  We present numerical simulations of ultrafast multiphoton ionization dynamics
in a two-dimensional atomic model driven by co- and counterrotating circularly
polarized single-color and bichromatic carrier envelope phase (CEP) stable
ultrashort laser pulse sequences. Taking into account phase variations due to
CEP fluctuations and the Gouy phase, our results accurately reproduce recently
measured photoelectron momentum distributions [Pengel et al., Phys. Rev. Lett.
118, 053003 (2017), Phys. Rev. A 96, 043426 (2017); Kerbstadt et al., Nat.
Comm. 10, 685 (2019), Adv. Phys. X 4, 1672583 (2019)]. The time evolution of
the complex-valued electron wave function in coordinate and momentum space is
calculated to study the bound state- and the vortex formation dynamics. The
non-vanishing azimuthal probability current density proves the vortex nature of
electron wave packets with odd-numbered rotational symmetry. Their angular
momentum expectation value assumes half-integer values of 3.5 (corotating) and
0.5 (counterrotating). Knowledge of the wave function allows us to analyze the
photoionization dynamics and to validate the physical pictures proposed in
previous experimental studies. As an outlook, we investigate how electron
vortices develop from the multiphoton- to the tunnel regime with increasing
laser intensity.
