Theses and Dissertations from UMD
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Item SPECTROSCOPY-BASED METHODS FOR BIOLOGICAL APPLICATIONS(2024) Zhou, Xuewen; Scarcelli, Giuliano; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Spectroscopy plays a crucial role in biological measurements by offering functional insights across various biological scales, from cellular to organ levels. It has become an important complement to imaging technologies. The thesis focuses on enhancing spectroscopy-based methods, addressing both technical advancements and application-oriented improvements. The first part of the research presents the development of standardized test methods for multispectral photoacoustic imaging (MPAI), a phantom-based test method specific for evaluating MPAI performance in breast cancer detection. This test method contributes to standardizing the evaluation of multispectral photoacoustic imaging across different research and clinical settings with better reliability and reproducibility. The next segment of the thesis introduces an innovative 2D-dispersion spectrometer, utilizing an etalon and grating setup. This instrument significantly improves the sensitivity in detecting Whispering Gallery Mode (WGM) microlasers by providing spectral resolution less than 0.3 pm. The enhanced sensitivity allows for the measurement of hyperfine refractive index and absorption changes in liquid environments, which expands the application limit of microlasers as biosensors. In the final part, the thesis extends the application of the 2D-dispersion spectrometer to detect Brillouin and low-frequency Raman signals. For the best of our knowledge, this is the first setup demonstrating simultaneous Brillouin and low-frequency Raman measurement, which can provide mechanical and chemical information of a sample such as the viscoelasticity and intermolecular dynamic.Item MEASUREMENTS OF AEROSOL PHYSICOCHEMICAL PROPERTIES(2022) Razafindrambinina, Patricia Nirina; Asa-Awuku, Akua A; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Tiny liquid or solid particles suspended in the air with sizes ranging between several nanometers to several microns, collectively referred to as aerosol particles, are ubiquitous in the atmosphere and have been shown to affect a planet’s radiative budget. Aerosol particles have the ability to directly reflect and absorb solar radiation leading to a cooling or heating the planet’s surface, respectively (aerosol direct effect). Aerosol particles can also indirectly affect the net radiative forcing through water uptake and cloud formation prior to reflecting and absorbing solar radiation (aerosol indirect effect). In my dissertation, I utilize lab-based measurements to measure the optical properties of mineral dust and Martian dust simulants, and quantify the water uptake and cloud condensation nuclei activity of secondary organic aerosols (SOA), and water-soluble organic compounds in various mixing states and relative humidities. This body of work provides directly-measured values that may reduce uncertainties in climate prediction when used as inputs in future climate and air quality models.Item TRANSIENT SPECTROSCOPY AND DYNAMICS OF OPTICALLY CENTRIFUGED MOLECULES(2020) Ogden, Hannah Marie; Mullin, Amy S; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation investigates the behavior of molecules with extreme amounts of rotational energy and oriented angular momentum. The molecules studied in this thesis are prepared in an optical centrifuge and are studied using high-resolution transient IR absorption spectroscopy. The optical centrifuge uses intense, shaped laser pulses to accelerate molecules angularly into extreme rotational states. Through the interaction with the field, anisotropic molecules are trapped and follow the field through its angular acceleration. The final angular frequency of the molecules depends on the spectral profile of the centrifuge. The result is an ensemble of molecules with highly oriented angular momentum and rotational energies far from that of equilibrium. High-resolution transient IR absorption spectroscopy is used to measure nascent distributions of optically centrifuged molecules, as well as the collisional dynamics of the centrifuged molecules as they relax toward equilibrium by means of a collisional cascade. These studies show that an optical centrifuge can launch N2O molecules into rotational states with J≥195, and that the mechanism for collisional energy transfer depends on the rotational energy and angular momentum of the molecules. We show that our optical centrifuge is capable of driving CO2 into rotational states up to J= 280, and we measure spectral perturbations that have not been observed previously. We measure the full rotational distribution of centrifuged CO and show that states up to J= 80 are populated using the full centrifuge bandwidth. This study of CO provides a lower limit to the final angular frequencies of N2O and CO2 prepared in our optical centrifuge. We have developed a tunable optical centrifuge to measure directly the nascent population distributions of centrifuged molecules, from which the capture and acceleration efficiencies for CO and CO2 are compared. Lastly, the effect of reactant rotational energy on bimolecular reactions is investigated. We control the rotational energy in CO reactants and measure the yield of C2 products. We find that CO rotational energy inhibits bimolecular reactions that form C2. This work provides new information about the properties and behavior of molecules in a previously inaccessible energy regime and lays the groundwork for future investigations.Item In Operando Mechanistic Studies of