Direct observation of weak state mixing in highly vibrationally excited acetylene Original Research Article

A pulsed-laser double-resonance technique probes the mixing of zero-order states in the 3νCH vibrational overtone (ϵvib ≈ 9640 cm−1) of X̃1Σg+ acetylene (C2H2), where the calculated vibrational state density is about three states/cm−1. Vibrational overtone excitation populates and laser induced fluorescence via the Ã1Au electronic state detects the molecular eigenstates, which have slightly mixed vibrational character because of weak interactions between the zero-order optically bright CH stretching state and optically dark background states. Observing the interacting states at low state density in the weak perturber limit dramatically simplifies the assignment and interpretation of the spectra. A two-state model recovers the important features of the experimental data including our prior observations of surprisingly intense à « X̃ electronic transitions originating from 3νCH, the anomalous rotational-level dependence of the electronic absorption cross sections, and small perturbations in the 3νCH line positions. A multi-state deperturbation analysis gives coupling matrix elements of 0.01–0.05 cm−1 that are consistent with those measured for weak interactions in other polyatomic molecules at higher state densities.