Transient X‑ray Absorption Signatures of Photodissociation Pathways and Conical Intersection Dynamics in Phenol

Area of Science:

• Physical Chemistry
• Photochemistry
• Spectroscopy

Background:

• Phenol UV photodissociation produces hydrogen atoms with bimodal kinetic energy release.
• The low-energy component's origin involves complex dynamics like tunneling and statistical decay, requiring further investigation.

Purpose of the Study:

• To elucidate the contested mechanistic origins and branching of phenol's low-kinetic energy hydrogen atom release.
• To establish an unambiguous spectral framework for disentangling competing ultrafast bond-cleavage mechanisms using theoretical simulations.

Main Methods:

• Simulating oxygen K-edge transient X-ray absorption spectroscopy (TXAS) along the O-H dissociation coordinate.
• Utilizing ab initio calculations to predict spectral fingerprints for different dissociation pathways.
• Performing systematic natural transition orbital (NTO) analysis to decode electronic character evolution.

Main Results:

• Distinct spectral signatures were predicted for three key branching pathways through the second conical intersection: excited phenoxyl, ground-state phenoxyl, and nonadiabatic recrossing.
• Signatures were also assigned to predissociation, tunneling through the first conical intersection, and statistical decay channels.
• NTO analysis provided orbital-resolved insights into the electronic basis of these spectral features.

Conclusions:

• The study provides a definitive, atomically resolved roadmap for understanding phenol photodissociation dynamics.
• The established TXAS spectral references offer a theoretical foundation for future time-resolved X-ray experiments.
• This work enables experimental differentiation of competing ultrafast bond-cleavage mechanisms in phenol.