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

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 P11 and 1s4p P11 states in helium are prepared by high-order harmonics and ionized with either 800, 400, or 267 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.