High resolution spectroscopic detection of acetylene–vinylidene isomerization by spectral cross correlation

Abstract

Information about the unimolecular acetylene (HC 3/4 CH)↔vinylidene (H₂C=C:) isomerization on the S₀ energy surface has been extracted from vibrationally unassigned high resolution stimulated emission pumping (SEP) spectra of acetylene. The combination of a new pattern recognition scheme, spectral cross correlation (SCC) with complete nuclear permutation‐inversion (CNPI) group theory is shown to be a powerful new technique for characterizing bond rearrangement in highly vibrationally excited normally rigid polyatomic molecules. SCC detects isomerization ‘‘resonances’’ which destroy an approximate vibrational symmetry (e.g., the number of cis‐bending quanta). The energies (relative to the zero point level of the stablest isomer) and widths of such resonances provide information about the ‘‘energies’’ of isomer rovibrational levels and the isomer‐level‐specific isomerization rate. Vinylidene isomerization resonances may be distinguished from ordinary acetylene Fermi or Coriolis perturbations by a unique rotational symmetry dependence due to the correlation between acetylene [D_∞h(M)] and vinylidene [C_2v(M)] levels in the CNPI group G₈. An SCC map of the HCCH S₀ 15 000–15 900 cm⁻¹ energy region above the zero point level was obtained by comparing SEP spectra recorded via S₁(Ã ¹A_u)3³ and 2¹6² K_a=1 intermediate levels. The predicted rotational symmetry dependence of the SCC was found between 15 410–15 640 cm⁻¹, but the vinylidene resonance line shape was obscured by Franck–Condon interference effects from well known perturbations between the 3³ and 2¹6² SEP intermediate levels. It was therefore not yet possible to determine the height of the isomerization barrier (or even whether a barrier exists) but an upper bound for the vinylidene zero point level of 15 525 cm⁻¹ was inferred from the SCC data.