Investigation of low-energy photoelectron dynamics accelerated in terahertz light fields using a novel Velocity-Map-Imaging spectrometer

In this work, the dynamics of low-energy photoelectrons, generated by multiphoton ionization from near-infrared (NIR) laser pulses, in the presence of intense near single-cycle carrier-envelope phase stable Terahertz (THz) pulses, were experimentally investigated. For certain time delays between the NIR and THz pulses, a strong modulation of the photoelectron momentum distribution was observed and attributed to rescattering from the ionic core. During the rescattering process, an additional momentum was transferred to the photoelectrons leading to a higher kinetic energy in the continuum in contrast to directly emitted photoelectrons that are not rescattered. In another experiment, the 51${}^{st}$ harmonic of the NIR laser, generated by a high-order harmonic generation source, was used to ionize 4d photoelectrons in Xe leading to single and double Auger decays. A dominant decay channel was a double Auger decay, where a slow Auger (SA) and a fast Auger electron are involved. The superposition with intense THz radiation led to a relative time shift between the time-dependent 4d${}_{5/2}$ photoelectron and the SA electron momentum distributions. This relative time shift can be explained by the Auger lifetime τ${}_{AE}$>23 fs of the SA electron as well as contributions of the Eisenbud-Wigner-Smith and Coulomb-laser coupling time delays, which are pronounced at low kinetic electron energies and long streaking wavelengths. In the both experiments, the angular momentum distribution of photoelectrons was measured by a velocity-map imaging spectrometer (VMIS) with a novel capillary gas injection. The VMIS provides high target gas densities while preserving its energy resolution and was embedded in a table-top THz streak camera.