Daniel A. Hartzler

As global demand for rare earth elements (REEs) increases, maintaining the production and supply chain is critical. Technologies capable of being used in the field and in situ in the subsurface for rapid REE detection and quantification facilitates the efficient mining of known resources and exploration of new and unconventional resources. Laser-induced breakdown spectroscopy (LIBS) is a promising technique for rapid elemental analysis both in the laboratory and in the field. Multiple articles have been published evaluating LIBS for detection and quantification of REEs; however, REEs in their natural deposits have not been adequately studied. In this work, detection and quantification of two REEs, La and Nd, have been studied in both synthetic and natural mineral matrices at concentrations relevant to REE extraction. Measurements were performed on REE-containing rock and coal samples (natural and synthetic) utilizing different LIBS instruments and techniques, specifically a commercial benchtop instrument, a custom benchtop instrument (single- and double-pulse modes), and a custom LIBS probe currently being developed for in situ, subsurface, borehole wall detection and quantification of REEs. Plasma expansion, emission intensity, detection limits, and double-pulse signal enhancement were studied. The limits of detection (LOD) were found to be 10/14 ppm for La and 15/25 ppm for Nd in simulated coal/rock matrices in single-pulse mode. Signal enhancement of 3.5 to 6-fold was obtained with double-pulse mode as compared to single-pulse operation.

Localized operating conditions inside boilers, heat recovery steam generators, or other large thermal systems have a huge impact on the efficiency, environmental performance, and lifetime of components. It is extremely difficult to measure species accurately within these systems due to the high temperatures and harsh environments, locally oxidizing or reducing atmospheres, ash, other particulates, and other damaging chemical species. Physical probes quickly suffer damage and are rendered nonfunctional. This work has attempted to adapt the measurement approach based on Scheimpflug light detection and ranging (S-LIDAR) for the remote sensing of gas species inside the high-temperature boiler environment. For a proof-of-concept, the detection of Raman signals of N₂, O₂, and CO₂ and their behavior with increasing temperature have been presented.

Sub-surface activity such as geologic carbon sequestration (GCS) has the potential to contaminate groundwater sources with dissolved metals originating from sub-surface brines or leaching of formation rock. Therefore, a Laser Induced Breakdown Spectroscopy (LIBS) based sensor is developed for sub-surface water quality monitoring. The sensor head is built using a low cost passively Q-switched (PQSW) laser and is fiber coupled to a pump laser and a gated spectrometer. The prototype sensor head was constructed using off the shelf components and a custom monolithic, PQSW laser and testing has verified that the fiber coupled design performs as desired. The system shows good calibration linearity for tested elements (Ca, Sr, and K), quick data collection times, and Limits of Detection (LODs) that are comparable to or better than those of table top, actively Q-switched systems. The fiber coupled design gives the ability to separate the PQSW LIBS excitation laser from the pump source and spectrometer, allowing these expensive and fragile components to remain at the surface while only the low-cost, all optical sensor head needs to be exposed to the hostile downhole environment.

A prototype laser-induced breakdown spectroscopy (LIBS) sensor is tested for the determination of rare earth elements (Eu and Yb) in liquid and solid samples. The sensor head, built using a monolithic passively Q-switched (PQSW) Nd:YAG laser, produced a 1064 nm laser beam with ns pulses and an energy of 4.2 mJ. The measurements show good calibration linearity for both Eu and Yb with R2 values above 0.99 for all analyzed spectral lines in liquid and solid samples. Limits of detection (LODs) obtained were as low as 1 ppm, which are comparable to or better than those reported previously by using table top actively Q-switched systems. This study aims to develop a high sensitivity, field deployable sensor for characterizing existing and new sources of rare earth elements.

X-ray free electron lasers (XFELs) provide ultrashort intense X-ray pulses suitable to probe electron dynamics but can also induce a multitude of nonlinear excitation processes. These affect spectroscopic measurements and interpretation, particularly for upcoming brighter XFELs. Here we identify and discuss the limits to observing classical spectroscopy, where only one photon is absorbed per atom for a Mn²⁺ in a light element (O, C, H) environment. X-ray emission spectroscopy (XES) with different incident photon energies, pulse intensities, and pulse durations is presented. A rate equation model based on sequential ionization and relaxation events is used to calculate populations of multiply ionized states during a single pulse and to explain the observed X-ray induced spectral lines shifts. This model provides easy estimation of spectral shifts, which is essential for experimental designs at XFELs and illustrates that shorter X-ray pulses will not overcome sequential ionization but can reduce electron cascade effects.

Artificial Photosynthesis carries promise to deliver abundant clean energy for the needs of a growing population. Deep mechanistic understanding is required to achieve rational design of fast and durable water oxidation catalysts. Here we provided first evidences for a new mechanism of the O-O bond formation via radical coupling of the oxidized metal=oxo of radicaloid character (RuIV=O) and ligand based radical ([ligand-NO]+• cation radical). O-O bond formation is facilitated via spin alignment and takes place via a virtually barrier less pathway inside the single metal complex. In situ reactive intermediate conversion was monitored by mass spectroscopy, resonance Raman (RR) and EPR. Computational analysis have shown that the formation of [ligand-NO]+• happens at a lower overpotential than the formation of the [RuV=O(ligand)]3+ intermediate. Overall, the presented paradigm for O-O bond formation opens new opportunities for rational catalyst design.