No abstract

Glasses of composition (x − 1)PbOؒ(100 − x)GeO 2 ؒ1Ln 2 O 3 , with x = 30 mol% (Ln = Nd, Eu, Er), 40 mol% (Ln = Pr, Nd, Sm, Eu, Dy, Ho, Er, Tm), and 50 mol% (Ln = Eu, Er), have been prepared by quenching the oxidic melts. From the optical absorption and emission spectra in the ultravioletvisible-near-infrared (UV-VIS-NIR) region, the intensity parameters, spontaneous emission probabilities, branching ratios, radiative lifetimes, and, for selected NIR transitions, peak stimulated emission cross sections have been obtained. The trends observed in the intensity parameters have been discussed, as a function of the number of f electrons as well as a function of the lead content. As the amount of lead increases, the covalency of the Ln−O bond increases, the symmetry of the rare-earth site increases, and the dopant site distribution narrows. The peak stimulated emission cross sections rank among the highest found for oxide glasses.

As long as product molecules of 2 are not yet aligned in their own crystal lattice, hut in the rebuilt lattice of 1, short-wavelength excitation can cause a photochemical reverse reaction. Only crystalline 2 photodimerizes exclusively to form the cyclobutane derivative [9].The lack of absorption of light at 2. = 365 nm by the crystalline product 2 prevents the formation of its [2 +2]-dimer. Irradiation of 1 (4 h) with light from a 150 W Hanau Hg high-pressure lamp passed through a Wertheimer UVW-55 bandpass filter (isolation of the 365nm light with 2.8% contribution from 334 nm light ( % transmission <1 at 315 nm and at 410 nm)) in a powder cuvette leads to a considerable shift of the diffuse reflectance spectrum to curve bin Figure I. whereas the broad-band irradiation (>. > 300 nm) ofcrystalline 2 produces a decrease in the long-wavelength absorption and an increase in a band of the photodimer 191 at 266 nm.

No abstract

No abstract