Heterogeneous Electrocatalysis on Solid Oxide Electrochemical Cell Materials(2017) Geller, Aaron; Eichhorn, Bryan W; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation details the development and utilization of in operando protocols for observing electrochemical reactions on solid oxide electrochemical cells (SOCs) in order to better understand the fundamental chemistry governing their operation. Two key reactions in SOC processes are studied using ambient pressure X-ray photoelectron spectroscopy (AP-XPS), the oxygen reduction and evolution reactions (ORR and OER). Measurements made on lanthanum strontium manganite (La1-xSrxMnO3±∂, LSM), a standard electrode material show that the surface composition does not match the bulk stoichiometry. Sr extrudes onto the LSM surface in the form of SrO and greater Mn reduction is observed. These phenomena are further augmented by application of a cathodic bias (promoting ORR), while an anodic bias (promoting OER) results in the oxidation of Mn and no significant changes in Sr segregation. Surface potentials on the LSM are measured to locate regions of electrochemical activity when promoting ORR and OER. These measurements yield in operando spectroscopic evidence that all electrochemical activity occurs at the electrode/electrolyte interface and that LSM is more electrocatalytically active toward ORR than OER. We further compare surfaces between a pure LSM material and a composite of LSM and yttria-stabilized zirconia ((ZrO2)1-2x(Y2O3)x, YSZ) in different gaseous environments which approximate standard operating conditions. The LSM/YSZ composite exhibits a larger concentration of surface oxygen vacancies in each environment allowing for greater oxygen reactivity. A method for measuring surface Co oxidation states with XPS is explored. In situ thermal redox studies on cathode material, lanthanum cobaltite (LaCoO3-∂), show a potential correlation between Co reduction and the Auger parameter. An in operando technique for monitoring SOCs with near infrared (NIR) imaging is presented. Ce oxidation states are tracked in an operating SOC using ceria (CeO2-x) electrodes in studies analogous with previous AP-XPS research. However, the NIR experiments take place in fully ambient conditions as opposed to the model, near ambient conditions used in the AP-XPS experiments. Ce reduction is observed within an electrochemically active region commensurate with that found with AP-XPS, simultaneously supporting the use of NIR imaging for in operando studies on these SOCs, and the model AP-XPS experiments previously conducted.Item ISOTOPE EFFECTS IN THE STATE-RESOLVED COLLISION DYNAMICS OF HIGHLY EXCITED MOLECULES(2014) Echebiri, Geraldine Onyinyechi; Mullin, Amy S; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The importance of highly excited molecules in the fields of combustion and atmospheric chemistry makes it essential to study pathways by which energy is lost from the excited molecule. One such pathway is by inelastic collisions with a bath molecule. In this dissertation, the collisional relaxation of highly excited pyrazine-h4 (Evib = 37900 cm-1) and pyrazine-d4 (Evib = 37900 cm-1) with HCl (300 K) is studied. The outcomes of the inelastic collision studies reveal quantum state-energy gaps of molecules and their intermolecular interactions affect the mechanism and dynamics of collisional energy transfer. The results from collisional relaxation of pyrazine-h4 (Evib = 37900 cm-1) with HCl were compared to those from collisional relaxation of pyrazine-h4 (Evib) with DCl in order to deduce the effects of quantum state-energy gaps on the dynamics of collisional energy transfer. The comparison shows the dynamics for collisional deactivation of pyrazine-h4 (Evib) with HCl and DCl are different, and are possibly due to their intermolecular interactions with pyrazine-h4 (Evib. The data for collisional relaxation of pyrazine-d4 (Evib = 37900 cm-1) with HCl were compared to those for pyrazine-h4 (Evib) + HCl collisions in order to determine the contributions of near-resonant vibrational energies of the collision partners on the collision dynamics. The comparison shows the energy transfer dynamics for collisional quenching of pyrazine-h4 (Evib) and pyrazine-d4 (Evib) with HCl are similar. The similarity in their energy transfer dynamics suggests near-resonance effects are not contributing significantly to the collision dynamics.Item Radiation Transport Measurements in Methanol Pool Fires with Fourier Transform Infrared Spectroscopy(2008) Yilmaz, Aykut; Jackson, Gregory S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Pool fires rely on radiation and conduction heat feedback from the combustion process to the liquid surface to vaporize the fuel. This coupled relationship determines the fuel burning rate and thus the fire structure and size. Radiative heat transfer is the dominant heat feedback in large pool fires. Species concentrations and temperatures have large influence on the radiative heat transfer in the fuel rich-core between the flame and the pool surface. To study radiative transport in the fuel-rich core, an experimental method was developed to measure radiative absorption through various pathlengths inside a 30 cm diameter methanol pool fire by using a Fourier Transform Infrared Spectrometer with N2 purged optical probes. The measured spectra are used to estimate species concentration profiles of methanol, CO, and CO2 in the fuel rich core by fitting predictions of a spectrally resolved radiation transport model to the measured spectra. Results show the importance of reliable temperature measurements for fitting the data and the need for further measurements to further