Neutral-to-ionic ZEKE transition dipole couplings beyond Koopmans' picture: Application to femtosecond pump-probe spectroscopy

We derive an expression to calculate approximate non-Condon, i.e., coordinate-dependent, dipole couplings, for transitions from excited neutral to ionic states with zero electron kinetic energy (ZEKE). Our expression can be considered beyond Koopmans' picture since it is obtained for cases in which the initial neutral state is described multiconfigurationally - as excited states usually are - and the final ionic state is well represented by a single Slater determinant with non-frozen orbitals. It is composed of three terms: (i) the configuration interaction coefficients corresponding to determinants of the neutral molecule which differ in only one spin orbital from the ionic determinant, (ii) determinants depending on the overlaps of non-frozen orbitals of the initial neutral molecule and of the final ion, occupied with α or β electrons, and (iii) the single electron matrix element between the initial and final orbitals, the latter representing the photo-detached electron. Exemplarily, transition probabilities are estimated from MR-CCI/CASSCF ab initio calculations for the recently investigated [C. Daniel, J. Full, L. González, C. Lupulescu, A. Meri, J. Manz, S. Vajda, L. Wöste, Science 299 (2003) 536] organometallic compound CpMn(CO)3 and used to predict non-Condon effects of femtosecond pump-probe spectra in different laser domains.