Continuum phase shifts and partial cross sections for photoionization from excited states of atomic helium measured by high-order harmonic optical pump-probe velocity map imaging

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Phase shift differences and ratios of radial dipole matrix elements of the outgoing $S$ and $D$ continuum waves from state-selected helium atoms are directly measured from the photoelectron angular distributions using pump-probe velocity map imaging. Aligned $1s3p$ $^{1}P_{1}$ and $1s4p$ $^{1}P_{1}$ states in helium are prepared by high-order harmonics and ionized with either 800, 400, or $267\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ light. The results allow for the determination of energy-dependent quantum defect differences and ratios of partial cross sections and agree favorably with theoretical calculations on electron scattering and photoionization.