Abstract
Ultrafast proton migration and isomerization are key processes for acetylene and its ions. However, the mechanism for ultrafast isomerization of acetylene [HCCH]2+ to vinylidene [H2CC]2+ dication remains nebulous. Theoretical studies show a large potential barrier ( > 2 eV) for isomerization on low-lying dicationic states, implying picosecond or longer isomerization timescales. However, a recent experiment at a femtosecond X-ray free-electron laser suggests sub-100 fs isomerization. Here we address this contradiction with a complete theoretical study of the dynamics of acetylene dication produced by Auger decay after X-ray photoionization of the carbon atom K shell. We find no sub-100 fs isomerization, while reproducing the salient features of the time-resolved Coulomb imaging experiment. This work resolves the seeming contradiction between experiment and theory and also calls for careful interpretation of structural information from the widely applied Coulomb momentum imaging method.
Highlights
Ultrafast proton migration and isomerization are key processes for acetylene and its ions
We model the dynamics on the core-hole state prior to the Auger decay that yields the dication
Our ab initio molecular dynamics simulations of core ionized acetylene cation start from initial conditions sampled from the vibrational ground-state harmonic
Summary
We present a complete theoretical time-resolved picture of the ultrafast X-ray pump/X-ray probe experiment on acetylene dication dynamics. We model the dynamics of the core-ionized cation, its Auger decay, the dynamics of the dication, and the momentum distribution in the time-resolved. Our results show that a sub 100 fs a. A second X-ray probe pulse with a variable delay further core ionizes the dication, which promptly undergoes further Auger decay and Coulomb explosion. The momentum of the resulting fragments is measured to create the momentum map described in the text. B The potential curves of the singlet dicationic states are plotted in the adiabatic representation The momentum of the resulting fragments is measured to create the momentum map described in the text. b The potential curves of the singlet dicationic states are plotted in the adiabatic representation
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