Femtosecond electron dynamics driven by ponderomotive gradients

Electrons driven by the oscillating electric field of intense femtosecond laser pulses experience an effective cycle-averaged ponderomotive potential. This thesis investigates the ponderomotive effects of circularly polarized few-cycle pulses and standing waves on photoionization, photoemission, and the subsequent electron motion. Reconstructing the time-dependent dipole moment of recolliding electron wave packets from an extreme ultraviolet absorption spectrum reveals an envelope-driven recollision channel in circularly polarized light, as predicted by the theory of recolliding periodic orbits. Furthermore, tomography of photoelectron angular distributions is achieved using velocity-map imaging within a femtosecond enhancement cavity operating at 100MHz. This cavity supports two counter-propagating pulses that form a transient standing wave at the focus. Exploiting the standing wave structure allows to probe the photoemission at different crystal facets of a tungsten nanotip and to minimize the source volume of above-threshold ionization. In addition, the high-intensity Kapitza-Dirac effect for resonantly ionized electrons in the picosecond regime is presented together with still unexplained diffraction results for femtosecond pulses. The intense intra-cavity standing waves are a promising tool for coherent control of electron matter waves, potentially advancing the development of a femtosecond electron interferometer at low energies.