Theoretical study of the radiative lifetime of the A ¹Π _u state of C₂

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 et al., 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 g angular 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.