Two-photon spectroscopy of dipole-forbidden transitions

Abstract

We have investigated the applicability of CNDO/S-type methods for the calculation of optical spectra of molecules with the special implication that the calculations should not only describe the intense, dipole-allowed transitions which dominate the one-photon absorption spectrum but also those transitions which are one-photon forbidden in first-order approximation. We show that such a method is well suited to predict dipole allowed and dipole forbidden transitions at a similar level of accuracy if double excited configurations are taken into account. In spite of the lack of perfect pairing in NDO methods there are still two types of states which exhibit a different sensitivity towards correlation effects. Therefore, the approximation by which we describe the R-dependence of the Coulomb repulsion gains much more importance than in cases where mainly dipole allowed transitions are of interest. These findings confirm results obtained earlier from theories for which the pairing theorem is valid. The calculated data show an excellent stability with respect to further increase of the number of configurations if at least about 200 energy selected configurations are taken into account.

TheT 1 state ofp-nitroaniline and related molecules: A CNDO/S study

“…Particular parametrizations are described in the text. Computations involving doubly excited states were carried out as described separately for singlets [32] and triplets [33].…”

“…We performed a cursory examination of the effects on the electronic structure of PNA of including doubly-excited states in the configuration interaction [32,33]. Briefly, we find relative to corresponding computations using singlyexcited states: (1) doubly-excited configurations do not alter the character of the 7"1 state; (2) the T1 -So splitting is significantly enhanced in the computation employing NM integrals due to a large selective stabilization of So;…”

Two-photon spectroscopy of dipole-forbidden transitions

“…and 6 it the twophoton polanzation parameter rs denved which serves as a symmetry indicator in many cases [6]. h pornt group Dzh, III which tramitlons from the ground state 1% to Ae and BE states are two-photon allowed, the upper limit for S2 of 15 should be observed for Bg states, while % states usurdly lead to miruma in n(v) with values down to 02.5 [6,9].…”

New experimental information from two-photon spectroscopy and comparison with theory

“…In this case we have hv = hv1 = hv2. The selection rules for one-and two-photon transitions to electronically excited states are similar to the selection rules for IR and Raman scattering (also a two-photon process) (9,17). This means that for molecules with a center of symmetry, the principle of mutual exclusion holds: from a gerade ground state, transitions are only allowed to ungerade excited states in one-photon absorption (OPA) and to gerade excited states in TPA.…”

“…In order to compare our experimental results with calculations which either include correlation effects or not we use the following two calculation schemes (17)…”