- Selective photolysis of a mixture of acetylene isotopomers prepared in rovibrational states lying in the energy window of the second overtone of CH stretches was performed. Tunable near infrared (NIR) photons prepared the isotopomers in initial states and ultraviolet photons both promoted them to the excited electronic trans -bent A ̃ 1 A u state and tagged the H(D) photofragments via (2+1) resonantly enhanced multiphoton ionization. Photoacoustic spectroscopy monitored the initial state preparation and action spectroscopy simultaneously tagged the H(D) photoproducts yield as a function of the wavelength of the exciting NIR laser. The measurements and analysis indicate differing dissociation efficacies in C 2 H 2 and C 2 HD revealing enhanced CH bond cleavage in the former and hindered CH(D) bond fission in the latter. The enhancement is a result of coincident resonances with the upper state leading to sharp structures in the photodissociation cross-section of C 2 H 2 .