Direct imaging of curve-crossing dynamics in IBr by attosecond transient absorption spectroscopy

The electronic character of photoexcited molecules can abruptly change at avoided crossings and conical intersections. Here, we report direct spectroscopic imaging of such nonadiabatic processes in a prototype molecule, iodine monobromide (IBr), using extreme-ultraviolet (XUV) attosecond transient absorption spectroscopy. A few-femtosecond visible pulse resonantly excites the B(3Π0+ ) state and the accompanying dissociation dynamics are tracked by an attosecond XUV pulse that simultaneously probes the I-4d and Br-3d core-level ab- sorption edges. Direct comparison with quantum mechanical simulations unambiguously identifies the core-level absorption features associated with adiabatic and diabatic channels at the avoided crossing that is accessed by one-photon and two-photon visible-excitation processes. The results delineate rapid switching of valence-orbital vacancies from σ∗ to π/π∗ ensuing at the avoided crossing.