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

Abstract

In typical carbonyl-containing molecules, bond dissociation events follow initial excitation to nπ C�O * states. However, in acetyl iodide, the iodine atom gives rise to electronic states with mixed nπ C�O * and nσ C−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 spinmixed state, we use a combination of time-dependent density functional theory (TDDFT)-driven nonadiabatic ab initio molecular dynamics and EOM-CCSD calculations of the N 4,5 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.

Generalization of One-Center Nonorthogonal Configuration Interaction Singles to Open-Shell Singlet Reference States: Theory and Application to Valence-Core Pump-Probe States in Acetylacetone

“…Another exciting avenue for development is the inclusion of spin–orbit coupling into the theory to calculate NEXAS beyond the K-edge. This would allow 1C-NOCIS 2eOS to serve TR-NEXAS experiments like those carried out recently at the iodine N 4,5 -edge. − …”

Femtosecond Core-Level Spectroscopy Reveals Involvement of Triplet States in the Gas-Phase Photodissociation of Fe(CO)5

“…The experimental apparatus is described in detail elsewhere . Briefly, the output of a Ti:sapphire amplifier (3 mJ, 35 fs, 1 kHz) is split equally into the pump and probe arms.…”

“…Section S2.4 of the Supporting Information shows the power dependence data verifying predominant one-photon absorption of the UV pump. The instrument response function (IRF) is measured by a ponderomotive shift of absorption lines in xenon gas to be 74 fs FWHM (as reported in ref ). Using the Beer–Lambert law, the 266 nm absorption cross section of 8 Mb, and a path length of 10 mm at a sample pressure of 1.5 Torr, we estimate 9% excitation fraction of 5 in the pump volume.…”

Early-Career and Emerging Researchers in Physical Chemistry Volume 2

“…Finally, the spin–orbit distributions of NO show the fingerprints of the Jahn–Teller effect in this high-symmetry complex. Work by Ramasesha and co-workers makes use of short pulses of extreme ultraviolet radiation to excite core-to-valence orbital transition in an iodine atom. As a result, they are able to observe the ultrafast cleavage of a C–I bond in UV-photoexcited acetyl iodide.…”