New experimental measurements of the Collision-Induced Absorptions of H2-H2 and H2-He in the 3600-5500 cm-1 spectral range from 120 to 500 K

The atmospheres of the gaseous and icy giant planets represent a high-density environment, whose composition is generally dominated by H₂ and He.

Consequently, the H₂ Collision-Induced Absorption (CIA) represents one of the main sources of opacity in the near-infrared spectral range between 1 and 5 μm, a spectral range widely used by remote sensing instruments.

To reduce the retrieval uncertainty of the atmospheric gases, it is very important to have experimental data on the CIA absorption compared with the available theoretical models. These models are necessary in any case where these are missing due to technological limits in the lab.

We measured in our lab the H₂ CIA fundamental band in the [3600, 5500] cm^-1 spectral range using an experimental setup called PASSxS (Planetary Atmosphere Simulation for Spectroscopy) (Snels et al, 2021).

This setup consists of a simulation chamber that contains a Multi-Pass cell coupled with a Fourier spectrometer and aligned to reach an optical path of 3.28 m. The chamber can be heated up to 550 K, cooled down to 100 K, and sustain pressures up to 70 bar.

We measured the H₂-H₂ and H₂-He binary absorption coefficients for temperatures going from 120 to 550 K by using a pure H₂ gas and an H₂-He mixture, as shown in Figures 1 and 2.

Figure 1: H₂-H₂ binary absorption coefficients

Figure 2: H₂-He binary absorption coefficients

A large water vapor absorption can be noted on the band’s wings, highlighted by the two light blue rectangles.

The results obtained have been recently published (Vitali et al., 2024) and the data can be downloaded from the Zenodo platform at the following link https://doi.org/10.5281/zenodo.13142014.

Those measurements can be of particular interest in the field of planetary atmospheres since they can be recombined to obtain the total absorption coefficients, at a fixed temperature, of a H₂-He mixture for any desired mixing ratio.

Moreover, these are essential input parameters when using radiative transfer models such as the NASA Planetary Spectrum Generator (PSG).

We plan to extend the investigation of the CIA in a wider spectral range, including the [7500, 9500] cm^-1 range interested by the H₂ CIA first overtone band, with a more detailed temperature scale for both spectral ranges.

Figures 1 and 2 show the so-called interference dips (Van Kranendonk, 1968) around 4150 cm^-1 and 4700 cm^-1, which represent a lack of absorption at specific wavelengths not taken into account by the CIA models.

A further investigation of the density dependence of those features is already in progress, by exploiting the new Fourier spectrometer at a higher spectral resolution and the entire optical path in vacuum.

We plan to perform high-resolution measurements at different densities using both a pure H2 gas and a H2-He mixture. We will use the line profile developed by Van Kranendonk to study the dependence of the inter-collisional halfwidth on density. Finally, we will also investigate the dip’s behavior while varying the He mixing ratio.

Acknowledgments: This work has been developed under the ASI-INAF agreement n. 2023-6-HH.0.