A method is described for an ab-initio Cl calculation of the decay of H2CO(1A2). The normal modes 4, 5 and 6 turn out to be about equally important for inducing the radiative transition, while the non-radiative decay is an example of the resonance case. So the experimentally determined exponential decay must be due to a coupling with the dissociative continuum.
van Dijk, J. M. F., Kemper, M. J. H., Kerp, J. H. M., Buck, H. M., & Visser, G. J. (1978). Ab-initio CI calculation of radiative and non-radiative decay of formaldehyde (1A2) with application to its photochemical decomposition. Chemical Physics Letters, 54(2), 353-356. https://doi.org/10.1016/0009-2614(78)80116-X