Anomalous gain in an isotopically mixed CO2 laser and application to absolute wavelength calibration

Measurements are reported on a grating-tuned CO₂ laser, containing an isotopic mixture of ¹⁶O¹²C¹⁶ O, ¹⁶O¹²C¹⁸O, and ¹⁸O ¹²C¹⁸O. The P₆ and R ₁₄ lines of ¹⁶O¹²C¹⁶O were found to have anomalously high intensities. These anomalies are produced by the near coincidence of the transition frequencies in two distinct isotopes, permitting them to act as a single indistinguishable population. These two lines can be used to identify the rotational quantum numbers in the P and R branch spectra, thereby permitting absolute wavelength calibration to be achieved