Frank Magnotta

INFRARED LASER MULTIPLE-PHOTON DISSOCIATION OF CDCl3, IN A MOLECULAR BEAM

Pulsed CO2 laser multiple‐photon dissociation of CDCl3 was examined in a molecular beam using 11 μ radiation. The only observed dissociation channel was hydrogen chloride elimination (≳99.1%), with no evidence of simple chlorine atom cleavage (<0.9%). Implications for isotope separation are discussed.

Infrared laser multiple‐photon dissociation of CTCl3: Wavelength dependence, collisional effects, and tritium/deuterium isotope selectivity

Isotopically selective infrared multiple‐photon dissociation of CTCl3 in CDCl3 was investigated using a pulsed CO2 laser‐pumped NH3 laser in the 12 μm region. The wavelength dependence of CTCl3 MPD in the ν4 absorption band was studied and the observed MPD peak was found to be shifted ∼10 cm−1 to the red of the absorption peak at 835 cm−1, consistent with the anharmonicity of the CTCl3 ν4 mode. The CTCl3 dissociation probability per pulse was investigated under various experimental conditions, including laser energy, pulse duration, and CDCl3 and buffer gas pressure. A rate equation model was used to analyze the CTCl3 experiments assuming various multiple‐photon absorption and collisional relaxation mechanisms; this modeling effort was extended to a brief, more general, investigation of multiple‐photon absorption by molecules with prespecified dissociation probability profiles. Lower limit single step T/D enrichment factors in chloroform MPD exceeding 15 000 were achieved under optimized conditions of the...

Highly selective tritium-from-deuterium isotope separation by pulsed NH3 laser multiple-photon dissociation of chloroform

Abstract Infrared multiple-photon dissociation of CTCl 3 was investigated using a pulsed CO 2 laser-pumped NH 3 laser at 12.08 μm. No evidence of any CDCl 3 depletion or decomposition product was observed in photolyzed CTCl 3 /CDCl 3 mixtures. A lower limit of the single-step T/D enrichment factor, β, was found to be ≈ 165, based on the sensitivity in measuring CDCl 3 depletion. The low-fluene CTCl 3 /CDCl 3 optical selectivity in absorptions is > 9000 at the 835 cm −1 CTCl 3 ν 4 peak.

Ge-Si alloy microstructure fabrication by direct-laser writing with analysis by Raman microprobe spectroscopy

Micron‐dimension structures of germanium‐silicon alloys of various compositions are formed by several direct‐laser writing techniques, including pyrolytic deposition of silicon from silane on laser‐melted germanium substrates and codeposition from silane/germane mixtures on different absorbing substrates. In situ composition analysis of these polycrystalline alloy microstructures is performed by Raman microprobe analysis. The measured Raman shifts and widths of the laser‐deposited alloys are found to be in better agreement with published Raman data on germanium‐silicon alloy films than with the Raman data on alloy bulk solids. In the codeposition of alloys, the decomposition of germane to form Ge is observed to be about six times faster than the decomposition of silane to form Si, independent of the silane/germane ratio, laser power, and substrate type.

Raman microprobe analysis during the direct laser writing of silicon microstructures

Raman microprobe techniques are used for the first time as a real‐time probe during local direct laser writing and also as an in situ probe after writing. The Stokes–Raman emission observed during pyrolytic deposition of micron‐dimension structures of silicon on germanium and vitreous carbon substrates is found to be weaker, more asymmetric, and to peak at a smaller Raman shift than the corresponding spectrum of the same structure similarly probed in situ after deposition. Results of detailed post‐deposition Raman analysis of these silicon microstructures are presented and compared to the Raman spectra of oven‐heated silicon. Potential applications of these techniques are discussed.

Direct‐laser writing of silicon microstructures: Raman microprobe diagnostics and modeling of the nucleation phase of deposition

Two topics vital to the analysis and applications of direct‐laser writing of silicon microstructures are addressed here: optical diagnostics and modeling. The use of real‐time and postprocessing Raman microprobe analysis of laser‐written doped polysilicon microstructures on CMOS gate arrays is shown to be a useful diagnostic tool. Also, the results of a Monte Carlo simulation of adatom migration and desorption are applied to the study of nucleation effects in direct‐laser writing, with specific reference to the deposition of silicon.

Cross-dispersion infrared spectrometry (CDIRS) for remote chemical sensing

The advent of high performance infrared detector arrays together with recent advances in coarse grating fabrication now makes possible the design and fabrication of infrared spectrometers that cover the mid-IR atmospheric windows at high resolution with no moving components. For applications involving small sources at large distances this instrument approach can provide a substantial increase in sensitivity over Fourier transform spectrometers at the same resolution and spectral coverage. We describe the design evolution of the LLNL cross dispersive infrared spectrometer (CDIRS) remote chemical sensors operating in the two atmospheric windows from 2.3 microns to 4.2 microns. The spectral and mechanical performance of the first generation 1 meter focal length prototype echelle grating spectrometer (EGS) is presented. A second generation cryogenic (150 K) spectrometer (mini-EGS) utilizes a high dispersion silicon immersion grating to provide a compact design. This instrument will be flown in the winter of 1995 as part of the instrumentation suite for the DOE airborne multisensor pod system (AMPS) effluent research program.

Midwave infrared DIAL noise phenomenology

LLNL has utilized optical parametric oscillator technology to develop and field a rapidly-tunable mid-wave IR DIAL system. The system can be tuned at up to 1 KHz over the 3.3- 3.8 micron spectral region, where hydrogen-bond stretching modes provide spectroscopic signatures for a wide variety of chemicals. We have fielded the DIAL system on the LLNL site on range, turbulence, and receiver aperture size. In this paper we describe the interplay of turbulence and speckle to produce the observed nose fluctuations at short range.

Tritium removal from contaminated water via infrared laser multiple-photon dissociation

Isotope separation by means of infrared-laser multiple-photon dissociation offers an efficient way to recover tritium from contaminated light or heavy water found in fission and fusion reactors. For tritium recovery from heavy water, chemical exchange of tritium into deuterated chloroform is followed by selective laser dissociation of tritiated chloroform and removal of the tritiated photoproduct, TCl. The single-step separation factor is at least 2700 and is probably greater than 5000. Here we present a description of the tritium recovery process, along with recent accomplishments in photochemical studies and engineering analysis of a recovery system.