Absolute frequency calibration of the CO2 isotope laser transitions

The frequencies of rare CO
2
isotope lasers are measured by comparison with
¹²
C
¹⁶
O
2
reference lines and with each other. Improved heterodyne techniques are used to generate difference frequencies in a liquid-nitrogen-cooled HgCdTe varactor photodiode. Microwave frequency counter measurements of the difference frequencies are then used to calculate the band centers, rotational constants and transition frequencies with an estimated accuracy of less than a few kilohertz. Selected applications of CO
2
isotope lasers in precision heterodyne calibration, spectroscopy, microwave and IR synthesis are described.