Photoelectron angular distributions from two-photon ionizations of atoms

Photoelectron angular distributions provide detailed information about interferences between different quantum pathways of photoionization. Measurements of photoelectron energies and angular distributions from two-color two-photon ionizations of atoms using ultrashort pulses of extreme ultraviolet and optical light are performed using a novel, homebuilt experimental instrument. The setup is composed of an amplified femtosecond laser system, a high-order harmonic generation source, and an interaction region with photoelectron velocity map imaging The experimental temporal resolution is determined to be approximately 100 fs. Two different types of two-photon ionizations are investigated. Photoelectron angular distributions from resonant two-photon ionizations of helium are measured using the 15th high-order harmonic to excite from the ground state to either the 1s3p 1P1 state at 23.1 eV or to the 1s4p 1 P1 state at 23.7 eV and either 800, 400, or 267 nm to ionize. The anisotropy parameters allow for the determination of the energy-dependent ratios of radial dipole matrix elements and the phase shift differences between the S and D partial waves. Using available total cross section measurements, the absolute partial cross sections of the 1s3p1P 1 state are obtained, providing the complete information on photoionization. The experimental results are in excellent agreement with theoretical predictions using the one-electron model. Additional experiments are aimed at studying atomic free-free transitions. Two-color two-photon above threshold ionizations of helium and argon are investigated using selected high-order harmonics and perturbative infrared dressing fields. The measured anisotropy parameters and cross section ratios of the positive and negative above threshold ionization sidebands are compared to theoretical predictions using second-order perturbation theory and the soft-photon approximation. In general, deviations between the experimental results and the theoretical predictions demonstrate that a more comprehensive theoretical treatment of two-color two-photon above threshold ionization is needed. Future studies on time-resolved chemical dynamics are possible using this experimental instrument.