Absolute frequency measurement for the emission transitions of molecular iodine in the 982 — 985 nm range

We report high-precision frequency measurements of the separate hyperfine structure (HFS) components of the emission B — X system transitions of ¹²⁷I₂ molecules in the 982 — 985 nm range. To resolve the HFS of the emission lines, advantage was taken of the method of three-level laser spectroscopy. The function of exciting radiation was fulfilled by the second harmonic of a cw Nd : YAG laser, and the probe radiation in the 968 — 998 nm range was generated by an external-cavity diode laser. The output Nd : YAG laser frequency was locked to an HFS component of the absorption transition and the probing laser radiation to the emission transition component. When both frequencies were locked to HFS components with a common upper level, the output diode laser frequency was precisely equal to the emission transition frequency. The output frequency of the thus stabilised diode laser was measured with the help of a femtosecond optical frequency synthesiser based on a Ti : sapphire laser. We present the results of the absolute frequency measurements of 20 HFS components belonging to six vibrational — rotational transitions of the B — X system of iodine [R56(32 — 48)a1, P58(32 — 48)a1, P85(33 — 48)a1, R87(33 — 48a1, R88(33 — 48)a10] and all 15 components of the R86(33 — 48) line. The relative measurement uncertainty is equal to 7×10^-10 and is determined by the frequency instability of the diode laser radiation.