Lorenzo Di Sipio

The evaluation of the one-centre integrals in the semi-empirical molecular orbital theory

An expression is given which fairly accurately provides valence-state energies. It is shown that one can formally attribute the meaning of the one-centre integrals to the coefficients in such an expression. In this way one also obtains the criteria of Pariser and Parr. Moreover, if one introduces such semi-empirical integrals in the expression which gives the vertical ionization potentials and electron affinities, one obtains the corresponding experimental values. On the basis of these facts usage of such semi-empirical integrals in the M.O.-L.C.A.O. theory is proposed. A discussion of the advantages and the limitations of its application is made.

The spectra and electronic structure of tetrahedral ions

An S.C.F.-M.O.-L.C.A.O. calculation in the Roothaan-Popie scheme, using the Pariser-Parr approximations, has been carried out for the ions [CrO4]-- and [MnO4]-. The necessary integrals have been evaluated semi-empirically. For a more exact evaluation of the spectral transitions, a configuration interaction calculation, extended to the most important mono-excited configurations, has been performed. The order of levels previously suggested by Ballhausen and Liehr and other authors is confirmed by the results of the calculations. A new spectral assignment is presented, attributing to each observed absorption band two electronic transitions occurring between one of the occupied M.O.s (t 1, 2t 2, 1t 2) and the two lowest empty M.O.s (2e, 3t 2).

Optical transition intensities of trivalent lanthanide ions in zinc and lead metaphosphate glasses

Abstract The Judd-Ofelt intensity parameters of the lanthanide ions Pr 3+ , Sm 3+ , Dy 3+ , Ho 3+ , Er 3+ and Tm 3+ in Zn(PO 3 ) 2 and Pb(PO 3 ) 2 glasses were evaluated from the absorption spectra. With the exception of Pr 3+ , the ω 2 parameter is significantly lower in Pb(PO 3 )2 than in Zn(PO 3 ) 2 , as previously found for the Nd 3+ ion, due to a weaker mixing with the opposite parity configurations. The monotonic dependence of ω 6 , on the number of f electrons suggests that the mechanism responsible for the spectral intensities is mainly due to static distortions. The intensity parameters are in general higher than for other common oxide glasses.

Indirect assignment of the infrared spectrum of K2ReF6

Abstract The infrared (IR) spectrum of K 2 ReF 6 was recorded at room temperature and 10 K. The spectral data confirm the presence of a trigonal distortion acting on the ReF 6 2− anion. By means of the analysis of the hot bands present in the polarized electronic absorption spectra measured at different temperatures, it was possible to unequivocally assign the symmetries of the internal and lattice modes active in the IR spectrum.

A study of the absorption and emission of U(VI) in some glassy matrices

Abstract The absorption and emission spectra of xSiO 2 /yNa 2 O/zPbO glasses doped with U(VI) show the influence of the lead oxide on the formation of the UO 2 2+ ion, that can exist only when PbO is absent or present at most in a mole percentage about 29%. This is due to a destruction of the random three-dimensional network by PbO and to the consequent steric impossibility for the hexavalent uranium to form the UO 2 2+ linear ion.

Vibrational dispersion in ground and excited states: the Γ 7(2T2g)⇐Γ8(4A2g) absorption and luminescence spectra of crystalline K2ReF6 at 4 K

Abstract Trigonal crystals of K 2 ReF 6 show intense Γ 7 ( 2 T 2g → Γ 8 ( 4 A 2g ) luminescence at 4 and 77 K under 514 nm excitation. Extensive vibronic structure is resolved which is dominated by the vibronic origins due to the trigonal components of the v 6 , v 4 and v 3 modes of the ReF 2− 6 ion. The vibrational dispersions of v 3 and v 4 are significantly greater in the ground state than in the excited states and the cause of this is discussed.