DIRECT OBSERVATION OF WEAK STATE MIXING IN HIGHLY VIBRATIONALLY EXCITED ACETYLENE

A pulsed-laser double-resonance technique probes the mixing of zero-or der states in the 3 nu(CH) vibrational overtone (epsilon(vib) approxim ate to 9640 cm(-1)) of (X) over tilde (1) Sigma(g)(+) acetylene (C2H2) , where the calculated vibrational state density is about three states /cm(-1). Vibrational overtone excitation populates and laser induced f luorescence via the (A) over tilde (1)A(u), electronic state detects t he molecular eigenstates, which have slightly mixed vibrational charac ter because of weak interactions between the zero-order optically brig ht C-H stretching state and optically dark background states. Observin g the interacting states at a 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 exp erimental data including our prior observations of surprisingly intens e (A) over tilde <-- (X) over tilde electronic transitions originating from 3 nu(CH), the anomalous rotational-level dependence of the elect ronic absorption cross sections, and small perturbations in the 3 nu(C H) line positions. A multi-state deperturbation analysis gives couplin g matrix elements of 0.01-0.05 cm(-1) that are consistent with those m easured for weak interactions in other polyatomic molecules at higher state densities.