VIBRONIC COUPLING IN THE GROUND STATE OF VINYLIDENE X̃ 1A1 H2CC

begin{wrapfigure}{r}{0pt}_x000d_ includegraphics[scale=.8, clip]{../images/2758_image.eps}_x000d_ end{wrapfigure}_x000d_ The nature of the isomeration process that turns vinylidene H$_2$CC to_x000d_ acetylene HCCH, requiring a 1,2-hydrogen atom shift across the molecule,_x000d_ is a long standing puzzle that has its origin in a 1989 photoelectron_x000d_ measurement of vinylidide (H$_2$CC$^-$)footnote{K. M. Ervin emph{et al.} emph{J. Chem. Phys.}textbf{91} 5974 (1991).}._x000d_ In recent years the photoelectron spectrum of vinylidide has been_x000d_ revisited, using improved experimental techniques, including velocity-map imaging for the detection of photoelectrons, low-temperature near-threshold methods (cryo-SEVI)footnote{J. A. De~Vine emph{et al.} emph{J. Am. Chem. Soc.} textbf{138} 16417 (2016).}, and sophisticated emph{ab inito} calculationsfootnote{L.~Guo emph{et al.} emph{J. Phys. Chem.} textbf{119} 8488 (2015).}. The simple normal-mode structure, 1064~nm velocity-map image illustrated, is proving a challenge to decipher. However, the dramatic change in the photoelectron angular distribution of the inner-ring structure is characteristic of vibronic couplingfootnote{A. Weaver emph{et al.} emph{J. Chem. Phys.} textbf{94} 1740 (1991).}. The lowest electronic state with the correct symmetry, $tilde{B}; ^{1}!B_2$, is 4eV higher in energy.makeatletter{renewcommand*{@makefnmark}{}footnotetext{Research supported by the Australian Research Council Discovery Project Grant DP160102585.}makeatother