Ultrafast isomerization in acetylene dication after carbon K-shell ionization

Area of Science:

• Physical Chemistry
• Chemical Physics
• Molecular Dynamics

Background:

• Ultrafast proton migration and isomerization are critical in acetylene and its ions.
• The mechanism of acetylene dication (HCCH²⁺) isomerization to vinylidene dication (H₂CC²⁺) is not well understood.
• Theoretical studies predict high energy barriers for isomerization, suggesting timescales longer than picoseconds.

Purpose of the Study:

• To investigate the contradiction between theoretical predictions and recent experimental findings on acetylene dication isomerization timescales.
• To provide a comprehensive theoretical study of acetylene dication dynamics following X-ray photoionization.
• To clarify the feasibility of sub-100 femtosecond isomerization in acetylene dication.

Main Methods:

• Theoretical study of acetylene dication dynamics after Auger decay following carbon K-shell photoionization.
• Simulations utilizing femtosecond X-ray free-electron laser experimental data.
• Analysis of Coulomb momentum imaging data for structural interpretation.

Main Results:

• No evidence of sub-100 femtosecond isomerization for acetylene dication was found.
• The study successfully reproduced key features observed in time-resolved Coulomb imaging experiments.
• Theoretical calculations indicate that isomerization on low-lying dicationic states is not possible within sub-100 fs.

Conclusions:

• The apparent contradiction between theory and experiment regarding acetylene dication isomerization is resolved.
• Sub-100 femtosecond isomerization is not possible for acetylene dication in lower electronic states.
• The findings necessitate a careful re-evaluation of structural information derived from Coulomb momentum imaging techniques.