Dispersed Fluorescence Spectrum of Acetylene from the à ¹A_u Origin: Recognition of Polyads and Test of Multiresonant Effective Hamiltonian Model for the X State

Abstract

We have recorded a high-resolution dispersed fluorescence (DF) spectrum of the à 1Au → X̃ transition of acetylene-h2, utilizing the zero-point level of the à 1Au state. Here we present only the analysis of the vibrational levels of the X̃-state with Evib < 10 000 cm-1. By comparing the observed and calculated spectral intensity patterns, we have estimated the previously undetermined pure bend vibrational constant, to be −11(2) cm-1. Unlike previously recorded DF spectra, this DF spectrum is uniquely suited for comparison with an effective Hamiltonian since (1) the Franck−Condon envelope facilitates observation of levels at lower Evib in the X̃-state, (2) the improved resolution is sufficient to observe intramolecular vibrational redistribution (IVR) at lower Evib, and (3) spectral features are no longer absent because of nodal patterns in the Franck−Condon envelope. Our comparison shows that our current effective Hamiltonian model, can qualitatively describe the IVR pathways on the X̃-state for chromostates (zero-order bright states) which contain high excitation in the trans-bend (v4 ≤ 12). The failure of our model for chromostates which contain excitation in both the CC stretch (v2) and the trans-bend (v4 > 8) may be attributed to a new stretch−bend or stretch-only resonance. Comparison of the present DF spectrum to prior SEP spectra has allowed us to make plausible rotational and vibrational assignments for previously unassigned transitions in the SEP spectra.