  Identifying multiple rival reaction products and transient species formed
during ultrafast photochemical reactions and determining their time-evolving
relative populations are key steps towards understanding and predicting
photochemical outcomes. Yet, most contemporary ultrafast studies struggle with
clearly identifying and quantifying competing molecular structures/species
amongst the emerging reaction products. Here, we show that mega-electronvolt
ultrafast electron diffraction in combination with ab initio molecular dynamics
calculations offer a powerful route to determining time-resolved populations of
the various isomeric products formed after UV (266 nm) excitation of the
five-membered heterocyclic molecule 2(5H)-thiophenone. This strategy provides
experimental validation of the predicted high (~50%) yield of an episulfide
isomer containing a strained 3-membered ring within ~1 ps of photoexcitation
and highlights the rapidity of interconversion between the rival highly
vibrationally excited photoproducts in their ground electronic state.
