Theoretical and Experimental Evidence of Hydrogen Migration rather than Isomerization in the Acetylene Dication

Theoretical calculations and experimental results in the acetylene dication have long agreed that isomerization after x-ray excitation occurs in the first singlet state, where the carbon-carbon bond lives long enough for isomerization to complete [1]. These same calculations predict that a large barrier to isomerization exists that would cause isomerization to occur in about a picosecond, while there is some evidence for ultrafast isomerization in under 100 fs.~However, new ab initio calculations of the acetylene dication reveal that ultrafast isomerization after x-ray excitation is unlikely.~In this talk, we present evidence that signatures of hydrogen migration observed in recent time resolved LCLS data [2] are mostly due to hydrogen migration in an excited state which dissociates too quickly for isomerization to complete. [1] T. S. Zyubina, Y. A. Dyakov, S. H. Lin, A. D. Bandrauk, and A. M. Mebel., ``Theoretical study of isomerization and dissociation of acetylene dication in the ground and excited electronic states,'' J. Chem. Phys. \textbf{123}, 134320 (2005). [2]~C. E. Liekhus-Schmaltz, et al, ``Ultrafast isomerization initiated by X-ray core ionization,'' Nat. Commun. \textbf{6}, 8199 (2015).