Effects of Plume Hydrodynamics and Oxidation on the Composition of a Condensing Laser-Induced Plasma.

High-temperature chemistry in laser ablation plumes leads to vapor-phase speciation, which can induce chemical fractionation during condensation. Using emission spectroscopy acquired after ablation of a SrZrO target, we have experimentally observed the formation of multiple molecular species (ZrO and SrO) as a function of time as the laser ablation plume evolves. Although the stable oxides SrO and ZrO are both refractory, we observed emission from the ZrO intermediate at earlier times than SrO.

A combined theoretical and experimental investigation of uranium dioxide under high static pressure.

We have investigated the behavior of uranium dioxide (UO2) under high static pressure using a combination of experimental and theoretical techniques. We have made Raman spectroscopic measurements up to 87 GPa, electrical transport measurements up to 50 GPa from 10 K to room temperature, and optical transmission measurements up to 28 GPa. We have also carried out theoretical calculations within the GGA + U framework.

H2O outgassing from silica-filled polysiloxane TR55.

Temperature-programmed desorption/decomposition (TPD) was employed to obtain the moisture content and outgassing kinetics of TR55, a silica-filled cross-linked polysiloxane. The total moisture content of TR55 in the as-received state and after 20-30 min of vacuum pumping in the load-lock prior to TPD was measured to be on the order of 0.35 wt%.

Laser-induced damage in deuterated potassium dihydrogen phosphate.

Laser-induced pinpoint bulk damage of deuterated potassium dihydrogen phosphate at 351 nm is shown to depend on the propagation direction relative to the crystallographic axes and on growth temperature in addition to the previously reported dependence on continuous filtration. Pulse-length scaling is also consistent with earlier reports. The leading hypothesis for the cause of pinpoint damage is absorbing nanoparticle impurities, and our results are consistent with but not conclusive for that model.

Viscoelastic properties of demineralized human dentin measured in water with atomic force microscope (AFM)-based indentation.

Using an atomic force microscope (AFM) with an attachment specifically designed for indentation, we measured the mechanical properties of demineralized human dentin under three conditions: in water, in air after desiccation, and in water after rehydration. The static elastic modulus (E(h)r = 134 kPa) and viscoelastic responses (tau(epsilon) = 5.1 s and tau(sigma) = 6.

Atomic force microscopy of mammalian sperm chromatin.

We have used the atomic force microscope (AFM) to image the surfaces of intact bull, mouse and rat sperm chromatin and partially decondensed mouse sperm chromatin attached to coverglass. High resolution AFM imaging was performed in air and saline using uncoated, unfixed and unstained chromatin. Images of the surfaces of intact chromatin from all three species and of an AFM-dissected bull sperm nucleus have revealed that the DNA is organized into large nodular subunits, which vary in diameter between 50 and 100 nm.

Analysis of synthetic DNAs and DNA-protamine complexes with the scanning tunneling microscope.

Three duplex DNAs 22, 47, and 100 base-pairs in length have been imaged with the scanning tunneling microscope (STM) after deposition on highly oriented pyrolytic graphite (HOPG). Images of the 47 base-pair (bp) molecules are resolved sufficiently to identify the two phosphodiester strands, the direction of helical coiling (this molecule contains three turns of left-handed helix), and single-stranded ends. Length measurements indicate that all three DNA sequences have adopted an "A-like" conformation.

Tip-radius-induced artifacts in AFM images of protamine-complexed DNA fibers.

Isolated DNA fibers complexed with protamine (the chromosomal protein that packages DNA in mammalian sperm) have been produced by partially decondensing the highly compacted mouse sperm chromatin particle on a glass coverslip. These DNA fibers were then scanned with the atomic force microscope (AFM). While the smallest of the fibers appear in AFM images as ribbon-like structures 250-350 A wide and 10-25 A high, experiments indicate that these images are the result of a convolution of the imaging-tip's shape with the object's actual shape.

Scanning tunneling microscope images of adenine and thymine at atomic resolution.

The scanning tunneling microscope has been used to obtain images of DNA that reveal its major and minor grooves and the direction of helical coiling, but sufficient resolution has not yet been achieved to identify its bases. To determine if this technology is capable of identifying individual DNA bases, we have examined the molecular arrangements of adenine and thymine attached to the basal plane of highly oriented pyrolytic graphite. Both molecules form highly organized lattices following deposition on heated graphite.

Direct observation of native DNA structures with the scanning tunneling microscope.

Uncoated double-stranded DNA dissolved in a salt solution was deposited on graphite and imaged in air with the scanning tunneling microscope (STM). The resolution was such that the major and minor grooves could be distinguished. The pitch of the helix varied between 27 and 63 angstroms in the images obtained.