  Recent experiments [D. Pengel, S. Kerbstadt, L. Englert, T. Bayer, and M.
Wollenhaupt,
\href{https://journals.aps.org/pra/abstract/10.1103/PhysRevA.96.043426}{{\PRA}
{\bf 96} 043426 (2017)}] have measured the photoelectron momentum distribution
for three-photon ionization of potassium by counter-rotating circularly
polarized 790-nm laser pulses. The distribution displays spiral vortices,
arising from the interference of ionizing wavepackets with different magnetic
quantum numbers. The high level of multidimensional detail observed in the
distribution makes this an ideal case in which to demonstrate the accuracy of
emerging theoretical techniques applicable to such problems. We use the
\(R\)-matrix with time dependence approach to investigate this process. We
calculate the full-dimensional photoelectron momentum distribution, and compare
against a set of planar projections of this distribution previously measured in
experiment.
