PHOTODISSOCIATION OF ACETYLENE - DETERMINATION OF D-0(0)(HCC-H) BY PHOTOFRAGMENT IMAGING

Acetylene cooled in a He supersonic expansion is photodissociated by excitation in the 201-216 nm region of the à 1Au - X̃ 1Σg+ transition. Subsequent ionization of the H-atom fragments by 2 + 1 (243 nm) REMPI, and mass-selected ion imaging allows analysis of the velocity distribution of H-atoms from the HCCH →hv C2H + H process. Measurement of the maximum velocity for H atoms from this channel produced by photodissociation of acetylene through the à 1Au - X̃ 1Σg+ V07K 01, 101V0 4K01, 201V 05K01 and V0 5K01 vibronic transitions gives a value for D00(HCC-H) of 131 ± 1 kcal/mol. Other channels producing hydrogen atoms (including HC2 → hv C2 + H and HCCH →hv HCCH+ →hv C2H+ + H) are detected at all photon fluxes used. These multiphoton channels produce hydrogen atoms with higher translational energy and therefore obscure measurement of the maximum velocity of H atoms produced by single-photon dissociation of acetylene. Reduction of photon flux by more than two orders of magnitude to ∼5×106 J/cm2 gives a background, multiphoton, H-atom intensity of ≤7% of the peak primary dissociation intensity. Because this multiphoton background limits the detectability of fast H atoms from single-photon dissociation of acetylene, the dissociation energy reported here is an upper limit. Calculations of potential rovibronic excitation of the C2H fragment are discussed. © 1990 American Institute of Physics.