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Absorption cells filled with diatomic iodine are frequently employed aswavelength reference for high-precision stellar radial velocity determinationdue their long-term stability and low cost. Despite their wide-spread usage inthe community, there is little documentation on how to determine the idealoperating temperature of an individual cell. We have developed a new approachto measuring the effective molecular temperature inside a gas absorption celland searching for effects detrimental to a high precision wavelength reference,utilizing the Boltzmann distribution of relative line depths within absorptionbands of single vibrational transitions. With a high resolution Fouriertransform spectrometer, we took a series of 632 spectra at temperatures between23{\deg}C and 66{\deg}C. These spectra provide a sufficient basis to test thealgorithm and demonstrate the stability and repeatability of the temperaturedetermination via molecular lines on a single iodine absorption cell. Theachievable radial velocity precision is found to be independent of the celltemperature and a detailed analysis shows a wavelength dependency, whichoriginates in the resolving power of the spectrometer in use and thesignal-to-noise ratio. Two effects were found to cause apparent absolute shiftsin radial velocity, a temperature-induced shift of the order of 1 m/s/K and amore significant effect resulting in abrupt jumps of 50 m/s is determined to becaused by the temperature crossing the dew point of the molecular iodine.

A systematic approach to determining the properties of an iodine absorption cell for high-precision radial velocity measurements