Polarizable Clusters Research Articles

Semiclassical Green function calculation of four wave mixing in polarizable clusters and liquids

A theory for the nonlinear optical response of an assembly of four level atoms representing an s–p transition, with dipole–dipole interactions is developed. Simulation results for two-pulse ultrafast four wave mixing measurements in atomic clusters and in liquids demonstrate that the electronic density of states can be obtained by combining absorption and four wave mixing experiments. We show that a sensitive direct probe for interatomic interactions and correlations is provided by reversing the pulse time ordering in a photon echo sequence. The effect of nuclear motions on the linear absorption spectrum is investigated.

Defect clustering in double doped fluorite crystals with Lu and Gd

Abstract Solid solutions Ca1 x-yLuxGdy F2+x+y for 10−4 ≤ x ≤ 2 × 10−2 and y=0.0001 have been studied by electron paramagnetic resonance (EPR) and ionic thermal currents (ITC). It has been found that the ITC spectrum from 77 to 420 K is very weak and the main peak is attributed to the relaxation of both Lu3+-F− x and Gd3+F− i nn dipoles. No polarizable clusters are present in the temperature range explored here. The EPR spectra show the presence of Gd3+ tetragonal and cubic centers due to the local and non local compensation, respectively. The continuous decrease in the molar fraction of Gd3+ tetragonal centers together with the low concentration of Lu nn dipoles is an evidence of the existence at these low and intermediate concentrations of large clusters such as the cubo-octahedral hexamer which has been proposed for CaF2 crystals very highly doped with small trivalent cations.