Calculating Time-Resolved Absolute Number Densities of Reactive Species via Cavity Ringdown Spectroscopy

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2021-05

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The Ohio State University

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To test proposed kinetic mechanisms, the measurement of absolute concentrations is important and often required. Numerous spectroscopic techniques have been used to follow the concentration of reactive species, such as radicals or excited states, throughout processes like chemical reactions or relaxations to the ground state. However, highly reactive states are difficult to measure absolute populations for. To obtain quantitative values for these populations, cavity ringdown spectroscopy experiments were performed by the Adamovich group. In conjunction with this experimental work, a series of calculations were performed to calculate the absolute populations of two species: metastable N2 and HO2 radical. To perform these calculations, a series of constants and parameters from prior experiments were used to form a simulation on the program Pgopher, which generated values used to calculate the absolute cross-sections required to convert the experimentally obtained absorption coefficients into absolute number densities. In the metastable N2 experiment and calculations, the absolute cross-sections were calculated using two different methods using values obtained from prior experiments before they were converted into number densities. In the HO2 experiment, the absolute cross-sections were instead determined by first fitting the dipole moments of a simulated spectrum to a set of empirical cross-sections from a prior experiment, then changing the conditions of the simulation to match those of the corresponding experiment, yielding a spectrum whose peaks were cross-sections that enabled number densities to be obtained from the observed absorption coefficients.

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Absolute populations, CRDS

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