Theoretical study of the radiative lifetime of the A 1∏u state of C2

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Abstract
We have studied the Phillips (A 1∏ u –X 1∑+ g ) band system of C2 in order to help resolve the discrepancy between the best theoretical lifetimes and those deduced recently from laser‐induced fluorescence(LIF) studies. Our calculated lifetime for the A 1∏ u (v’=0, J’=1) state is 13.0 μs, in excellent agreement with the recent calculations of ONeil e t a l., but considerably shorter than the direct experimental measurements. A measure of the accuracy of the complete‐active‐space self‐consistent‐field (CASSCF) multireference configuration‐interaction (MRCI) results in this work is obtained by a series of calibration calculations. Convergence in the n‐particle space is demonstrated by comparison with full configuration interaction calculations in a double‐zeta plus polarization basis, while the convergence of the one‐particle basis set is demonstrated by systematically expanding the one‐particle basis set up through gangular momentum functions. Furthermore, a coupling of the one‐ and n‐particle spaces is shown to be unimportant by systematically expanding the active space in the CASSCF/MRCI treatment. Given that our theoretical results are estimated to be accurate to about 5%, the LIF lifetimes would appear to contain some systematic error.

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