Anomalously slow intramolecular vibrational redistribution in the acetylene X̃ 1Σg+ state above 10 000 cm−1 of internal energy

Abstract

We have identified, in dispersed fluorescence spectra of acetylene à 1Au→X̃ 1Σg+ emission, a series of bright states between 10 000 and 15 000 cm−1 of internal energy which display anomalously slow intramolecular vibrational redistribution. That is, these bright states display virtually no fractionation at internal energies at which the majority of other observed bright states are fractionated over several hundred cm−1 in a complicated fashion. The anomalous bright states are distinguished from the other nearly isoenergetic bright states only by the way in which the vibrational excitation is distributed among the CC stretch and trans bend modes; specifically, the anomalous bright states have relatively low excitation in the trans bend mode (v4⩽8), with the remainder of the vibrational excitation in the CC stretch mode (v2⩽6). A refinement of the acetylene global effective Hamiltonian permits detailed insight into the mechanism of the anomalously slow intramolecular vibrational redistribution, and reveals that the relatively simple fractionation patterns of these bright states can be adequately described in terms of a system of 4 interacting zero-order states which are coupled by Darling-Dennison bending resonances, vibrational l-resonance, and an anharmonic “3,245” resonance. The refined effective Hamiltonian also permits the assignment of similar, minimally perturbed bright states up to at least 17 500 cm−1 of internal energy.