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Li, Hui (2016): Ultrafast dynamics from quantum to classical regime. Dissertation, LMU München: Faculty of Physics



This thesis presents work on the interaction of waveform controlled few-cycle pulses with various systems, from simple diatomic molecules through hydrocarbons and C60 fullerene, to nano-sized silver clusters. By employing the single-shot phase-tagged velocity-map imaging (VMI) technique, the angular distributions of ionic fragmentations and electrons can be obtained. Various electronic and molecular dynamics can be manipulated by controlling, e.g. carrier envelope phase (CEP), laser intensity, and polarization of few-cycle pulses. The dissociative ionization in the prototype molecular system D2 and multi-electron diatomic system DCl are investigated as a function of CEP and laser intensity. In D2, the population mechanism of highly excited 3σ state is identified by the analysis of the asymmetry in the emission of D+ ions. We find that at low intensities (up to about 2.0×1014 W cm−2), recollisional excitation followed by laser-field excitation can lead to population of the 3σ states. Direct excitation by electron recollision plays a role at higher laser intensities. The maximum asymmetry of about 44% is obtained for the recollisional channel at an intensity of around 1.3×1014 W cm−2 for a 4 fs pulse which indicates strong CEP-control on the electron localization. The dissociative ionization of DCl in 4 fs laser fields is studied for an intensity range of (1.3-3.1)×1014 W cm−2. D+ ions with kinetic energies above 15 eV are obtained for higher laser intensities. By comparison to quantum dynamical simulations we identify the high kinetic energy signal to originate from double ionization induced by rescattered electrons, exhibiting a characteristic CEP-dependence. Besides electron localization, CEP-controlled few-cycle pulses can also manipulate the rearrangement of molecular structures. Sub-cycle control of proton migration is demonstrated in allene and toluene. Quantum dynamical calculations performed on allene show that a superposition of vibrational modes can be created by waveform controlled few-cycle laser fields, which will result in a directionality of the hydrogen migration. As we move on to ever more complex molecular systems such as fullerenes, collective electron motion becomes important which can be controlled by tailoring the CEP and polarization. Quantum dynamical calculations show that the asymmetry parameter of electron emission from C60 can reflect the transient localization of the coherent electronic wave packet at the time of ionization. On the other hand, electron emission with high kinetic energy can be obtained in C60 with circularly polarized few-cycle pulses, which is dominated by multiple small-angle recolllision with the C60 shell. This provides a solid example for the collective motion of electrons in other complex systems. For silver nano-clusters, the angular distribution of electron emission is studied in dual-pulse laser fields and transient resonant plasmon excitation is observed at an optimal delay. The asymmetry oscillations show clear delay-dependent phase offsets for the low energy electron emissions. These electrons, which exhibit characteristic spectral features, may have a contribution from a correlated electron decay process involving two Rydberg state electrons. The work in this thesis advanced the CEP-control on various processes towards complex systems. Theory for small systems is capable of reproducing the results. However, this is still a challenge for the more complex systems.