  Photoinduced isomerization reactions, including ring-opening reactions, lie
at the heart of many processes in nature. The mechanisms of such reactions are
determined by a delicate interplay of coupled electronic and nuclear dynamics
unfolding on the femtosecond scale, followed by the slower redistribution of
energy into different vibrational degrees of freedom. Here we apply
time-resolved photoelectron spectroscopy with a seeded extreme ultraviolet free
electron laser to trace the ultrafast ring opening of gas phase thiophenone
molecules following photoexcitation at 265 nm. When combined with cutting edge
ab initio electronic structure and molecular dynamics calculations of both the
excited and ground state molecules, the results provide unprecedented insights
into both electronic and nuclear dynamics of this fundamental class of
reactions. The initial ring opening and non-adiabatic coupling to the
electronic ground state is shown to be driven by ballistic SC bond extension
and to be complete within 350 femtoseconds. Theory and experiment also allow
clear visualization of the rich ground-state dynamics involving formation of,
and interconversion between, several ring opened isomers and the reformed
cyclic structure, and fragmentation (CO loss) over much longer timescales.

Chemical Physics