Differing reactivities in the predissociation of acetylene isotopomers pre-excited with three CH stretching quanta

Selective photolysis of a mixture of acetylene isotopomers prepared in rovibrational states lying in the energy window of the second overtone of CH 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 à 1Au 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 C2H2 and C2HD revealing enhanced CH bond cleavage in the former and hindered CH(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 C2H2.