Theoretical determination of the radiative lifetime of the A 2Σ+ state of OH

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Abstract
The radiative lifetime of the A 2Σ+(v′=0, N′=1) state of OH has been computed, using the theoretical potentials and transition moment, to be 673 ns, in excellent agreement with two of the experimental values (686±14 and 693±10 ns) determined by laser excitation fluorescence. The electron correlation treatment has been calibrated using full CI (FCI) calculations in a [4s3p2d/2s1p] Gaussian basis set. When a δ orbital is included in the CASSCF/MRCI treatment, the transition moment is within 0.2% of the FCI value. A basis set study shows that the [6s5p4d2f1g/4s3p2d] ANO basis set employed in our study is probably within 1% of the basis set limit for the transition moment. The theoretical radiative lifetimes, which are expected to be lower bounds and accurate to 2%, are sufficiently accurate to rule out the slightly lower value determined by Hanle effect studies, and the higher values determined for the v′=0 N′=1 level by the high frequency deflection technique (HFD). The HFD lifetimes for higher N′ values, however, are in relatively good agreement with theory.