Imaging the photodissociation dynamics of internally excited ethyl radicals from high Rydberg states.

The site-specific hydrogen-atom elimination mechanism previously reported for photoexcited ethyl radicals (CHCH) [D. V. Chicharro , , 2019, , 6494] is interrogated in the photodissociation of the ethyl isotopologues CDCD, CHCD and CDCH through the velocity map imaging (VMI) detection of the produced hydrogen- and deuterium-atoms. The radicals, generated from photolysis of a precursor using the same laser pulse employed in their excitation to Rydberg states, decompose along the C-H/D and C-H/D reaction coordinates through coexisting statistical and site-specific mechanisms. The experiments are carried out at two excitation wavelengths, 201 and 193 nm. The comparison between both sets of results provides accurate information regarding the primary role in the site-specific mechanism of the radical internal reservoir. Importantly, at 193 nm excitation, higher energy dissociation channels (not observed at 201 nm) producing low-recoil H/D-atoms become accessible. High-level calculations of potential energy curves and the corresponding non-adiabatic interactions allow us to rationalize the experimental results in terms of competitive non-adiabatic decomposition paths. Finally, the adiabatic behavior of the conical intersections in the face of several vibrational modes - the so-called vibrational promoting modes - is discussed.