Laser Ablation Millimeter-Wave Instrumentation for in Situ Exploration of the Solar System
Raymond, Alexander W.
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CitationRaymond, Alexander W. 2018. Laser Ablation Millimeter-Wave Instrumentation for in Situ Exploration of the Solar System. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
AbstractDetermining the chemical makeup of different sites throughout the solar system is an on-going endeavor. The major portion of the thesis relates to instrumentation for determining molecular composition, but we begin on a purely scientific topic with the discussion of a numerical model of the pulsed glow discharge plasma in a laboratory experiment intended to mimic the chemistry of the upper atmosphere of Saturn's moon Titan. Titan's upper atmosphere contains rich organic chemistry, which is the progenitor to the aerosol haze that blankets the surface. The experiment, known as the Titan Haze Simulation (THS), is being conducted at NASA Ames Research Center. The model accomplishes two things. First, it identifies some of the reaction pathways that lead to the positive ions that have been detected using the THS time-of-flight mass spectrometer. Second, it compares the electron density and temperature of the laboratory analog with those measured by the Casinni spacecraft at Titan and establishes that the plasma environments are similar in important ways.
The next chapter of the thesis presents a detailed characterization of the Fabry-Perot cavity, which is a key component in a new miniature rotational spectrometer being developed by the Spectroscopy Lab at the Jet Propulsion Laboratory. The spectrometer, known as Spec-Chip, operates at millimeter-wave frequencies around 100 GHz and is capable of detecting the rotational transitions of polar molecules. The strength of the rotational spectroscopy approach is that it is highly specific: the spatial arrangement of the atoms in a molecule lead to a unique set of transitions that allow for the precise determination of sample makeup. We map the cavity modes, measure the quality factor, and document the detection of a new gas. The Spec-Chip is intended for deployment on space missions to measure the atmospheric or outgassing composition of planetary bodies or comets.
Spec-Chip is meant for measuring gas-phase composition; however, to expand the number of possible sites to which it might be deployed, we consider the possibility of using it to identify the volatilized products of pulsed laser ablation. Before testing the instrument directly, we describe an experiment using a laboratory-scale rotational spectrometer operated by the McCarthy Lab at the Harvard-Smithsonian Center for Astrophysics, which addresses the question: what are the conditions inside the plume created by the nanosecond ablation of a molecular solid? When volatilized by a laser, the molecular bonds of the analyte species can break to form fragments; a process that can complicate interpretation of the rotational spectrum. The nanosecond ablation of mixtures of alanine amino acid and copper powder produce several neutral molecular fragments that we detect in comparable abundance to the parent molecules. To help explain the cause of the fragmentation and identify the spatio-temporal origin of the volatilize analyte molecules, we do pump-probe shadowgraphy of the expanding ablation plume. We find that the ablation produces a shockwave moving at 5 to 15 km/s depending on the precise laser and atmospheric conditions. Using that speed, we infer the initial conditions of the laser-vaporized material and show that the unfragmented parent molecules probably originate from material in the superheated liquid phase some time after the ablation plume has substantially cooled.
In the final part of the thesis, we demonstrate that Spec-Chip can be coupled to a laser-ablation source to detect neutral gas-phase molecules. Rotational transitions of two salts, NaCl and KCl, are measured using Spec-Chip. As with the Cu/alanine ablation in Chapter 4, we find that a significant amount of the salt is ejected in the form of microparticles, which, if fully vaporized, opens the possibility of improving the volatilizaiton yield and the instrument sensitivity along with it. Mass spectrometry measurements of ablated salts with nanosecond and femtosecond pulses show that dissociation occurs in these materials.
Citable link to this pagehttp://nrs.harvard.edu/urn-3:HUL.InstRepos:42015042
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