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We theoretically study the photoelectron angular distributions (PADs) fromtwo-color two-photon near-threshold ionization of hydrogen and noble gas (He,Ne, and Ar) atoms by a combined action of femtosecond extreme ultraviolet (EUV)and near-infrared (IR) laser pulses. Using the second-order time-dependentperturbation theory, we clarify how the two-photon ionization process dependson EUV-IR pulse delay and how it is connected to the interplay between resonantand nonresonant ionization paths. Furthermore, by solving the time-dependentSchr\"odinger equation, we calculate the anisotropy parameters $\beta_2$ and$\beta_4$ as well as the amplitude ratio and relative phase between partialwaves characterizing the PADs. We show that in general these parameters notablydepend on the time delay between the EUV and IR pulses, except for He. Thisdependence is related to the varying relative role of resonant and nonresonantpaths of photoionization. Our numerical results for H, He, Ne, and Ar show thatthe pulse-delay effect is more pronounced for $p$-shell ionization than for$s$-shell ionization.

Theoretical study of pulse delay effects in the photoelectron angular distribution of near-threshold EUV+IR two-photon ionization of atoms