Dissociative ionization and post-ionization alignment of aligned O₂ in an intense femtosecond laser field

We examined dissociative ionization of O₂ in an intense femtosecond laser field (782 nm, 120 fs, 4 × 10¹⁴ W cm^-2) by recording the kinetic energy distribution of O⁺ emitted along the laser polarization direction as a function of the delay time between the pump pulse (9 × 10¹³ W cm^-2) for the molecular alignment and the probe pulse for the dissociative ionization. We found the two distinct rotational revival patterns which are out-of-phase by π with each other in the kinetic energy distribution of O⁺. One of the patterns shows the dissociative ionization is enhanced when the O₂ axis is parallel to the laser polarization direction, suggesting that the ionization is induced by the electron emission from the 3σ_g orbital. On the other hand, the other pattern shows that the dissociative ionization is enhanced when the O₂ axis is perpendicular to the laser polarization direction, suggesting that the ionization is induced by the electron emission from the 1π_u orbital. Because of the collection efficiency of the time-of-flight mass spectrometer, the enhancement of the O⁺ yield at the anti-alignment time delay indicates that the electron emission from the 1π_u orbital is followed by the molecular alignment of O₂⁺ in the course of the dissociation. We performed classical trajectory Monte-Carlo simulation of O₂⁺ with the dissociation and rotational coordinates in the light-dressed potential to evaluate the effect of the post-ionization alignment by the probe pulse.