The electronic character of photoexcited molecules can abruptly change at avoided crossings and conical intersections. Here, we report direct mapping of the coupled interplay between electrons and nuclei in a prototype molecule, iodine monobromide (IBr), by using attosecond transient absorption spectroscopy. A few-femtosecond visible pulse resonantly excites the (B3Π0+), Y(0+), and Z(0+) states of IBr, and the photodissociation dynamics are tracked with an attosecond extreme-ultraviolet pulse that simultaneously probes the I-4d and Br-3d core-level absorption edges. Direct comparison with quantum mechanical simulations unambiguously identifies the absorption features associated with adiabatic and diabatic channels at the B/Y avoided crossing and concurrent two-photon dissociation processes that involve the Y/Z avoided crossing. The results show clear evidence for rapid switching of valence-electronic character at the avoided crossing.
Department of Chemistry

Kobayashi, Y. (Yuki), Chang, K.F. (Kristina F.), Zeng, T, Neumark, D.M. (Daniel M.), & Leone, S.R. (Stephen R.). (2019). Direct mapping of curve-crossing dynamics in IBr by attosecond transient absorption spectroscopy. Science, 364(6448), 79–83. doi:10.1126/science.aax0076