UC Berkeley

The development of ultrafast light sources has made it possible to directly observe electron dynamics as they unfold on their natural timescales. Resolving the full richness of these dynamics, including the influence of the excited state orbital compositions and spin, requires experimental probes sensitive to more than just absorption intensity. The results presented in this work demonstrate that control over the polarization state of the pump and probe pulses in attosecond transient absorption spectroscopy provides access to these additional degrees of freedom.In atomic helium, elliptically polarized XUV pulses are used to prepare oriented ensembles of 1snp Rydberg states. The polarization-dependent couplings induced by an elliptically polarized NIR pulse are understood using the selection and propensity rules, establishing a foundation for polarization-resolved investigations of coupling pathways in complex systems. The method is then extended to the condensed phase, where control over the relative XUV and NIR polarizations reveals a strong polarization dependence in the coupling between bright and dark core exciton states in LiF. In nickel, the spin-asymmetric electronic density of states near the Fermi level gives rise to magnetic circular dichroism. The temporal evolution of this dichroism provides insight into ultrafast demagnetization. The continuous nature of the XUV probe spectrum reveals a significant probe-energy-dependent apparent delayed onset of the change in magnetic circular dichroism. Rather than an underlying magnetization dynamic, this apparent delay is shown to arise from overlapping magnetic and non-magnetic spectral features with distinct kinetics around the M3 edge. This finding has direct implications for ongoing investigations of sublattice-dependent demagnetization timescales in magnetic alloys.Together, these results establish the laser polarization as an indispensable experimental degree of freedom in attosecond transient absorption spectroscopy, providing sensitivity to both spin- and orbital-dependent ultrafast dynamics across a broad range of systems.