Theoretical study of the radiative lifetime of the a 1Π u, state of C2

We have studied the Phillips (A ¹Π_u-X ¹Σ_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 ¹Π_u (υ′ = 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.