ANOMALOUSLY SLOW INTRAMOLECULAR VIBRATIONAL REDISTRIBUTION IN THE ACETYLENE (X)OVER-TILDE-(1)SIGMA(-)1 OF INTERNAL ENERGY()(G) STATE ABOVE 10000 CM()

We have identified, in dispersed fluorescence spectra of acetylene (A) over tilde (1)A(u) --> (X) over tilde (1)Sigma(g)(+) emission, a seri es of bright states between 10 000 and 15 000 cm(-1) of internal energ y which display anomalously slow intramolecular vibrational redistribu tion. That is, these bright states display virtually no fractionation at internal energies at which the majority of other observed bright st ates are fractionated over several hundred cm(-1) in a complicated fas hion. The anomalous bright states are distinguished from the other nea rly isoenergetic bright states only by the way in which the vibrationa l excitation is distributed among the CC stretch and trans bend modes; specifically, the anomalous bright states have relatively low excitat ion in the trans bend mode (upsilon(4)less than or equal to 8), With t he remainder of the vibrational excitation in the CC stretch mode (ups ilon(2)less than or equal to 6). A refinement of the acetylene global effective Hamiltonian permits detailed insight into the mechanism of t he anomalously slow intramolecular vibrational redistribution, and rev eals that the relatively simple fractionation patterns of these bright states can be adequately described in terms of a system of 4 interact ing zero-order states which are coupled by Darling-Dennison bending re sonances, vibrational I-resonance, and an anharmonic ''3,245'' resonan ce. The refined effective Hamiltonian also permits the assignment of s imilar, minimally perturbed bright states up to at least 17 500 cm(-1) of internal energy. (C) 1998 American Institute of Physics. [S0021-96 06(98)01234-3].