Photodissociation of acetylene: Determination of D00 (HCC–H) by photofragment imaging

Abstract

Acetylene cooled in a He supersonic expansion is photodissociated by excitation in the 201–216 nm region of the à 1Au −X̃ 1∑+g 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 hν→ C2H+H process. Measurement of the maximum velocity for H atoms from this channel produced by photodissociation of acetylene through the à 1Au −X̃ 1∑+g V70K10, 110V40K10, 210V50K10 and V50K10 vibronic transitions gives a value for D00 (HCC–H) of 131±1 kcal/mol. Other channels producing hydrogen atoms (including HC2 hν→ C2+H and HCCHhν→ HCCH+ hν→ 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.