Continuous-Wave Terahertz Spectroscopy of Plasmas and Biomolecules
Abstract
Continuous-wave linear-absorption spectroscopy based on THz radiation generated by solid-state photomixers has been applied to the investigation of the dynamics of biomolecules in polyethylene matrices and to line shape studies of HF for diagnostics of semiconductor etching plasmas. The THz spectra of biotin and myoglobin have been obtained using a variable-temperature, cryogenic sampling system. The spectrum of biotin displays a small number of discrete absorptions over the temperature range from 4.2 K to room temperature while the spectrum of myoglobin has no obvious resonance structure at the >10% fractional absorption level. Spectral predictions from the lowest energy ab initio conformations of biotin are in poor agreement with experiment, suggesting the need to include condensed-phase environmental interactions for qualitative predictions of the THz spectrum. Vibrational anharmonicity is used to model the line shapes that result from drastic changes in vibrational sequence level populations of biotin over this temperature range. Anharmonicity factors (χeωe/ωe) at the levels of 0.1% to 0.8% are obtained from non-linear least squares fits of the observed resonances and illustrate their important for refining model predictions. Application of the photomixer system to line shape studies in etching plasmas has been used to study the formation efficiency and translational temperature of HF at 1.2 THz under different operating conditions. These results will aid in understanding the chemistry of industry-standard fluorocarbon and oxygenated fluorocarbon etching plasmas.
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