Ultrafast Isomerization in Acetylene Dication: To Be or Not To Be

Experimental evidence has pointed toward the existence of ultrafast proton migration and isomerization as a key process for acetylene and its ions, however the actual mechanism for ultrafast isomerization of the acetylene [HCCH]2+ to vinylidene [H2CC]2+ dication remains nebulous. Theoretical studies show a high potential barrier of over 2eV for the isomerization pathways on the low lying dicationic states, implying that the corresponding isomerization should take picoseconds or even longer according to transition state theory. However a recent experiment at a femtosecond X-ray free electron laser (XFEL) [Nature Commun. 6, 8199 (2015)] suggests that large amplitude hydrogen migration proceeds on a sub-100 femtosecond time scale. In order to resolve the contradiction, we present 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 that isomerization does not occur on the sub-100 fs timescale and is not required to explain the time-resolved Coulomb imaging experiment. This study resolves the seeming contradiction between experiment and theory concerning the isomerization time scale in acetylene dication. This work calls for careful interpretation of structural information from the widely applied Coulomb momentum imaging method but also points out its strengths in mapping out momentum dispersion dynamics even when structural variation is minor.