Revealing Ultrafast Population Transfer between Nearly Degenerate Electronic States

The response of a molecule to photoexcitation
is governed by the
coupling of its electronic states. However, if the energetic spacing
between the electronically excited states at the Franck–Condon
window becomes sufficiently small, it is infeasible to selectively
excite and monitor individual states with conventional time-resolved
spectroscopy, preventing insight into the energy transfer and relaxation
dynamics of the molecule. Here, we demonstrate how the combination
of time-resolved spectroscopy and extensive surface hopping dynamics
simulations with a global fit approach on individually excited ensembles
overcomes this limitation and resolves the dynamics in the n3p Rydberg
states in acetone. Photoelectron transients of the three closely spaced
states n3px, n3py, and n3pz are used to validate
the theoretical results, which in turn allow retrieving a comprehensive
kinetic model describing the mutual interactions of these states for
the first time.