Excited-State Dynamics during Primary C–I Homolysis in Acetyl Iodide Revealed by Ultrafast Core-Level Spectroscopy

In
typical carbonyl-containing molecules, bond dissociation events
follow initial excitation to nπCO* states.
However, in acetyl iodide, the iodine atom gives rise to electronic
states with mixed nπCO* and nσC–I* character,
leading to complex excited-state dynamics, ultimately resulting in
dissociation. Using ultrafast extreme ultraviolet (XUV) transient
absorption spectroscopy and quantum chemical calculations, we present
an investigation of the primary photodissociation dynamics of acetyl
iodide via time-resolved spectroscopy of core-to-valence transitions
of the I atom after 266 nm excitation. The probed I 4d-to-valence
transitions show features that evolve on sub-100-fs time scales, reporting
on excited-state wavepacket evolution during dissociation. These features
subsequently evolve to yield spectral signatures corresponding to
free iodine atoms in their spin–orbit ground and excited states
with a branching ratio of 1.1:1 following dissociation of the C–I
bond. Calculations of the valence excitation spectrum via equation-of-motion
coupled cluster with single and double substitutions (EOM-CCSD) show
that initial excited states are of spin-mixed character. From the
initially pumped spin-mixed state, we use a combination of time-dependent
density functional theory (TDDFT)-driven nonadiabatic ab initio molecular dynamics and EOM-CCSD calculations of the N4,5 edge to reveal a sharp inflection point in the transient XUV signal
that corresponds to rapid C–I homolysis. By examining the molecular
orbitals involved in the core-level excitations at and around this
inflection point, we are able to piece together a detailed picture
of C–I bond photolysis in which d →
σ* transitions give way to d → p excitations as the bond dissociates. We also report theoretical
predictions of short-lived, weak 4d → 5d transitions in acetyl iodide, validated by weak bleaching
in the experimental transient XUV spectra. This joint experimental–theoretical
effort has thus unraveled the detailed electronic structure and dynamics
of a strongly spin–orbit coupled system.