Competition of sequential and direct paths in two-photon ionization of He

We study the photoelectron angular distribution (PAD) from the two-photon ionization (TPI) of He by femtosecond EUV pulses. The calculation with the time-dependent Schrodinger equation (TDSE) and the measurement at the SPring-8 EUV-FEL show reasonable agreement. The extracted relative phase δ between the s and d wave packets are distinct from that between the corresponding eigenstates, due to the competition between sequential and direct ionization paths. When the pulse is resonant with an excited level, the sequential and direct TPI compete with each other, and δ and the PAD depend on the pulse width. On the other hand, when the Ry-dberg manifold is coherently excited, δ does not vary with the pulse width.