Experimental results on the H2-H2 and H2 -He collisional-induced absorption coefficients at typical Jupiter’s upper tropospheric conditions. 

Jupiter’s atmosphere is primarily composed of molecular hydrogen and helium with a mixing ratio almost identical to our Sun. Since the atmosphere of this gaseous giant represents a high-density environment, spanning many orders of magnitude along its radius, the H2-H2 and H2-He Collision Induced Absorption (CIA) bands represent its main source of opacity in the infrared part of the spectrum, particularly between 1 and 5 μm, a spectral range widely used by remote sensing instruments. Thus, it is of primary importance to have experimental data on the CIA absorption and to compare them with the theoretical models present in the literature, to have the most accurate estimation of Jupiter’s atmospheric opacity, at least for the CIA. Consequently, measurements of the hydrogen CIA fundamental band have been performed at three different temperatures and pressure conditions typical of the Jovian upper-tropospheric profile [1], using an H2-He mixture with 13.8 % helium, a typical value of Jupiter’s atmosphere. For this scope, we used an experimental setup called PASSxS, which allows us to record absorption spectra in the spectral range from 1 to 6 μm. It comprises two stainless steel concentric vessels, as shown in Figure 1. The inner one contains the gas or mixture of gases under investigation, while the external one can be evacuated to ensure thermal insulation of the sample chamber from the external environment. The inner vessel contains a Multi-Pass cell, characterized by an optical path of about 3.2 m, coupled with an FT-IR spectrometer. The spectral resolution achievable with the present FT-IR spans from 0.06 to 10 cm-1. For a more detailed description of the experimental setup refer to [2]. Figure 2 shows the experimental absorption coefficients (blue curve) acquired at 402 K and 19.2 bar, compared with Abel’s theoretical model [3] (red curve). The band shows a maximum absorption around 4200 cm-1 where the absorption coefficients reach a value of  5.8 10-4 cm-1. Some discrepancies between the data and the model are evident and have to be further investigated. This work presents the first experimental study of the CIA fundamental band of H2-He at this high temperature. Since our experimental setup can reach temperatures up to 550 K, one of the main objectives will be to perform measurements at still higher temperatures, to further investigate a not-yet explored temperature range (work in progress at the time of this abstract). Figure 2 also shows the so-called interference dips [4] around 4150 cm-1, which represent a lack of absorption at specific wavelengths. The spectral resolution of our setup allowed, for the first time, to resolve four interference dips. A further investigation of these features might be important for future modeling. Acknowledgments: This work has been developed under the ASI-INAF agreement n. 2023-6-HH.0. References: [1] A. Seiff (1997), Science Vol 276, pp.102-104. [2] M. Snels et al. (2021), AMT 14, 7187–7197. [3] M. Abel et al. (2012), The Journal of Chemical Physics, 136. [4] J. Van Kranendonk (1968), Canadian Journal of Physics Vol. 46, N. 10.