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Role of excited states in molecular alignment-dependent ionization.

We introduce an ab initio approach and the modified strong-field approximation to investigate the alignment-dependent ionization of H2+(1πu) exposed to different few-cycle laser fields. The ab initio calculations are performed by the B-splines one-center method and the Crank-Nicolson method in spherical coordinates. It is shown that the peak ionization probabilities appear around alignment angles 50° and 40° at the laser intensities 3×1013 W/cm2 and 5×1013 W/cm2, respectively, and the above distinct features come from the resonant excitation of the molecular ion, which is confirmed by calculation including and excluding the state 2σg in the basis expansion. Furthermore, the results obtained by including the state 2σg in the ab initio simulations can be qualitatively reproduced by the modified molecular length gauge strong-field approximation (SFA) taking account of the 1πu and 2σg states simultaneously. Analysis indicates that a part of electron is directly emitted from the 1πu orbital and another portion of electron is released from 2σg orbital and other excited state after the single-photon resonant transition between 1πu and 2σg orbitals.