Fig. 1: Attosecond transient absorption of ICl molecules.

a Key components of the ATAS experiments. The interaction of the IAP with the sample creates the free induction decay (FID) shown in purple. Delayed NIR pulses perturb the FID, which affects the spectrum of the IAP recorded on the XUV camera and interferes with the incident pulse. The NIR pulse is removed after the sample using an aluminum filter (cf. text for details). Changes in absorption are isolated using an optical shutter to modulate the NIR pulse (not shown). b Iodine N_4,5 edge absorption of ICl centered around 4d⁻¹6p transitions obtained with the described experiment (dashed line). The orange and blue features are the fits to transitions to the indicated 4d⁻¹6p core-excited states. The lineshapes include lifetime broadening, finite spectral resolution and vibronic progressions as described in the text. The solid black line is the total fit. c Core-hole orbital electronic density (cf. SM3 for computation details), separated into core-orbitals perpendicular to the molecular (blue box, blue features in b), and orbitals aligned along the molecular axis (orange box, orange features in b). The energies are determined based on the fittings to the experimental absorption spectrum. d ATAS spectrum of ICl at the iodine N_4,5 edge in ΔmOD (changes in milli-optical density, cf. text for details).