Using a one-dimensional particle-in-cell code, which includes field ionization and electron collisional ionization, as well as elastic binary Coulomb collisions, we study the acceleration of electrons and ions by the interaction of an intense laser pulse (1017—1018W/cm2, 26.7fs) with a helium gas target (1021—1022/cm3, 0.8 μm thick). It is shown that field ionization appears quickly at the target front surface. Collisional ionization is found inside the target by energetic electrons, which are accelerated by the laser fields at the front surface and transported into the target. Part of these electrons transmits through the target rear surface and induces an electrostatic field there. This field further leads to field ionization at the rear surface. Meanwhile, it accelerates new-born ions produced there through the field ionization. Because of the electrostatic fields induced at the two target surfaces, some electrons oscillate between them, resulting in oscillating energy exchange between electrons, ions and the electrostatic fields. Particular attention is also paid to the origins of accelerated ions. Under certain conditions, ions accelerated inside the target are more energetic than those accelerated at the target surfaces.