modulations of the dication yield with periodicities of 4.3 fs for phenylalanine and 3.9 fs for tryptophan. This fast dynamics can only be assigned to a pure electron dynamics, since nuclear dynamics usually comes into play on a longer temporal scale.
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Link isic.epfl.ch
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