Frequency dispersion of sound propagation in Rouse polymer melts via generalized dynamic random phase approximation.

An extended generalization of the dynamic random phase approximation (DRPA) for L-component polymer systems is presented. Unlike the original version of the DRPA, which relates the (LxL) matrices of the collective density-density time correlation functions and the corresponding susceptibilities of concentrated polymer systems to those of the tracer macromolecules and so-called broken-links system (BLS), our generalized DRPA solves this problem for the (5xL) x (5xL) matrices of the coupled susceptibilities and time correlation functions of the component number, kinetic energy and flux densities. The presented technique is used to study propagation of sound and dynamic form-factor in disentangled (Rouse) monodisperse homopolymer melt.

Laser detection of the rare isotope (3)He at concentrations as low as 10(-9).

The method of collinear laser photoionization of atoms in a modulated fast beam is used to detect the rare isotope (3)He, with high-repetition-rate lasers being applied to improve the detection sensitivity. The method has made it possible to detect (3)He at relative abundances as low as 10(-9).