The overall rate constants (k,) for the quenching of a number of aromatic hydrocarbon and ketone triplets by di-tert-butyl nitroxide (DTBN) in fluid solution have been measured. The values of k , decrease in ( I ) Part I: R.

The third-order nonlinear optical (NLO) response of colloidal “quantum-dot” cadmium sulfide (CdS) nanocrystals has been measured both in solution and in Langmuir−Blodgett (L−B) multilayers of a polydiacetylene (PDA) “alloy” NLO polymer. The PDA “alloy” was composed of a tailored mixture of diacetylene monomers having carboxylic acid “head” groups and benzamide “head” groups, in each case separated from the diacetylene moiety by an eight-carbon spacer group, so as to disrupt the strong aggregation, leading to excessive light scattering, that normally occurs with L−B films of carboxyl-terminated PDAs. It is shown that at 530 nm the nonlinear refractive index (n 2) of a nanocomposite PDA film containing thiophenol-capped CdS nanocrystals, as measured by both Z-scan and degenerate four-wave mixing (DFWM) techniques, is 11 × 10-8 cm2/MW, while that of an undoped PDA film is only 3 × 10-8 cm2/MW. This demonstrates the concept of a “nonlinear−nonlinear” optical material, in which the NLO response of an embedded quantum-dot semiconductor can add constructively to that of a NLO polymer having complementary wavelength characteristics.

Full experimental and kinetic details will be published in the full paper.(10) Obtained from Diaprep, Inc., in purity satisfactory for direct use.

The extended x-ray absorption fine structure (EXAFS) spectrum of aluminum has been measured with a nanosecond pulse of soft x-rays generated by a laser-produced plasma. This technique provides a practical alternative to synchrotorn radiation for the acquisition of EXAFS data. It also provides a unique capability for the analysis of molecular structure in highly transient chemical species.

The polydiacetylenes have received much attention in recent years because of their large non -resonant third -order nonlinear optical response and because of the flexibility they offer in designing tailored molecular structures for specific applications. However, the fabrication of these materials into devicequality thin films has proven to be difficult, largely because of their propensity for forming highly scattering polycrystalline solids (with the exception of the few soluble polydiacetylenes, such as 3-or 4 -BCMU, which can be spin-coated by conventional techniques, albeit in the less-active yellow form of the polymer).We have addressed this problem by developing a family of polydiacetylene "alloys ": polymers derived from mixtures of similar diacetylene monomers having identical positioning of the diacetylene unit within the hydrocarbon chain, but with different "head" groups that provide varying degrees of affinity to water. The rationale here has been to design mixtures that would form a uniform, quasiamorphous monolayer on the water surface in Langmuir-Blodgett multilayer fabrication, and thus avoid the scattering losses associated with grain boundaries that form within the monolayer on the water surface. To date, multilayer films up to 0.4 um thick have been fabricated by this approach, and photolithographic techniques have been developed for patterning strips down to 4 um wide in the films, without converting the blue form of the polydiacetylene to the red form in the process. Moreover, high quality films of polydiacetylene have been deposited successfully onto curved surfaces, such as polycarbonate lens blanks, using these compositions.