Electron Wave Packet Dynamics on the Attosecond Time Scale

Sammanfattning: One objective of attosecond science is to study electron dynamics in atoms and molecular systems on their natural time scale. This can be done using attosecond light pulses. Attosecond pulses are produced in a process called high-order harmonic generation, in which a short, intense laser pulse interacts with atoms or molecules in a highly nonlinear process, leading to the generation of high-order frequencies of the fundamental laser with a large spectral bandwidth, supporting pulses with attosecond duration. In some condition the harmonics are locked in phase leading to a train of attosecond pulses or, in some cases, to a single attosecond pulse. This thesis presents experiments based on interferometry to study electron dynamics using attosecond pulses. The first part describes a series of experiments, in which the dynamics of electrons was studied after photoionization with an attosecond pulse train. The time resolution in these experiments was achieved by measuring the accumulated phase of the free electron wave packet after photoemission using an interferometric technique. The phase carries temporal information about the ionization process, from which the delay in photoemission can be determined with a much better time resolution than that given by the temporal structure of the pulse train. The same technique was applied to investigate the phase behavior of resonant two-photon ionization in helium atoms. The second part describes the application of an interferometric pump-probe technique to characterize bound electron wave packets. Single attosecond pulses are used to excite a broad electron wave packet containing bound and continuum states. The bound part of the wave packet is further ionized by an infrared laser with a variable delay. Analysis of the resulting interferogram allows for full reconstruction of the bound wave packet, since both the amplitude and the phase of all ingoing states in the wave packet are encoded in the interference pattern.