Monitoring the Evolution of Relative Product Populations at Early Times during a Photochemical Reaction

Abstract

Identifying multiple rival reaction products and transient species formed during ultrafast photochemical reactions and determining their time-evolving relative populations are key steps toward understanding and predicting photochemical outcomes. Yet, most contemporary ultrafast studies struggle with clearly identifying and quantifying competing molecular structures/species among 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 timeresolved 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 three-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.

Perspective on Theoretical and Experimental Advances in Atmospheric Photochemistry

“…The nonadiabatic relaxation can lead to the formation of ground-state molecules with internal energies deviating significantly from a Boltzmann distribution at early times, which can result in athermal reactivity and the formation of (sometimes) unexpected products . Such processes were discussed in the context of gas-phase photochemistry , and also observed for nitroaromatic compounds of atmospheric importance …”