  The effect of ion mass on pair production in the interaction of an
ultraintense laser with overdense plasmas has been explored by particle-in-cell
(PIC) simulation. It is found that the heavier ion mass excites the higher and
broader electrostatic field, which is responsible for the enhancement of
backward photon number. The pair yields are also reinforced due to the increase
of head-on collision of backwards photon with incoming laser. By examining the
density evolution and angle distribution of each particle species the origin of
pair yields enhancement has been clarified further.
