  We studied the recollision dynamics between the electrons and D$_2^+$ ions
following the tunneling ionization of D$_2$ molecules in an intense short pulse
laser field. The returning electron collisionally excites the D$_2^+$ ion to
excited electronic states from there D$_2^+$ can dissociate or be further
ionized by the laser field, resulting in D$^+$ + D or D$^+$ + D$^+$,
respectively. We modeled the fragmentation dynamics and calculated the
resulting kinetic energy spectrum of D$^+$ to compare with recent experiments.
Since the recollision time is locked to the tunneling ionization time which
occurs only within fraction of an optical cycle, the peaks in the D$^+$ kinetic
energy spectra provides a measure of the time when the recollision occurs. This
collision dynamics forms the basis of the molecular clock where the clock can
be read with attosecond precision, as first proposed by Corkum and coworkers.
By analyzing each of the elementary processes leading to the fragmentation
quantitatively, we identified how the molecular clock is to be read from the
measured kinetic energy spectra of D$^+$ and what laser parameters be used in
order to measure the clock more accurately.
