1：Experimental Design and Method Selection:

The study employs saturation absorption spectroscopy with a tunable Nd : YAG-laser with an intracavity frequency doubler, stabilised at a wavelength of 532 nm by saturated absorption resonances in molecular iodine I2. The method involves observing resonances in luminescence and stabilisation by third harmonic zero in response to a probe frequency modulation of laser radiation.

2：The method involves observing resonances in luminescence and stabilisation by third harmonic zero in response to a probe frequency modulation of laser radiation. Sample Selection and Data Sources:

2. Sample Selection and Data Sources: The hyperfine structure component a1 of the molecular iodine absorption line R(56) 32 – 0 is used for measurements.

3：List of Experimental Equipment and Materials:

Includes two tunable Nd : YAG-lasers with ring cavities and intracavity frequency doubling, a Fabry – Perot interferometer, luminescence iodine cell, and electronic system for automatic frequency tuning.

4：Experimental Procedures and Operational Workflow:

The radiation frequency of the first laser is stabilised by a resonance frequency of the interferometer according to the Pound – Drever – Hall method. The second laser's emission is tied to the first laser's by phase self-tuning. The luminescence signal is recorded by a photomultiplier and detected in the third harmonic of the modulation frequency.

5：Data Analysis Methods:

The frequency shifts are measured as functions of the molecular iodine vapour pressure and probe modulation frequency deviation.