Inner ionization ionization mechanisms of methane clusters driven by an ultrashort intense laser pulse

By use of three-dimensional particle dynamics simulations, the explosion dynamics of methane clusters driven by an ultrashort intense laser pulse has been analyzed. Especially the influences of some classical inner ionization mechanisms on the charged state and kinetic energy of the ions produced in the laser-cluster interaction process have been discussed in detail. It is presented that when the laser intensity is low, the atoms are ionized by the laser field mainly through the tunnel ionization while the barrier suppression ionization dominates for the inner ionization as the laser intensity increases. Under the irradiation of the same laser pulse with high intensity, the cluster can attain highly-ionized charged states by the barrier suppression ionization rather than by the tunnel ionization. After the cluster is ionized, the ignition ionization effect from the inner Coulomb field of the cluster and the electron impact ionization effect from the free electrons resided inside the cluster will facilitate the process that the cluster can be ionized further.