Unlike most hydrocarbons, acetylene does not dissociate rapidly at energies just above the R—H thermodynamic dissociation threshold. In its S1 excited electronic state, the dissociation threshold lies slightly below the trans↔cis isomerization transition state. Predissociation in the S1 state is slow and the rate is strongly vibration-rotation level dependent. Predissociation is slow because the HCCH S1 state belongs to a σ2π3π* electronic configuration and the ground state of CCH belongs to a σπ4 configuration. Whether predissociation proceeds through the nonplanar T3 state or through the cis-bent conformation of the S1 state is probed by a variety of action spectroscopies: H atom REMPI, H atom fluorescence, and extremely strong C2 C1Πg-A1Πu and d3Πg-a3Πu fluorescence. The C2 fluorescence results from three-photon excited fluorescence, resonance enhanced in the HCCH S1 state at the one-photon level.

Always acetylene!

253rd National Meeting (2017)

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