Competition of Resonant and Nonresonant Paths in Resonance-Enhanced Two-Photon Single Ionization of He by an Ultrashort Extreme-Ultraviolet Pulse

We theoretically study the pulse-width dependence of the photoelectron angular distribution ( PAD) from the resonance-enhanced two-photon single ionization of He by femtosecond (less than or similar to 20 fs) extreme-ultraviolet pulses, based on the time-dependent perturbation theory and simulations with the full time-dependent Schrodinger equation. In particular, we focus on the competition between resonant and nonresonant ionization paths, which leads to the relative phase delta between the S and D wave packets distinct from the corresponding scattering phase shift difference. When the spectrally broadened pulse is resonant with an excited level, the competition varies with pulse width, and, therefore, delta and the PAD also change with it. On the other hand, when the Rydberg manifold is excited, delta and the PAD do not much vary with the pulse width, except for the very short-pulse regime.