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An experimental study of excited state level populations in the argon inductively coupled plasma Walker, Zane Harry


In an effort to elucidate excitation and ionization of analyte in the inductively coupled plasma, excited state level populations have been determined for three analytes, Fe, Ba, and Cr, from experimentally measured line emission intensities at various rf input powers and spatial positions. Measurement of line intensities was achieved using a 4096 pixel linear photodiode array spectrometer. The photodiode array spectrometer proved to be very advantageous in the collection of data, allowing the simultaneous measurement of line intensities from a wavelength window approximately 50 nm in width. Population plots were constructed from relative level populations for both atom and ion species of the three analytes and their dependence on rf input power was examined. The spatial dependence of the Fel and Fell populations was also studied. Excitation temperatures determined from the atom and ion level populations of Fe, Ba and Cr were examined. The results strongly support the existence of partial local thermodynamic equilibrium in the ICP. Such an equilibrium condition is characterized by an overpopulation of low energy atom levels and the presence of Saha equilibrium between high energy atom levels and the ground state ion. The principal means of analyte excitation and ionization appears to be a result of inelastic electron collisions.

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