Imaging conical intersection dynamics during azobenzene photoisomerization by ultrafast X-ray diffraction.

Area of Science:

• Physical Chemistry
• Chemical Physics
• Molecular Dynamics

Background:

• X-ray diffraction traditionally analyzes static molecular structures.
• Advanced X-ray sources, like free-electron lasers, allow time-resolved scattering experiments.
• Studying photochemical reactions requires capturing ultrafast molecular dynamics.

Purpose of the Study:

• To simulate and interpret X-ray diffraction patterns during azobenzene isomerization.
• To understand the role of vibronic coherences in ultrafast photochemical processes.
• To develop methods for retrieving detailed molecular information from scattering data.

Main Methods:

• Femtosecond X-ray diffraction simulations.
• Analysis of X-ray scattering contributions (elastic and inelastic).
• Modeling of molecular charge density and electronic transitions.
• Investigation of vibronic coherences at conical intersections.

Main Results:

• Azobenzene isomerization reveals rich information beyond charge density changes.
• Vibronic coherences at conical intersections contribute significantly to diffraction signals.
• These coherences induce phase modulations in momentum space, reflecting real-space electronic transitions.
• Intense elastic scattering can mask subtle inelastic signals.

Conclusions:

• Time-resolved X-ray diffraction offers a dynamic view of molecular isomerization.
• Vibronic coherences are key observables for non-adiabatic dynamics.
• Using very hard X-rays can enhance the retrieval of dynamic information from scattering data.