Highly vibrationally excited 12C2H2 in the X̃ 1∑+g state: Complementarity of absorption and dispersed fluorescence spectra

Abstract

Direct absorption and dispersed fluorescence (DF) spectra sample the 12C2H2 X̃ 1Σ+g potential energy surface via profoundly different zero-order bright states. Despite the complementary nature of the data sets, a polyad model based on the approximate quantum numbers, Ns=v1+v2+v3, Nr=5v1+3v2+5v3+v4+v5, l=l4+l5, accounts for the energies (±0.35 cm−1) and relative intensities for all transitions into pure bending levels (Ns=0) at Evib≤12 000 cm−1 and Nr≤15, l=0 and 2. The parameters that define this model are obtained by fitting 41 pure bending levels, of which 9 are J=0 energies derived from components of the [Ns=0, Nr=8, 10, 12, l=0, g] polyads observed in the DF spectrum. This polyad model provides a basis for extrapolating a description of otherwise indescribably complex spectra and dynamics to the Evib≊16 000 cm−1 region of the barrier to acetylene ↔ vinylidene isomerization, and could therefore provide a basis for detecting the pattern-breaking signature of the onset of isomerization.