Attosecond X-ray absorption spectroscopy of ionic dynamics induced by strong field ionization

Abstract

Real-time observation of electronic dynamics in ionic molecules induced by strong-field ionization (SFI) is essential for understanding the photoinduced chemical reactions and physical processes. The advent of ultrafast X-ray spectroscopy has opened new avenues for directly probing the electronic and nuclear dynamics in ions [1,2]. In a recent experiment [3], the population distribution of electronic states in air lasing of nitrogen ion was measured for the first time. Here we establish a theoretical approach to simulate the ionic dynamics, which simultaneously addresses strong-field ionization of the neutral molecules and electronic-vibrational coupled dynamics of the ions. We successfully reproduced the X-ray transient absorption spectra (XAS) of nitrogen ion observed in the experiment. By separating the contributions of different electronic states, our study revealed the important role of nuclear vibrational motion in strong-field induced non-equilibrium dynamics. It is found that the electronic population distributions strongly depend on the alignment angle of the molecular axis relative to the laser polarization. With the time-integrated XAS, we achieve the vibrational-resolved measurement of electronic state populations. The modulation of the absorbance with the time delay originates from the vibrational dynamics. Our work sheds light on ultrafast probing of ionic dynamics under strong laser field.