The Dependence on Initial Configuration of Strong Field-Driven Isomerization of C 2 H 2 Cations and Anions.

We have investigated the femtosecond laser-induced fragmentation of C$_2$H$_2^{q}$ ion beam targets in various initial configurations, including acetylene (linear HCCH), vinylidene (H$_2$CC), and \emph{cis}/\emph{trans}. The initial configuration is shown to have tremendous impact on the branching ratio of acetylene-like (CH$^{q_1}$~+~CH$^{q_2}$) and vinylidene-like (C$^{q_1'}$~+~CH$_2^{q_2'}$) dissociation of a specific C$_2$H$_2^{q}$ molecular ion. In particular, whereas C$_2$H$_2^+$ generated from C$_2$H$_2$, a linear HCCH target, exhibits comparable levels of acetylene-like and vinylidene-like fragmentation, vinylidene or \emph{cis}/\emph{trans} configuration ion beams preferably undergo vinylidene-like fragmentation, with an acetylene branching ratio ranging from 13.9\% all the way down to zero.