The demonstration that quantitative and sensitive analysis can be carried out using surface enhanced resonance Raman scattering (SERRS) prompted a discussion and investigation of the main variables which are within the control of the analyst using colloidal silver as the substrate. Previous papers have dealt with the crucial need to obtain good chemisorption of the analyte to the surface and have reported the use of specially designed dyes for SERRS. One of the most variable processes is the aggregation of the colloid. Here, we investigate the addition of controlled amounts of an organic aggregating agent, poly-L-lysine, at concentrations which reduce the zeta potential in a controlled manner, thus aiding aggregation control. The relationship between the excitation frequency, the surface plasmon resonance frequency of the silver colloid and the frequency of the maximum absorbance of the molecular chromophore is studied using low concentrations of dye and no aggregating agent. Under these conditions, little to no aggregation is expected. The magnitude of the enhancement is strongly dependent on the frequency of the molecular chromophore as well as the plasmon resonance frequency. However, when sodium chloride is used to aggregate the colloid, a larger enhancement is obtained and the strong dependence on the molecular chromophore largely disappears. A much broader enhancement profile is obtained which appears to be related more to the specific enhancement processes caused by aggregation than the frequency of the chromophore. However, the total enhancement for SERRS is higher than for SERS thus indicating that the chromophore is still important to the process.

Age momentum correlation experiments in kapton has revealed the existence of two positron states corresponding to the trapping of positrons on the partially negatively charged O and/or N sites in the polymer chain. A young age broadening of the annihilation line shape is also observed and is attributed to the annihilation in flight of the positrons.

Positron annihilation lifetime spectroscopy measurements of polystyrene and poly(methyl methacrylate) have been performed at 20 K during $400 h. An increase of the probability of positronium (Ps) formation with irradiation time is observed. This effect is due to the accumulation of shallow-trapped electrons which are stable at low temperatures. Detailed analyses of the lifetime spectra within the blob model framework show that these electrons induced by the previous positron tracks act as extra blob electrons: they either recombine with a cation or form Ps. This kinetic analysis introduced by the blob model indicates that no formation of Ps occurs after the blob recombination in these amorphous polymers. An increase of the free positron lifetime with the measuring time is also observed. This effect can not be explained in terms of artifact of discrete analysis caused by the existence of an ortho-positronium lifetime distribution.