Two-photon spectroscopy of dipole-forbidden transitions. II. Calculation of two-photon cross sections by the CNDO–CI method

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Abstract
In the first paper of this series we investigated the applicability of a CNDO/S scheme including double excited configurations for the calculation of excitation energies of larger unsaturated molecules. In this paper we show that the same scheme is very useful for the prediction of two‐photon transition probabilities. If the proper expansion is used, the results converge quite well with increasing number of intermediate states. We also show that the inclusion of double excited configurations is not only necessary to obtain better energies for dipole‐forbidden transitions to ’’covalent’’ excited states but also to obtain the correct order of magnitude for two‐photon cross sections.