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 ¹A_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) photo-products 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.