High-Order Above-Threshold Ionization of H₂⁺ in Intense Laser Field

The nonperturbative quantum electrodynamics method proposed by Fu et al. Phys. Rev. A 75 (2007) 063419 is employed to study the high-order above-threshold ionization (ATI) of a diatomic molecule. Based on this frequency-domain theory, the high-order ATI process can be regarded as ATI followed by laser-assisted collision, where the total transition amplitude is the coherent summation of the contributions from each ATI channel. The angular-resolved ATI spectrum, which agrees with the results by Becker et al. based on the time-domain method, is obtained by this frequency domain theory. Furthermore, it is demonstrated that the interference characteristics representing the molecular structure in the ATI spectrum originates from the recollision of the electron with two-centre ion in each ATI channel.