Ultrafast structural dynamics in electronically excited many-body systems

This thesis reports on results of three different experiments of photo-induced structural dynamics in the condensed phase, investigated by time-resolved pump-probe spectroscopy with femtosecond time-resolution. In the first part, we address the ultrafast dynamics of a quantum solid : crystalline hydrogen. This is accomplished by optical excitation of a dopant molecule, Nitric Oxide (NO), to a large orbital Rydberg state, which leads to a bubble-like expansion of the species surrounding the impurity. The dynamics is directly inferred from the time-resolved data, and compared with the results of molecular dynamics simulations. We report the presence of three time-scales in the structural relaxation mechanism: the first 200 fs are associated with the ultrafast inertial expansion of the first shell of lattice neighbors of NO. During the successive 0.6 ps, as the interactions between the molecules of the first and of the successive shells increase, we observe a progressive slowing-down of the bubble expansion. The third timescale (~ 10 ps) is interpreted as a slow structural re-organization around the impurity center. No differences were observed between the dynamics of normal- and para-hydrogen crystals, justifying the simplified model we use to interpret the data, which ignores all internal degrees of freedom of the host molecules. The molecular dynamics simulations reproduce fairly well the static and dynamic features of the experiment. In line with the measurements, they indicate that the quantum nature of the host medium plays no role in the initial ultrafast expansion of the bubble. In the second part, we present the results of our study on the photo-physics of triangular-shaped silver nanoparticles upon intraband excitation of the conduction electrons. The picosecond dynamics is dominated by periodic shifts of the surface plasmon resonance, associated with the size oscillations of the particles, triggered by impulsive lattice heating by the laser pulse. The oscillation period compares very well with the lowest totally symmetric vibrational frequency of a triangular-plate, which we calculated improving an existing elastodynamic model. We propose an explanation for the unusual phase behavior of the oscillations, based upon the non-spherical shape, and size-inhomogeneity of the sample. Taking into account these effects, we are able to reproduce spectrally and temporally our data. In the last part, we present a comparative study of the ligand dynamics in heme proteins. We studied the photo-induced spectroscopic changes in the ferric CN complexes of Myoglobin and Hemoglobin I upon photo-excitation of the porphyrin ring to a low-lying electronic state (Soret), monitoring the UV-visible region of the Soret band, and the mid-infrared region of the fundamental C=N vibrational stretch. The transient response in the UV-visible spectral region does not depend on the heme pocket environment, and is very similar to that known for ferrous proteins. The infrared data on the MbC=N stretch vibration provides a direct measure for the return of population to the ligated electronic (and vibrational) ground state with a 3 ps time constant. In addition, the CN stretch frequency is sensitive to the excitation of low frequency heme modes, and yields independent information about vibrational cooling, which occurs on the same timescale. The similarity between ferrous and ferric hemes rules out the charge transfer processes commonly invoked to explain the ligand dissociation in the former.

Chergui, Majed
Lausanne, EPFL
Other identifiers:
urn: urn:nbn:ch:bel-epfl-thesis3148-9

 Record created 2005-03-16, last modified 2018-03-17

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