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 ¹A_u −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 ¹A_u −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 HCCH^hν_→ 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.