<p>
</p><p>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), using attosecond transient
absorption spectroscopy. A few-femtosecond visible pulse resonantly excites the
B(3_0+) state of IBr and the accompanying photodissociation
dynamics are tracked by an attosecond extreme-ultraviolet pulse that
simultaneously probes the I-4d
and Br-3d corelevel
absorption edges. Direct comparison with quantum mechanical simulations
unambiguously identifies the core-level 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 molecularorbital
occupations at the avoided crossing.</p>
<p><br></p>
Direct Mapping of Curve-Crossing Dynamics in IBr by Attosecond Transient Absorption Spectroscopy