Measurement of the Hydrogen Cyanide Absorption Lines' Centers with the Potential for Mise en Pratique

We built a linear absorption spectroscopy setup to measure hydrogen cyanide (H13C14N) 2ν3 rotational-vibrational band. The laser’s frequency was locked to the minimum of the absorption line by using the 1f technique. We used a commercially available lock-in amplifier combined with a custom-made PID controller allowing for the short-term frequency stability of about 1exp(-10). The laser frequency was determined by measuring the beat note between the laser and the optical frequency comb. The laser was locked to each absorption line for about 8 hours and the final time record allowed us to determine the absorption line central frequency with the uncertainty of about 40 kHz. This shows significant improvement compared to the data already available for HCN. Our work can potentially lead to the acceptance of HCN for Mise en Pratique (MeP), which means that laser locked to one of the HCN lines can become the official source of the one-meter unit.