Target dressing effects in laser-assisted x-ray photoionization

Photoelectron angular distributions and cross section ratios of two-color two-photon above threshold ionization of argon

Anisotropy parameters and cross section ratios of two-color two-photon above threshold ionization sidebands from argon are measured using photoelectron velocity map imaging with the selected 13th or 15th high-order harmonics in a perturbative 800 nm dressing field. A new data analysis technique determines accurate anisotropy parameters of the photoelectron angular distributions for each sideband by subtracting a sequence of percentages of the single-photon ionization background from the above threshold ionization signal to correct for the angular averaging of overlapping photoelectron energies. The results provide a fundamental test of theoretical predictions based on second-order perturbation theory with a one-electron model and the soft-photon approximation and show agreement with theory for the cross section ratios. However, discrepancies between the theoretically predicted and experimentally determined photoelectron angular distributions demonstrate the need for a more comprehensive theoretical description of two-color two-photon above threshold ionization.