understand the structure of fuel rich cores in pool fires.Item Barrier Heights and Diffusion Coefficients in Protein Folding(2007-08-03) Naganathan, Athi Narayanan; Munoz, Victor; Biochemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A widely held view with respect to the folding of single-domain proteins is that they are two-state. In other words, it is seemingly sufficient to invoke just two thermodynamic macrostates - folded and unfolded - to explain the experimental data with a transition-state like picture. Unfortunately, a chemical two-state model and the resulting conventional analyses do not estimate the barrier height which is essential in determining whether protein folding can be approximated as a two-state, all-or-none transition. However, the energy landscape theory of protein folding predicts small and even zero folding free energy barriers (downhill folding) because of partial or complete compensation between large enthalpic and entropic terms as folding proceeds. They have been recently validated by the thorough experimental characterization of proteins that fold globally downhill (BBL) and those that fold over marginal free energy barriers. In light of these findings, the question of whether this observation is an exception or merely the tip of the iceberg assumes primary importance. Analyzing the experimental data on previously characterized proteins with statistical mechanical models, it is shown here that the barrier to folding are indeed small and the folding phase space can be quantitatively classified into four regimes - global downhill, marginal barrier, twilight-zone and two-state like. The average effective diffusion coefficient to folding (Deff) is predicted to be strongly temperature dependent changing from 1/(20-25 microseconds) at 298 K to 1/(2 microseconds) at ~330-340 K. The activation term on the Deff is found to scale linearly with the protein size while the folding rates themselves scale inversely with the square root of protein length. This work further highlights the importance of baselines and proposes additional thermodynamic and kinetic signatures of downhill folding. A comprehensive experimental and theoretical characterization of PDD, a structural and functional homolog of BBL is also presented. The results indicate that PDD folds downhill at 298 K while crossing a marginal barrier at the apparent Tm. The evolutionary conservation of downhill folding indirectly suggests that this folding behavior has a functional consequence. In short, this work underlines the need for a fundamental shift towards physical models in characterizing protein folding processes.Item THE OPTICAL KERR EFFECT OF LIQUIDS(2006-12-19) Zhu, Xiang; Mullin, Amy; Fourkas, John T; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Optical Kerr effect (OKE) spectroscopy has found broad use in monitoring ultrafast dynamics in transparent media. I demonstrated that by using two pump pulses with independently-controllable polarizations, intensity and timing, different contributions to the OKE signal in liquids can be enhanced and suppressed, and I characterize in detail perpendicularly-polarized pulses used for the excitation step in OKE spectroscopy. The results indicate that the signal can be described well as arising from the sum of independent third-order responses initiated by each pump pulse. OKE spectroscopy has been used to study the orientational dynamics of benzene and benzene-d6 confined in nanoporous sol-gel glass monoliths with a range of average pore sizes. The orientational dynamics are described well by the sum of two exponentials, one of which depends on pore size. Comparision to Raman linewidth data suggests that the liquid exhibits significant structuring at the pore walls, with the benzene molecules lying flat on the surfaces of unmodified pores. OKE spectroscopy has also been used to study the temperature-dependent orientational dynamics of a series of nitriles with n-alkyl chains ranging from one to 11 carbons in length. In all cases the orientational diffusion is found to be described by a single-exponential decay. Analysis of the orientational correlation times using the Debye-Stokes-Einstein equation suggests that the molecules adopt extended configurations and reorient as rigid rods. The liquids with shorter alkyl chains undergo an apparent ordering transition as they are cooled.Item Heat Transfer Analysis for Improved In-situ Infrared Tempertaure Diagnostics in Microcombustors(2006-05-05) Veeraragavan, Ananthanarayanan; Cadou, Christopher; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis investigates heat transfer processes occurring in microcombustors. First, a simple 2D model is developed for predicting temperature profiles in a premixed laminar flame propagating between two parallel plates. The model is used to generate a correlation for the variation in Nusselt number with downstream distance which is useful for numerical simulations. It also shows that the temperature profile across the channel is well approximated using either 2nd or 4th order polynomials. Second, the functional form of the gas temperature profile is used to demonstrate a new diagnostic technique for making non-intrusive measurements of gas temperature and wall heat fluxes. The technique is applied in a silicon-walled microcombustor (5 cm x 2 mm x 5.5 cm). The gas temperature and wall heat flux measurements are combined with measurements of the wall temperature distribution to develop a complete picture of heat transfer in the microcombustor. The results show that thermal feedback from the post-flame to the pre-flame via the structure is the dominant heat transfer path in microcombustors.