  Tunneling times in atomic ionization are studied theoretically by a virtual
detector approach. A virtual detector is a hypothetical device that allows one
to monitor the wave function's density with spatial and temporal resolution
during the ionization process. With this theoretical approach, it becomes
possible to define unique moments when the electron enters and leaves with
highest probability the classically forbidden region from first principles and
a tunneling time can be specified unambiguously. It is shown that neither the
moment when the electron enters the tunneling barrier nor when it leaves the
tunneling barrier coincides with the moment when the external electric field
reaches its maximum. Under the tunneling barrier as well as at the exit the
electron has a nonzero velocity in the electric field direction. This nonzero
exit velocity has to be incorporated when the free motion of the electron is
modeled by classical equations of motion.
