Effect of ion mass on pair production in the interaction of an   ultraintense laser with overdense plasmas
  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. 
F. Wan, C. Lv, M. R. Jia, H. Y. Wang, and B. S. Xie
physics.plasm-ph quant-ph