Attosecond interferometry: techniques and spectroscopy

Abstract: The interaction between an intense laser pulse and a gas medium leads to the emission of coherent bursts of light in the extreme ultraviolet range. This process, known as high-order harmonic generation, has today, almost three decades after its discovery, developed into a reliable source of extremely short (on the order of 100 as) pulses of electromagnetic radiation, with a wide range of applications in the atomic, molecular and optical sciences. The access to radiation with attosecond duration opens up new possibilities for studying and even controlling electronic processes that take place on this timescale. This thesis presents a series of experiments where sequences of attosecond pulses, attosecond pulse trains, are used to perform photoelectron interferometry. Free electronic wave packets, launched via photoabsorption of a coherent train of ionizing attosecond pulses, are manipulated by an infrared laser field and brought to interfere. From the resulting interferogram the phase of the escaping wave packets can be partly reconstructed. This phase in turn carries a signature of the interactions that lead to the ejection of the electron. Under certain conditions the measured phase can be related to a delay of the wave packet, corresponding to the time it takes for it to escape the ionic binding potential, called photoionization time delay. This method was applied to a range of atomic systems and ejection mechanisms in order to study the influence of atomic electronic structure on the ejection of electrons. Since the composition of the electronic wave packets is partly determined by the temporal structure of the ionizing radiation, a comparative approach was applied to isolate the effect of the ion-electron interactions. The photoionization time delay for ionization from the 3s subshell of argon was measured relative to that of the 3p shell. In another experiment the delay of a two electron wave packet resulting from double ionization of xenon was referenced to single ionization from the valence shell. In an iterative measurement procedure, interferograms were cross-referenced from ionization of the valence shells of argon, helium and neon. Finally, the significant phase distortion resulting from an autoionizing resonance in argon was mapped out by stepwise tuning the central frequency of the exciting pulse train. The interferometric method was also utilized to study the temporal synchronization between the attosecond pulse train and the laser pulse used to produce it. The results show that the synchronization is dependent on the density of the gaseous medium due to the specific dispersion properties of the gas.