  The longitudinal momentum of the electron at the tunneling exit is a useful
quantity to make sense of the tunneling ionization process. It was usually
assumed to be zero from a classical argument, but recent experiments show that
it must be nonzero in order to explain the measured electron momentum
distributions. In this article we show that the flow momentum of the
probability fluid is a sensible quantum mechanical definition for
tunneling-exit momentum, and it can be (and in general is) nonzero at the
tunneling exit point where the kinetic energy is zero by definition. We show
that this longitudinal momentum is nonzero even in the static or adiabatic
limit, and this nonzero momentum is a purely quantum mechanical effect
determined by the shape of the wave function in the vicinity of the tunneling
exit point. Nonadiabaticity or finite wavelength may increase this momentum
substantially, and the detailed value depends on both the atomic and the laser
parameters.
