Acetylene: Isomerization and Dissociation

Abstract

AbstractThe Stimulated Emission Pumping (SEP) spectrum at 15600 cm−1 above the HCCH X1σ+g zero point level contains features diagnostic of isomerization to vinylidene. It also illustrates a powerful new way, spectral cross‐correlation, for extracting dynamical information from high resolution spectra. SEP spectra of the same X̃‐state energy region, from two different intermediate levels (33 vs. 2142), are cross‐correlated. The correlation index is expected and found to be insignificant, except for isolated energy regions where a vinylidene basis state promotes acetylene mode mixing. The energy width of the correlated region is a direct measure of the distribution of vinylidene character into predominantly acetylene eigenstates. SEP spectra of HCCD provide additional information about the acetylene‐vinylidene isomerization process. The HCC‐H dissociation energy is poorly known but crucial to kinetic models for hydrocarbon combustion. The presence of a low lying excited state in HCC and HCC+ complicates the interpretation of dissociation and ionization threshold experiments. Zeeman Anticrossing Spectroscopy provides an opportunity to detect the onset of predissociation in a single rovibronic level of the HCCH Ã state by inducing anticrossings with levels of the T1 and T2 surfaces (all three isomers of each, cis‐HCCH, trans‐HCCH, and vinylidene) and, indirectly, with high vibrational levels of the X̃ state. We have detected one predissociative anticrossing in HCCH Ã υ3 = 4, J = Ka = 1.