  We report on electron momentum distributions from single ionization of Ar in
strong orthogonally polarized two-color (OTC) laser fields measured with the
COLTRIMS technique. We study the effect of Coulomb focusing whose signature is
a cusp like feature in the center of the electron momentum spectrum. While the
direct electrons show the expected strong dependence on the phase between the
two colors, surprisingly the Coulomb focused structure is almost not influenced
by the weak second harmonic streaking field. This effect is explained by the
use of a CTMC simulation which describes the tunneled electron wave packet in
terms of classical trajectories under the influence of the combined Coulomb-
and OTC laser field. We find a subtle interplay between the initial momentum of
the electron upon tunneling, the ionization phase and the action of the Coulomb
field that makes the Coulomb focused part of the momentum spectrum apparently
insensitive to the weaker streaking field.

Atomic Physics