Ultrafast electron dynamics in laser-driven atoms and molecules

Abstract

Laser light is able to produce field strengths capable of ionizing atoms, molecules and other complex multi-electron systems like clusters of atoms. Whilst ionization of atoms is understood relatively well, understanding the electron dynamics determining ionization of spatially extended systems remains a challenge. This dynamics takes place on the timescale of the light frequency which is about 1 femtosecond. Inner-shell relaxation processes in atoms are even faster and take place in the attosecond regime. In order to resolve such time-scales with pump-probe experiments, pulses in the extreme ultraviolet (xuv) spectral range with durations of a few hundred attoseconds are necessary. Recently the first measurements of single attosecond pulses using laser dressed single photon xuv ionization of gas atoms were reported. The determination of the xuv pulse duration from the electron spectrum was carried out with a classical theory. Although classical models are known to give a qualitatively correct description of strong laser atom interaction, the range of validity for accurate determination of sub-fs pulses must be scrutinized by a quantum mechanical analysis. In this thesis a theoretical framework for the accurate temporal characterization of attosecond xuv pulses, using a Fourier Bessel expansion of the xuv electron spectrum under the strong field approximation and a semi--classical derivation is established, putting earlier results on a rigorous theoretical footing. The analysis reveals an improved scheme that is by more than an order of magnitude more efficient than the one used so far and allows -- for the first time -- for direct experimental discrimination between single and multiple attosecond pulses. To study the laser induced electron dynamics in molecules it has turned out that it is indispensible to abandon the single active electron approximation, where only the weakest bound electron of a system is considered to interact with the laserfield in an avarage potential of all other electrons and which has proven to give good results for atoms. Because for the interaction of matter with strong laser fields the Schr\"odinger equation must be solved in a non-perturbative way and since there are no analytical methods available for molecules, numerical methods must be used. A full numerical solution of the time-dependent Schr\"odinger equation is, with the computer power available at the moment, only possible for at maximum two electrons in a strong field. Therefore efficient numerical methods capable of treating multiple electrons and taking into account the correlated electron-response must be developed. A promising approach based on Hartree-Fock theory but extended to several configurations for considering electron-correlation and able to deal with time dependent problems (Multi-configuration Time-dependent Hartree-Fock, MCTHDF), limited to one spatial dimension, will be introduced. With the help of the developed MCTDHF method, laser induced electron dynamics in spatially far extending systems like extremely big molecules, quantum dots and other structures in semiconductors, is studied. It is found, that if the system is excited with a photon frequency bigger than a characteristic frequency, the electron dynamics is comparable to the one of a free electron. This means, the motion of the electrons exhibits a phase factor of pi to the laser field, completely analogous to the behaviour of a classical system of second order when excited with a frequency higher than the resonance frequency. The characteristic frequency for this dynamics is the difference between the first excited state and the ground state for the one-electron case. For the behaviour of a cloud of electrons it is the plasma frequency. Due to their closely spaced energy level structures the above mentioned big systems are suitable candidates for studying this 'overresonant' electron dynamics. By systematically calculating the ionization probability of one- and multi-electron systems as a function of the system size, the ionization potential, the photon energy and the laser intensity and -duration it was possible to prove that the electron dynamics can be modeled by classical mechanics and to identify the ionization mechanism in this special frequency range. That is, the electrons pick up energy when they "hit" the potential barrier and are dephased from the laser field. This mechanism may eventually lead to ionization. A parameter range, where the quantum mechanical electron dynamics can be described by classical mechanics is identified. As a fourth point of this thesis tunneling ionization in molecules in the quasistatic frequency regime is investigated. Recent experiments show that ionization of molecules cannot be described by the quasistatic tunneling theory of Ammosov-Delone-Krainov (ADK) but exhibits a factor of 5-10 higher saturation intensities than predicted by this theory. The electron dynamics taking place during tunneling ionization is investigated with the MCTDHF method. It will be shown that the laser induced polarization of the molecule is responsible for the differences between experiment and theory. That is, the polarization potential of the remaining electron cloud adds to the effective potential felt by the tunneling electron. This boost of the Coulomb barrier decreases the tunneling probability and thus increases the saturation intensity. Further on it will be shown that results obtained with ADK-theory, which was developed for atoms, and experimental results for molecules cannot directly be compared to each other to quantify the influence of the induced polarization. To do so an effective one-electron potential must be used.