- C2H2 is prepared in the 2030000 (five quanta of C–H stretch) vibrational state and photodissociated by 243.135 nm photons that also probe the H photofragments via (2+1) resonance-enhanced multiphoton ionization (REMPI) in a time-of-flight mass spectrometer. The production of H atoms is greatly enhanced upon rovibrational excitation. The REMPI action spectrum shows the characteristic features of a Σu+–Σg+ band and mimics the absorption spectrum, except that the R(13) line intensity is an order of magnitude higher than that expected for a Boltzmann distribution. The maximum translational energy of the H atoms obtained from dissociation of the regularly distributed rotational states is 0.67±0.10, whereas for R(13) it is 1.34±0.10 eV. The observed intensities and linewidths indicate the existence of two photodissociation pathways following the preparation of C2H2, where the C2H fragment is produced in two different states. In the R(13) pathway an additional bent state is prepared, or an accidental coincidence resonance is involved. © 1997 American Institute of Physics.