  We present an ellipticity resolved study of momentum distributions arising
from strong-field ionization of Helium at constant intensity. The influence of
the ion potential on the departing electron is considered within a
semi-classical model consisting of an initial tunneling step and subsequent
classical propagation. We find that the momentum distribution can be explained
by the presence of a longitudinal momentum spread of the electron at the exit
from the tunnel. Our combined experimental and theoretical study provides an
estimate of this momentum spread.
