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

We examined dissociative ionization of O in an intense femtosecond laser field (782 nm, 120 fs, 4 × 10 W cm) 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 × 10 W cm) 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 O axis is parallel to the laser polarization direction, suggesting that the ionization is induced by the electron emission from the 3σ orbital. On the other hand, the other pattern shows that the dissociative ionization is enhanced when the O axis is perpendicular to the laser polarization direction, suggesting that the ionization is induced by the electron emission from the 1π 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 orbital is followed by the molecular alignment of O in the course of the dissociation. We performed classical trajectory Monte-Carlo simulation of O with the dissociation and rotational coordinates in the light-dressed potential to evaluate the effect of the post-ionization alignment by the probe pulse.