Photodissociation of C2H at 193 nm: Branching ratios for the formation of C2 in the X 1Σg+, A 1Πu, and B′ 1Σg+ states

The ratio of the nascent population of C2 (X 1Σg+) to C2 (A 1Πu) to C2 (B′ 1Σg+) produced from the photodissociation of C2H at 193 nm has been measured using laser induced fluorescence. This technique is typically used to measure rotational and vibrational distributions in a given electronic state. Here, we have extended the technique to measure the relative electronic distributions in the C2 photofragment. From the simultaneous measurement of the Mulliken (X 1Σg+–D 1Σu+) and Freymark (A 1Πu–E 1Σg+) systems, the nascent population ratio of C2 (A 1Πu) to C2 (X 1Σg+) molecules was determined. Similarly, from the measurement of the Deslandres–D’Azumbuja (A 1Πu–C 1Πg) and the LeBlanc (B′ 1Σg+–D 1Σu+) systems, the nascent population ratio of C2 (A 1Πu) to C2 (B′ 1Σg+) was determined. The overall ratio for the production of C2 in the X:A:B′ electronic states was found to be 1:19:1.4. These results along with the results of high quality ab initio calculations of Cui and Morokuma (unpublished) are used to discuss the photodissociation dynamics of C2H at 193 nm. Furthermore, these results should aid in the analysis and modeling of cometary spectra of C2 .