Probing competing relaxation pathways in malonaldehyde with transient X-ray absorption spectroscopy

Area of Science:

• Physical Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• Excited-state intramolecular hydrogen transfer (ESIHT) is crucial in chemistry and biology.
• Malonaldehyde is the simplest ESIHT system, but its excited-state dynamics are poorly understood.
• Proposed decay pathways include internal conversion, C=C torsion, and intersystem crossing.

Purpose of the Study:

• To investigate the potential of oxygen K-edge transient X-ray absorption spectroscopy (TRXAS) for monitoring ultrafast decay pathways in malonaldehyde.
• To computationally simulate TRXAS signals from lowest valence states after photoexcitation to the S2(ππ*) state.
• To provide a pathway-specific mapping of TRXAS signals for experimental interpretation.

Main Methods:

• In silico transient X-ray absorption spectroscopy (TRXAS) experiment.
• Restricted active space perturbation theory and algebraic-diagrammatic construction for polarization propagator calculations.
• Ab initio multiple spawning simulations to model reaction coordinates and trajectories.

Main Results:

• Oxygen K-edge TRXAS can distinguish the hydrogen transfer intersection and population transfer to the S1(nπ*) state.
• Intersystem crossing to the T1(ππ*) state is detectable via changes in pre-edge signatures and intensities.
• Torsional deactivation leads to charge redistribution and significant shifts in pre-edge features.

Conclusions:

• Oxygen K-edge TRXAS is a promising experimental technique to resolve ultrafast excited-state decay channels in malonaldehyde.
• The study provides a detailed mapping of TRXAS signals to specific decay pathways.
• This work facilitates future experimental investigations of ESIHT dynamics in prototypical systems.