  Strong-field ionization of atoms by circularly polarized femtosecond laser
pulses produces a donut-shaped electron momentum distribution. Within the
dipole approximation this distribution is symmetric with respect to the
polarization plane. The magnetic component of the light field is known to shift
this distribution forward. Here, we show that this magnetic non-dipole effect
is not the only non-dipole effect in strong-field ionization. We find that an
electric non-dipole effect arises that is due to the position dependence of the
electric field and which can be understood in analogy to the Doppler effect.
This electric non-dipole effect manifests as an increase of the radius of the
donut-shaped photoelectron momentum distribution for forward-directed momenta
and as a decrease of this radius for backwards-directed electrons. We present
experimental data showing this fingerprint of the electric non-dipole effect
and compare our findings with a classical model and quantum calculations.
