  We introduce a method to investigate the longitudinal momentum spread
resulting from strong-field tunnel ionization of Helium which, unlike other
methods, is valid for all ellipticities of laser pulse. Semiclassical models
consisting of tunnel ionization followed by classical propagation in the
combined ion and laser field reproduce the experimental results if an initial
longitudinal spread at the tunnel exit is included. The values for this spread
are found to be of the order of twice the transverse momentum spread.
