Optical and quantum interferences in strong field ionization and optimal control

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Foote, David B.
Hill, Wendell T.
For decades, ultrafast laser pulses have been used to probe and control strong-field molecular dynamics, including in optimal control experiments. While these experiments successfully recover the optimal control pulses (OCPs), they have a limitation -- it is generally unknown how the OCP guides the target system to its final state. This thesis is concerned with "unpacking" OCPs to explain how they achieve their control goals. The OCPs that inspired this work consisted of pulse trains; a twin-peaked pulse (TPP) is the simplest example. Consequently, TPPs with variable interpeak delay and relative phase were employed in this work to study ionization, the first step in many control experiments. Two types of interference influence ionization from a TPP: optical interference (OI) between the electric fields of the two peaks, and quantum interference (QuI) between the electron wavepackets produced by the two peaks. Two sets of experiments were performed to determine what roles OI and QuI play in controlling ionization from a TPP and how they in turn influence subsequent molecular dynamics. The first set of experiments measured the total ionization yield induced by the TPPs. It was found that OI was principally responsible for changing the ion yield; QuI-induced oscillations were not observed. Small imperfections in the shape of the TPP (i.e., pedestals and subordinate peaks) were found to have a surprisingly large influence in the OI, highlighting the need for researchers in molecular control experiments to characterize the temporal profile of their pulses accurately. A time-dependent perturbation theory simulation showed that the signatures of QuI in the ionic continuum vanish when measuring {\it total} electron yield, but appear in {\it energy-resolved} electron yields. The second set of experiments measured photoelectron energy distributions from a TPP with a velocity map imager to search for QuI. The experiments were performed at high intensities (~10^14 W/cm^2) where the ponderomotive energy tends to wash out the fine energy structures of QuI. The thesis ends by proposing a modified, low-intensity experiment that will allow for the first unambiguous observation of QuI in non-resonant, multiphoton ionization.