Gary L. Loper

Water-vapor continuum CO 2 laser absorption spectra between 27°C and −10°C

Water continuum CO2 laser absorption spectra are reported for temperatures between 27 and −10°C. The continuum is found to possess a negative temperature coefficient. The results obtained suggest that the magnitude of this temperature coefficient increases with increasing water pressure and decreasing temperature. The temperature coefficients between 27 and 10°C for air mixtures containing 3.0- and 7.5-Torr water vapor are −2.0 ± 0.4 and −2.9 ± 0.5%/°C, respectively. For mixtures with 3.0-Torr water the 10–0°C temperature coefficient is −7.7 ± 0.2%/°C. The temperature and water pressure dependencies observed for the continuum suggest that while both collisional broadening and water dimer mechanisms contribute to the continuum, the dimer mechanism is more important over this temperature range.

Carbon dioxide laser absorption spectra and low ppb photoacoustic detection of hydrazine fuels

Absorption cross-section data are reported for the toxic rocket fuels hydrazine, monomethylhydrazine, and unsymmetrical dimethylhydrazine (UDMH), as well as for their selected air oxidation products dimethylamine, trimethylamine, and methanol at up to seventy-eight CO(2) laser wavelengths each. These data are important for the assessment of the capability of CO(2) laser-based spectroscopic techniques for monitoring low levels of hydrazine-fuel vapors in the ambient air. Interference-free detection sensitivities of <30 ppb have been demonstrated for UDMH using a laboratory photoacoustic detection system.

Carbon dioxide laser absorption spectra of toxic industrial compounds

CO(2) laser absorption cross-section data are reported for acrolein, styrene, ethyl acrylate, trichloroethylene, vinyl bromide, and vinylidene chloride. These data indicate that sub parts per billion level, interference-free detection limits should be possible for these compounds by the CO(2) laser photoacoustic technique. Photoacoustic detectabilities below 40 ppb should be possible for these compounds in the presence of ambient air concentrations of water vapor and other anticipated interferences. These compounds are also found not to be important interferences in the detection of toxic hydrazine-based rocket fuels by CO(2) laser spectroscopic techniques.

Biodetection using fluorescent quantum dots

Multi-pathogen biosensors that take advantage of sandwich immunoassay detection schemes and utilize conventional fluorescent dye reporter molecules are difficult to make into extremely compact and autonomous packages. The development of a multi-pathogen, immunoassay-based, fiber optic detector that utilizes varying sized fluorescent semiconductor quantum dots (QDs) as the reporter labels has the potential to overcome these problems. In order to develop such a quantum dot-based biosensor, it is essential to demonstrate that QDs can be attached to antibody proteins, such that the specificity of the antibody is maintained. We have been involved in efforts to develop a reproducible method for attaching QDs to antibodies for use in biodetection applications. We have synthesized CdSe/ZnS core-shell QDs of differing size, functionalized their surfaces with several types of organic groups for water solubility, and covalently attached these functionalized QDs to rabbit anti-ovalbumin antibody protein. We also demonstrated that these labeled antibodies exhibit selective binding to ovalbumin antigen. We characterized the QDs at each step in the overall synthesis by UV-VIS absorption spectroscopy and by picosecond (psec) transient photoluminescence (TPL) spectroscopy. TPL spectroscopy measurements indicate that QD lifetime depends on the size of the QD, the intensity of the optical excitation source, and whether or not they are functionalized and conjugated to antibodies. We describe details of these experiments and discuss the impact of our results on our biosensor development program.

Use of Photoluminescence to Investigate Apparent Suicides by Firearms

A photoluminescence technique, which detects lead and antimony in gunshot residue, was evaluated for use in investigations of apparent gunshot suicides. The study was conducted in conjunction with forensic science laboratories in five U.S. cities. Samples were collected by the adhesive lift method from the backs of the hands of 67 gunshot suicide victims, 41 subjects who died of other causes, and 31 live subjects with high occupational exposure to lead and antimony. Tentative simultaneous threshold levels of 0.85 microgram for lead and 0.01 microgram for antimony were selected as criteria for presuming the presence of gunshot residue on samples from the hands of these suicide victims. Although blood decreases the detectability of lead and antimony in the samples, 48% of the suicide cases involving handguns other than .22 revolvers exceeded the threshold levels for both lead and antimony. Methods are suggested for eliminating the effects of blood, which should significantly increase the success rate for cases involving these guns. A much lower success rate was obtained for cases involving .22 revolvers and long guns, as expected from the sparse amounts of residue found in previous test firings of these guns.

Submicrometer Linewidth Production On Integrated Circuit Materials By Uv Laser Radical Etching

We have developed ultraviolet laser-induced, radical-etching processes that can provide practical etch rates and selectivities for most of the important substrate combinations used in silicon microelectronic devices. These processes have been demonstrated, in simple proximity and projection exposure experiments, to produce etch features on surfaces with dimensions of a few tenths of a micrometer. The technique could significantly simplify the fabrication of submicrometer linewidth devices by eliminating photoresist patterning and acid or plasma etching processing steps.

Absorption Spectra Of Toxic Compounds At CO2 Laser Wavelengths

Absorption coefficient data are reported for acrolein, styrene, ethyl acrylate, trichloroethylene, vinyl bromide, and vinylidene chloride at up to seventy-two CO2 laser wavelengths each. These compounds are toxic industrial substances for which improved ambient air detection methods are desired. Absorption data for these compounds are needed to determine their detectabilities by CO2 laser-based photoacoustic, long-path absorption, and laser radar (lidar) techniques. The absorption data obtained for these compounds indicate that sub parts-per-billion (ppb) level, interference-free detection limits should be possible for these compounds by the CO2 laser photoacoustic technique. CO2 laser photoacoustic detectabilities of 40 ppb or less should be possible for these compounds in the presence of expected ambient air concentrations of water vapor and other anticipated interferences. In addition, absorption data on the first four compounds are needed to assess the capability of using CO2 laser spectroscopic techniques to detect low levels of the toxic hydrazine-based rocket fuels in air samples containing these compounds as interferences. The absorption data obtained for these four compounds indicate that the hydrazine-fuels should be detectable by the CO2 laser photoacoustic technique at concentrations below proposed workplace standards for hydrazines as low as 30 ppb in the presence of expected airborne concentrations of these compounds together with other expected interferences.

Fluorine Atom Production Mechanisms From COF2 and NF3 in Uv Laser Etching of Poly-Silicon and Molybdenum

Ultraviolet laser-induced, radical-etching processes developed by us can provide practical etch rates and selectivities for most of the important film layer combinations used in silicon microelectronic devices. These processes have been demonstrated in simple proximity and projection exposure experiments to produce etch features on surfaces with dimensions of a few tenths of a micrometer. Mechanistic studies suggest that, in our etching processes for polysilicon and molybdenum, fluorine atoms responsible for etching are primarily produced from the precursors COF 2 and NF 3 on the surface rather than in the gas phase. The predominant production process appears to be photodecomposition of surface adsorbed precursor. Contributions due to precursor pyrolysis or precursor reaction with photogenerated charge carriers are found to be unimportant.