Attosecond transient absorption spectroscopy of core-excited states of acetonitrile in the soft X-ray region

We present a combined experimental and theoretical study of attosecond transient absorption spectroscopy in acetonitrile (ACN) in the soft X-ray region. Due to its high ionization potential and polarizable valence manifold, ACN provides a well-suited platform for investigating field-driven core-excited dynamics in molecules. The spectra exhibit characteristic oscillations arising from the interplay between the soft X-ray pulse and an intense 1700-nm infrared field, with an energy dependence reflecting contributions from different core-excited states. Building on these observations, we perform first-principles modeling of the transient absorption response to elucidate the underlying electronic mechanisms and the role of strong-field-induced couplings between core and valence states. Our calculations reproduce the main experimental features and attribute the observed 2.8-fs beating signal to a well-defined electronic coherence induced by the soft X-ray pulse. Together, these results provide insight into the field-driven modulation of valence electronic states in ACN.