Transferring the high accuracy of the 10 μm CO2 laser bands to far infrared region with internal calibration of CS2

The main aim of the work is to transfer the high accuracy of the CO₂ laser bands around 10 μm to far infrared regions around 400 and 250 cm⁻¹ for secondary standards. The bands ν₁ + ν₂ and 3ν₂ of CS₂ were measured on the Bruker IFS 120 HR Fourier spectrometer in Oulu with special care and calibrated against CO₂. In the second stage the ν₂ region around 400 cm⁻¹ was measured at a resolution of 0.001 cm⁻¹. This spectrum was calibrated against 3ν₂ internally with the CS₂ band system using ladders formed with rotational lines in the bands ν₂, 2ν₂ − ν₂ and 3ν₂ − 2ν₂. Further, the difference band ν₁ − ν₂ at 263 cm⁻¹ together with accompanying hot bands was measured on a similar spectrometer in Lund, Sweden, but with a synchrotron radiation source. Using corresponding chains of lines as above this region was calibrated with ν₁ + ν₂. In this way, problems with conventional calibration could be avoided. Without the effect of the pressure shifts the absolute accuracy of 2.0 × 10⁻⁶ and 8.4 × 10⁻⁶ cm⁻¹ has been achieved at 400 and 250 cm⁻¹, respectively. Simultaneously the same calibration accuracy is also transferred to residual water lines around the CS₂ far infrared bands and the best H₂O lines will be given with literature comparisons. In addition to the calibration new results from the observed hot bands of CS₂ in the region of the bands ν₁ + ν₂ and 3ν₂ will be given.