  The quasistatic limit of the velocity-gauge strong-field approximation
describing the ionization rate of atomic or molecular systems exposed to linear
polarized laser fields is derived. It is shown that in the low-frequency limit
the ionization rate is proportional to the laser frequency, if a Coulombic
long-range interaction is present. An expression for the corresponding
proportionality coefficient is given. Since neither the saddle-point
approximation nor the one of a small kinetic momentum is used in the
derivation, the obtained expression represents the exact asymptotic limit. This
result is used to propose a Coulomb correction factor. Finally, the
applicability of the found asymptotic expression for non-vanishing laser
frequencies is investigated.
