Rapid-phase modulation of terahertz radiation for high-speed terahertz imaging and spectroscopy

Rapid voltage-controlled phase modulation of cw terahertz (THz) radiation is demonstrated. By transmitting an infrared beam through a lithium niobate phase modulator the phase of the THz radiation, which is generated by the photomixing of two infrared beams, can be directly modulated through a 2pi phase shift. The 100 kHz modulation rate that is demonstrated with this technique is approximately 3 orders of magnitude faster than what can be achieved by mechanical scanning.     

Bimodal Nanoparticle Size Distributions Produced by Laser Ablation of Microparticles in Aerosols

Silver nanoparticles were produced by laser ablation of a continuously flowing aerosol of microparticles in nitrogen at varying laser fluences. Transmission electron micrographs were analyzed to determine the effect of laser fluence on the nanoparticle size distribution. These distributions exhibited bimodality with a large number of particles in a mode at small sizes (3–6-nm) and a second, less populated mode at larger sizes (11–16-nm). Both modes shifted to larger sizes with increasing laser fluence, with the small size mode shifting by 35% and the larger size mode by 25% over a fluence range of 0.3–4.2-J/cm². Size histograms for each mode were found to be well represented by log-normal distributions. The distribution of mass displayed a striking shift from the large to the small size mode with increasing laser fluence. These results are discussed in terms of a model of nanoparticle formation from two distinct laser–solid interactions. Initially, laser vaporization of material from the surface leads to condensation of nanoparticles in the ambient gas. Material evaporation occurs until the plasma breakdown threshold of the microparticles is reached, generating a shock wave that propagates through the remaining material. Rapid condensation of the vapor in the low-pressure region occurs behind the traveling shock wave. Measurement of particle size distributions versus gas pressure in the ablation region, as well as, versus microparticle feedstock size confirmed the assignment of the larger size mode to surface-vaporization and the smaller size mode to shock-formed nanoparticles.     

Transverse asymmetry AT' from the quasielastic 3He(e,e') process and the neutron magnetic form factor

We have measured the transverse asymmetry A(T') in 3He(e,e(')) quasielastic scattering in Hall A at Jefferson Laboratory with high precision for Q2 values from 0.1 to 0.6 (GeV/c)(2). The neutron magnetic form factor G(n)(M) was extracted based on Faddeev calculations for Q2 = 0.1 and 0.2 (GeV/c)(2) with an experimental uncertainty of less than 2%.     

Evaluation of chlorofusin, its seven chromophore diastereomers, and key analogues

Chlorofusin, its seven chromophore diastereomers, and key analogues were comparatively examined for inhibition of MDM2-p53 binding revealing that the chromophore, but not simple replacements, contributes significantly to the natural products properties, and that this contribution is independent of its relative and absolute stereochemistry.     

The Boolean algebra of the theory of linear orders

We characterize the isomorphism type of the Boolean algebra of sentences of the theory of linear orders. It is isomorphic to the sentence algebras of the theory of equivalence relations, the theory of permutations and the theory of well-orderings. This work was partially supported by the National Science Foundation under research grant MCS 76-07249.     

Virtual hydrocarbon and combinatorial databases for use with CAVEAT

Three new virtual databases have been developed for use with the bond-orientation-based database searching program CAVEAT. These consist of a database of trisubstituted monocyclic hydrocarbons having ethyl, vinyl, and phenyl substituents; a database of unsubstituted bicyclic hydrocarbons; and a database of core structures from established combinatorial synthetic methods having hydrogen, ethyl, vinyl, and phenyl substituents at the readily varied positions. Each collection of molecules was subjected to a batch conformational search, minimization, and conversion to a vector database for use with CAVEAT.     

Total synthesis and examination of three key analogues of ramoplanin: a lipoglycodepsipeptide with potent antibiotic activity

The total synthesis and evaluation of three key ramoplanin aglycon analogues are detailed. The first (5a) represents replacement of the labile depsipeptide ester with a stable amide (HAsn2 --> Dap2) with removal of the HAsn pendant carboxamide, and it was found to be slightly more potent than the natural aglycon in antimicrobial assays providing a new lead structure with an improved profile and a more stable and accessible macrocyclic template on which to conduct structure-function studies. In contrast, a second amide analogue 5b which contains a single additional methylene relative to 5a (HAsn2 --> Dab2) was found to be inactive in antimicrobial assays (>100-fold loss in activity). The third key analogue 5c in which the Asn1 lipid side chain was replaced with an acetyl group revealed that it contributes significantly to the antimicrobial activity (16-fold) of the ramoplanins, but is not essential.     

Design of a Thin Film Infrared Barcode on a Flexible Substrate

We report the design, fabrication and characterization of an infrared barcode. This barcode is composed of a bilayer of titanium and amorphous silicon on a flexible Kapton substrate. Information encoded in the barcode shows high contrast when viewed with an infrared imaging system in the 8 to 12 m spectral region. The barcode information is concealed under visible viewing conditions, i.e., the barcode appears as an untreated, uniform metal sheet to a detector of visible radiation (400 to 700nm).