Enhanced ionization of acetylene in intense laser fields

Acetylene (C2H2), described by a one-dimensional model, is shown to be extremely efficiently ionized by an intense laser field when the two C-H distances are symmetrically stretched to twice the equilibrium distance. Up to six electrons are ejected from the three highest occupied molecular orbitals by tunneling ionization through a reduced potential barrier via temporal localization of the electron density at either of the two proton sites. The third highest occupied molecular orbital, which is initially localized in the area around the two C atoms, is found to act as an electron reservoir, through which the electron density is transferred to the proton sites. This electron transfer mechanism is considered to be universal and can be applied to interpret enhanced ionization found in a variety of hydrocarbon molecules.