Nuclear correlation in ionization and harmonic generation of H₂⁺ in short intense laser pulses

The dynamics of H₂⁺ in short intense laser pulses with wavelengths of 532 nm and 800 nm is considered in two approaches, including and not including the motional degree of freedom of the two nuclei. In the situation with moving nuclei, referred to as the non-Born-Oppenheimer approach, the ionization is substantially increased with respect to the case with no motional degree of freedom of the nuclei, and consequently the nuclear separation also increases with rising laser intensity. This separation is accompanied by complex relative accelerations of the nuclei due to a strongly varying mutual shielding via the laser-driven electron. In the harmonic spectrum the irregularity of the electronic dynamics induced by the nuclear motion leads to a broadening of the harmonic peaks. In particular, the correlated nuclear motion is shown to lead to an earlier onset of the harmonic cutoff than for fixed nuclei, but also to a wider cutoff regime with significantly higher harmonics.