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 H-2(+) (1 pi(u)) 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 degrees and 40 degrees at the laser intensities 3 x 10(13) W/cm(2) and 5 x 10(13) W/cm(2), 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 2 sigma(g) in the basis expansion. Furthermore, the results obtained by including the state 2 sigma(g) in the ab initio simulations can be qualitatively reproduced by the modified molecular length gauge strong-field approximation (SFA) taking account of the 1 pi(u) and 2 sigma(g) states simultaneously. Analysis indicates that a part of electron is directly emitted from the 1 pi(u) orbital and another portion of electron is released from 2 sigma(g) orbital and other excited state after the single-photon resonant transition between 1 pi(u) and 2 sigma(g) orbitals. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement