UV short-pulse fragmentation of isotopically labeled acetylene: Studies of emission with subnanosecond resolution

We have studied fragment emission in the spectral range 200 to 900 nm, following photolysis of acetylene (1–10 Torr) by short ultraviolet pulses (25 ps, 266 nm, 2.5–12 mJ). The dominant component of emission is the C2 d 3Πg→a 3Πu Swan system. Weak singlet C2 emission is observed. CH is also observed, in the A 2Δ→X 2Π and C 2Σ+→X 2Π systems. With isotopic labeling and with streak camera recording, we have demonstrated distinct unimolecular and intermolecular processes yielding C2 d 3Πg. The unimolecular channel proceeds with risetime τ1 = 215±15 ps, while the pressure-dependent intermolecular component develops over a period of nanoseconds. We discuss the underlying kinetics and the mechanistic implications of the 25 ps excitation.