Pure bending dynamics in the acetylene (X)over-tilde1Σg+ state up to 15,000 cm-1 of internal energy

We investigate the large-amplitude bending dynamics of acetylene, in its ground electronic state, using an effective Hamiltonian model that reproduces all relevant experimental data, up to 15 000 cm(-1) in internal energy, with 1.4 cm(-1) accuracy (1 sigma). The experimental data which make this analysis possible are derived from the dispersed fluorescence (DF) data set that we recently reported [J. P. O'Brien et al., J. Chem. Phys. 108, 7100 (1998)] for the acetylene (A) over tilde (1)A(u) --> (X) over tilde (1)Sigma(g)(+) system, which includes DF spectra recorded from five different vibrational levels of the (A) over tilde (1)A(u) state. A numerical pattern recognition technique has permitted the assignment of polyad quantum numbers to observed transitions in these spectra, with up to 15 000 cm(-1) in internal energy. Here we analyze a special subset of the identified polyads, those which involve excitation exclusively in the trans and cis bending modes: the pure bending polyads. The bending dynamics that is encoded in these polyads is analyzed using both frequency and time-domain formalisms. Among the conclusions of this analysis is that, in many ways, the observed bending dynamics is somewhat simpler at 15 000 than it is at 10 000 cm(-1); this rather surprising result is explained in terms of qualitative changes in the structures of the pure bending polyads as a function of increasing internal energy. (C) 1998 American Institute of Physics. [S0021-9606(98)01425-1].