Vacuum-Ultraviolet Photolysis of Acetylene

Abstract

The vacuum‐ultraviolet photolysis of acetylene has been investigated at the Kr (1236 Å, 1165 Å) and Xe (1470 Å, 1295 Å) resonance lines. Evidence is presented for production of a long‐lived excited state of acetylene. Decomposition of the excited state at low pressure leads to the formation of molecular hydrogen and excited C2. At the Kr resonance lines and at the 1295‐Å Xe line, an emission consistent with Swan bands (A 3π g →X 3π u ) of C2 is observed at low pressures. No emission is observed in the photolysis of acetylene at 1470 Å. Intensity of the emission, the quantum yield for hydrogen production, and the quantum yield for acetylene disappearance are all greatly reduced by the addition of a few millimeters of a quenching gas. Decomposition to H2 and C2 also occurs in the solid phase. The over‐all photolysis is explicable in terms of an excited molecule mechanism. Formation of C2 in excited A 3π g state depends on the energy of the incident photon: C 2 H 2 +hv→ C 2 H 2 * , C 2 H 2 * + M → C 2 H 2 + M , C 2 H 2 * + C 2 H 2 → C 4 H 4 * , C 4 H 4 * → C 2 H 2 ‡ + C 2 H 2 ‡ , C 2 H 2 * → C 2 (A 3 π g )+ H 2 , C 2 (A 3 π g )→ C 2 (X 3 π u )+hv, C 2 H 2 * → C 2 + H 2 , C 2 H 2 * → C 2 H + H , H + C 2 H 2 → C 2 H 3 , C 4 H 4 * + C 2 H 2 → C 6 H 6 * , etc ., C 4 H 4 * + C 2 H 2 → C 4 H 4 ( vinyl acetylene )+ C 2 H 2 , C 6 H 6 * + C 2 H 2 → C 6 H 6 + C 2 H 2 , 2 C 2 H → C 4 H 2 ( diacetylene )