Lasers especially multiple laser beams demonstrate unique advantages as energy sources in diamond synthesis. However, the fundamental mechanisms involved in the laser-assisted processes are not Well understood. In a reported amazingly-fast multiple laser coating technique, CO2 gas is claimed as the sole precursor or secondary precursor, which remains poorly understood and unverified. The absorption coefficient changes under the irradiation of the multiple lasers are one of the keys to resolve the mysteries of multiple laser beam coating processes. This study investigates the optical absorption in CO2 gas at the CO2 laser wavelength. This resonance absorption process is modeled as an inverse process of the lasing transitions of CO2 lasers. The well-established CO2 vibrational-rotational energy structures are used as the basis for the calculations with the Boltzmann distribution for equilibrium states and the three-temperature model for non-equilibrium states. Based on the population distribution, our predictions of CO2 absorption coefficient changes as the function of temperature are in agreement with the published data.

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