  The average dwell time of an electron in a potential barrier formed by an
external electric field and the potential of a helium atom is evaluated within
a semi classical one-dimensional tunneling approach. The tunneling electron is
considered to interact with a nuclear charge screened by the electron in the
helium ion. It is found that screening leads to smaller average dwell times of
the tunneling electron. The dissipative effect of the environment on the
tunneling electron is taken into account within a semiclassical model involving
a velocity dependent frictional force and is found to change the average dwell
time significantly.
