- Rovibrational excitation enables molecules to reach regions of the ground potential energy surface that are unavailable to vibrationless ground state species. Consequently, the photodissociation outcome may be affected via the Franck–Condon factor, which effectively influences the electronic excitation, depending on the initially prepared rovibrational state. Here we focus on the rovibrationally mediated photodissociation of C 2 HD/C 2 H 2 and compare action spectra to rovibrational absorption spectra to study the photodissociation dynamics. Rovibrationally excited C 2 HD (in the 15637–15735 and 15256–15337 cm −1 regions) and C 2 H 2 (in the 15480–15723 cm −1 region) are promoted to the excited electronic trans -bent states A 1 A u / B 1 B u and photodissociated by ∽243.1 nm photons that also interrogate the H/D fragments. The yield of both H and D photofragments is greatly enhanced upon rovibrational excitation of both isotopomers showing both vibrational and rotational state dependencies. The mechanism for photoproduct propensity in C 2 HD and for intensity enhancement and photodissociation pathways in C 2 H 2 is discussed.