HCOOH high resolution spectroscopy in the 9.18$\mu$m region

TL;DR

This paper presents highly accurate frequency measurements of formic acid lines in the 9.18 μm region using a femtosecond frequency comb, providing precise references for advanced spectroscopy.

Contribution

It reports the first high-precision absolute frequency measurements of HCOOH lines in this spectral region with 1-2 kHz accuracy, enhancing molecular spectroscopy references.

Findings

Achieved 1 kHz accuracy for 6 lines and 2 kHz for 17 lines.

Demonstrated stable HCOOH laser references with Allan deviation of 4.5×10^{-12}.

Lines are suitable for H₂⁺ Doppler-free two-photon spectroscopy.

Abstract

We report on higly accurate absolute frequency measurement against a femtosecond frequency comb of 6 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) CO$_2$ laser emission lines and are probed either by a CO$_2$ or a widely tunable quantum cascade laser phase locked to a master CO$_2$ laser. The relative stability of two HCOOH stabilized lasers is characterized by a relative Allan deviation of 4.5 10$^{-12}$ $\tau^{-1/2}$. They give suitable frequency references for H$_2^+$ Doppler free two-photon spectroscopy.