Core-Valence Attosecond Transient Absorption Spectroscopy of Polyatomic Molecules

Tracing ultrafast processes induced by interaction of light with matter is often very challenging. In molecular systems, the initially created electronic coherence becomes damped by the slow nuclear rearrangement on a femtosecond timescale which makes real-time observations of electron dynamics in molecules particularly difficult. In this work, we report an extension of the theory underlying the attosecond transient absorption spectroscopy (ATAS) for the case of molecules, including a full account for the coupled electron-nuclear dynamics in the initially created wave packet, and apply it to probe the oscillations of the positive charge created after outer-valence ionization of the propiolic acid molecule. By taking advantage of element-specific core-to-valence transitions induced by x-ray radiation, we show that the resolution of ATAS makes it possible to trace the dynamics of electron density with atomic spatial resolution.

Figure 1

Ionization spectrum of propiolic acid computed using the ab initio many-body Green’s function ADC method. Three energy windows illustrate the valence region (10.5–12.5 eV), core ionic states resulting from ionization out of $1s$ orbitals of triple-bond carbon atoms (293.1–293.25 eV), and oxygen atoms (539–542 eV). In all three energy ranges, the closest ionic states are located at least $\sim 2.5\mathrm{eV}$ above the presented lines. The spectral intensity is defined as the combined weight of all one-hole configurations in the configuration-interaction expansion of the ionic state. The relevant molecular orbitals are also depicted.

Figure 2

Top: electronic coherence measured by the time-dependent overlap $⟨{\chi }_1(\mathbf{R},\tau )|{\chi }_3(\mathbf{R},\tau ){⟩}_{\mathbf{R}}$ of the nuclear wave packets propagated in the first and third cationic states of propiolic acid after the removal of an electron from the HOMO. Middle: time-resolved absorption cross section as a function of the photon energy and time delay, plotted for the energy window corresponding to transitions between initially populated valence ionic states and the core states resulting from ionization out of $1s$ orbitals of carbon atoms forming the triple bond. Bottom: absorption cross section plotted for the energy range covering transitions to core ionic states of the oxygen atoms. The ionic states involved in the corresponding transitions are also shown. The transient absorption traces shown are rotationally averaged and thus describe randomly oriented molecules.