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\bibitem{Siemsen_etal2001} Siemsen, K. J., Bernard, J. E., Madej, A. A., \& Marmet, L. (2001). Absolute frequency measurement of a CO$_{2}$/OsO$_{4}$ stabilized laser at 28.8 THz. \textit{Appl. Phys. B: Lasers and Optics}, \textit{72}, 567--573.

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