Electronic-vibrational dynamics and coherence in x-ray transient absorption of N2+ induced by strong-field ionization

TL;DR

This paper develops a model to analyze the coupled electronic-vibrational dynamics in nitrogen ions during strong-field ionization, revealing new spectral features and coherence effects in x-ray transient absorption.

Contribution

It introduces an ionization-coupling model for ATAS, providing new insights into electronic-vibrational interactions and spectral features in nitrogen ions under strong laser fields.

Findings

Coherent vibrational wave packets are created with broad vibrational level distribution.

Significant role of the excited state A^2Π_u in spectral features.

Identification of absorption from forbidden transitions and vibronic coherence modulations.

Abstract

Attosecond transient absorption spectroscopy (ATAS) is becoming an indispensable and powerful tool in the emerging field of attochemistry, while the interpretation of measurements often requires full considerations of the coupling among various freedoms of motion. Here we develop the ionization-coupling model to incorporate the transient absorption and explore the coupled electronic-vibrational dynamics of nitrogen ions induced by strong-field ionization (SFI), which has been investigated in the recent transient x-ray K-edge absorption experiment [PRL 129, 123002 (2022)]. It is found the coherent vibrational wave packet on the involved electronic state is created with broad distribution of vibrational levels which leads to the spectral overlap on the K-edge absorption. By identifying the contributions of each electronic state, the study provides a different interpretation revealing the significant role of the excited state $A^2\Pi_u$ arising from the electronic-vibrational coupling in strong laser fields. We uncover new features of absorption from forbidden transitions during the laser pulse and confirm the vibronic coherence induced modulations of absorbance after SFI. A new scheme is proposed to avoid the spectral overlap and determine the population among the states which is crucial to resolve the debate on nitrogen air lasing. The work lays down the framework to research the ionic coherence in ATAS and offers valuable insights into the intricate interplay between electronic and vibrational dynamics.