  Electrons in atoms and molecules can not react immediately to the action of
intense laser field. A time lag (about 100 attoseconds) between instants of the
field maximum and the ionization-rate maximum emerges. This lag characterizes
the response time of the electronic wave function to the strong-field
ionization event and has important effects on subsequent ultrafast dynamics of
the ionized electron. The absolute time lag is not accessible in experiments.
Here, a calibrated attoclock procedure, which is related to a simple
Coulomb-induced temporal correction to electron trajectories, is proposed to
measure the relative lag of two different ionization events. Using this
procedure,the difference (i.e., the relative lag) between the ionization time
lags of polar molecules in two consecutive half laser cycles can be probed with
a high accuracy.
