Two-photon spectroscopy of dipole-forbidden transitions. I. Dipole-forbidden transitions and double excited configurations in the CNDO-CI methods

The applicability was investigated of CNDO/S-type methods for the calcn. of optical spectra of mols. with the special implication that the calcns. should not only describe the intense, dipole-allowed transitions which dominate the 1-photon absorption spectrum but also those transitions which are 1-photon forbidden in 1st order approxn. Such a method is well suited to predict dipole allowed and dipole forbidden transitions at a similar level of accuracy if doubled excited configurations are taken into account. In spite of the lack of perfect pairing in NDO methods there are still 2 types of states which exhibit a different sensitivity towards correlation effects. Therefore, the approxn. used to 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 calcd. data show an excellent stability with respect to further increase of the no. of configurations if at least ~200 energy selected configurations are taken into account

Two-photon spectroscopy of dipole-forbidden transitions. The low-lying singlet states of anthracene

The 2-photon excitation spectrum of anthracene in soln. from 29,000 to 47,000 cm-1 is reported. Two-photon allowed transitions to 1B3g(35,800 cm-1), 2Ag (38,000 cm-1), and 3Ag (43,000 cm-1) were assigned by polarization measurements. These results, with those from 1-photon spectroscopy, agree with calcns. Theor. data suggest assignment of a B3g state to a shoulder at 39,000 cm-1 in the 2-photon spectrum

Importance of initial and final states as intermediate states in two-photon spectroscopy of polar molecules

Two-photon transitions may proceed via a mechanism not involving virtually excited intermediate states. This mechanism may account for the largest contribution to the 2-photon cross section in polar mols. if the dipole moment strongly alters with excitation. Possible consequences for the 2-photon detection of forbidden states in polar polyene systems are discussed

Calculation of transition metal compounds using an extension of the CNDO formalism. II. Metal to metal bonding in binuclear transition metal compounds

A recently developed extension of the CNDO-method (Freund and Hohlneicher, 1979) is used to study the electronic structure of a no. of binuclear transition metal carbonyls and carboxylates with 4-fold or quasi-4-fold symmetry. The results are compared to those available from nonempirical calcns. Special attention is paid to the nature of the metal-metal bond. Connections with qual. MO-considerations allow a fairly general discussion of metal-metal bonding in binuclear transition metal complexes with basic 4-fold symmetry. A few, up to now unknown, but possibly existing, complexes are considered

Two-photon spectroscopy of the biphenyl chromophore. The electronic excited states of biphenyl and fluorene below 50000 cm-1

The two-photon excitation spectra of biphenyl and fluorene in dil. soln. were measured up to 50,000 cm-1. Both spectra exhibit a medium intense band system in the range 32,000-42,000 cm-1, and a strong band above 45,000 cm-1. The lowest frequency feature is assigned to a B3 symmetry transition in biphenyl and the corresponding B2 transition in fluorene. The polarization of the higher bands leads to the assignment of 2 A states at 38,000 and 47,000 cm-1. The origin of the electronically excited states of the biphenyl chromophore is discussed by simple composite mol. considerations as well as CNDO CI calcns. The latter give a semiquant. picture of transition energies and transition probabilities for 1- and 2-photon allowed excitations. A compilation of 1-photon spectra and calcns. from the literature is included in the anal. to provide a consistent picture of the electronically excited states of the biphenyl chromophore up to 50,000 cm-1

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

From a specially designed expt., allowing the measurement of 2-photon absorption spectra and 2-photon polarization parameters over a wide spectral range, new exptl. information is obtained on the location of electronically excited states usually not seen in the UV spectrum. By comparison with theory it is shown that in most of the examples investigated up to now the ordering of the low lying excited states is different from what is usually expected. Correlation strongly influences the ordering esp. in those systems which consist of loosely coupled p-systems

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

The applicability of a CNDO/S scheme including double excited configurations for the calcn. of excitation energies of larger unsatd. mols. was studied. The same scheme is very useful for the prediction of 2-photon transition probabilities. If the proper expansion is used, the results converge quite well with increasing no. of intermediate states. 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 2-photon cross sections