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SUMMARY: Tracking electron dynamics induced by XUV attosecond  pulses in m
 olecules.
DTSTART:20151217T163000
DTEND:20151217T173000
DTSTAMP:20260508T083526Z
UID:3f3ee30a3ef6e643930728146a5c5cc87468d8f5b3d9e9bdd3637675
CATEGORIES:Conferences - Seminars
DESCRIPTION:Dr. Francesca Calegari\nNational Research Council of Italy CNR
 -IFN\nPhysics Dept.\, Politecnico di Milano\nDynamical processes in molecu
 les occur on an ultrafast temporal scale\, ranging from picoseconds (1ps=1
 0-12 s) to femtoseconds (1fs =10-15 s) when concerning with a structural c
 hange\, down to attoseconds (1as = 10-18 s) when dealing with electrons. E
 lectron dynamics plays a very important role in bond-formation and bond-br
 eakage\, thus determining the final chemical reactivity of the molecule. R
 ecently\, theoretical studies have pointed out that after sudden ionizatio
 n of a large molecule very efficient “charge migration” can occur alon
 g the molecular backbone on a temporal scale ranging from few femtoseconds
  down to tens of attoseconds [1].\nIn this talk I will first show that XUV
  attosecond pulses in combination with NIR/VIS few-cycle pulses can be use
 d to investigate with extreme time-resolution the dissociative ionization 
 dynamics of molecular nitrogen. The time-dependent measurements reveal the
  presence of NIR/VIS-induced transitions between N2+ ionic states together
  with an interference pattern that carries the signature of the potential 
 energy curves activated by the XUV pulse. We show that the sub-fs characte
 rization of the interference pattern is essential for a semi-quantitative 
 determination of the repulsive part of these curves.\nI will then show tha
 t the same time-resolved measurement can be exploited to investigate charg
 e migration in more complex molecules such as aromatic amino acids. By mea
 suring the time evolution of the yield of the doubly charged immonium ion 
 for phenylalanine and tryptophan we were able to identify the presence of 
 fast modulations of the dication yield with periodicities of 4.3 fs for ph
 enylalanine and 3.9 fs for tryptophan. This fast dynamics can only be assi
 gned to a pure electron dynamics\, since nuclear dynamics usually comes in
 to play on a longer temporal scale [2].\nFinally\, the same experimental a
 pproach allowed us to measure in real-time hydrogen migration occurring in
  5-halo-uracils such as 5FU and 5BrU. In the case of 5FU\, the XUV pulse i
 onizes the molecule catalyzing the hydrogen migration from C6 to C5 at the
  first step of the reaction\, leading to the fragment m/z=44. The pump-pro
 be measurement reveals that at early times\, the NIR-VIS pulse inhibits th
 e hydrogen migration process thus resulting in a sudden reduction of fragm
 ents m/z=44\, and a consequent increase of fragment m/z=43. The rising (de
 caying) dynamics associated to fragment m/z=44 (m/z=43) occurs with a time
  constant of about 35 fs which can be associated to the time required by t
 he hydrogen atom to migrate from the C6 site to the C5 site of the molecul
 e. Similar behaviors were observed for the equivalent 5BrU fragments.\n[1]
  L. S. Cederbaum et al Chem. Phys. Lett. 307\, 205 (1999)\n[2] F. Calegari
  et al.\, Science 346\, 336 (2014)
LOCATION:CH G1 495 https://plan.epfl.ch/?room==CH%20G1%20495
STATUS:CONFIRMED
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