HCOOH high-resolution spectroscopy in the 9.18 μm region

We report on highly accurate absolute frequency measurement against a femtosecond frequency comb of six saturated absorption lines of formic acid (HCOOH) with an accuracy of 1 kHz. We also report the frequency measurement of 17 other lines with an accuracy of 2 kHz. Those lines are in quasi coincidence with the 9R(36) to 9R(42) CO2 laser emission lines and are probed either by a CO2 or a widely tunable quantum cascade laser phase locked to a master CO2 laser. The stability of HCOOH stabilized lasers is characterized by a fractional Allan deviation of 3.1 × 10−12 τ−1/2. They give suitable frequency references for H2+ Doppler-free two-photon spectroscopy.