Collision-induced absorption in planetary atmospheres: present data and future perspectives

Our solar system’s planetary variety stems from distinct evolutionary paths that also influenced the presence of atmospheres. They represent primary determinants of planetary environments and potential habitability, making their characterization essential in both planetary and exoplanetary science.

Central to this study is having detailed spectroscopic databases, such as HITRAN [1]. Specifically, the Collision-Induced Absorption represents a critical opacity source, most of all in high-density atmospheres. Despite its importance, significant gaps remain in both experimental and theoretical CIA data. To address this, we performed new experimental measurements of H₂+H₂ and H₂+CO₂ mixtures, important for jovian, early martian, and exoplanetary atmospheres, covering previously unexplored temperature and spectral ranges.

We used an experimental setup called PASSxS which can be visualized in Figure 1.

Figure 1: A picture of the experimental setup

It consists of an atmospheric simulation chamber containing the gas or mixture of gases under investigation, which can sustain pressures up to 70 bar. It can be heated up to 550 K and cooled down to 100 K. It also contains a Multi-Pass cell, characterized by an optical path of 3.27 m, coupled with a high-resolution FT-IR spectrometer through a series of transfer optics placed in a vacuum chamber. The maximum resolution achievable is 0.002 cm^-1.

For pure H₂ (99.9999% purity), we measured the fundamental band of the H₂ CIA between 4000 and 5500 cm^-1 at a resolution of 0.05 cm^-1, across seven temperatures (116- 498 K), and various densities. Fig. 2 shows the absorption coefficients measured at each considered temperature and corresponding pressures. The high resolution allowed us to resolve the so-called interference dips, visible on the blow-up of Fig. 2.

Figure 2: CIA absorption coefficients for a pure H₂ gas measured at seven different temperatures

They represent a lack of absorption observed at the same spectral position as the H₂ quadrupolar lines, coming from the interference phenomenon between the induced dipole moments acquired by the H₂ molecules in consecutive collisions [3]. They have been previously observed in other experimental works [4-8] but only at temperatures equal to and less than 300 K. Applying the theoretical profile developed by Kelley et al. [6], we fitted the resolved interference dips at each density to retrieve the intracollisional halfwidth parameter. It exhibited a linear density dependence across all temperatures, as illustrated in Fig. 3 for the Q(1) interference dip at 4155 cm^-1.

Furthermore, the integrated absorption coefficients within the spectral range of the interference dips followed a cubic trend with density.

These results provide new insights into the density-dependence of those features currently unaccounted for in existing theoretical models. A paper containing these new results is currently in preparation.

The next step will be to perform the same high-resolution measurements using a H₂-He mixture to investigate the effect of He on the behavior of those features.

Figure 3: Q(1) intracollisional halfwidth as a function of the density for each investigated temperature.

In the same spectral region, new measurements of the H₂+CO₂ CIA have been performed. We used a total pressure of 12 bar, with 17% of volume mixing ratio (VMR) of CO₂ for six temperatures, from 241 K to 498 K, at a resolution of 1 cm^-1.

While room-temperature measurements have been recently reported [9-10], there is a lack of experimental data for other temperatures.

Fig. 4 shows the measured CO₂-H₂ CIA binary absorption coefficients for all six temperatures explored.

Figure 4: CO₂-H₂ CIA binary absorption coefficients

At room temperature, our experimental data have been compared with the existing experimental data, showing a good agreement. We also compared our results with semi-empirical calculations based on the approach described in [11] at each investigated temperature.

Although the theoretical band shape showed significant discrepancies with the experimental results, the integrated band intensity showed reasonable agreement. Finally, following the method employed in [12], we fitted the experimental BACs with a temperature-dependent exponential profile. This procedure allowed us to obtain a set of wavenumber-dependent coefficients that can be used to calculate the BACs at every temperature inside the investigated range. The wavenumber-dependent coefficients, along with the measured BACs, are now available on the Zenodo platform (https://doi.org/10.5281/zenodo.18327685).

These results, obtained in collaboration with Dr. Tran (LMD, Paris) and published in [13], highlight the necessity for more refined theoretical models, even if they can be used to provide a reasonable estimate of the integrated band intensity.

Future work will focus on high-resolution measurements of H₂-CO₂ mixtures to determine reliable H₂-broadening coefficients for CO₂ lines, which is so far not available at different temperatures.

Acknowledgements:  This work has been developed under the ASI-INAF agreement n. 2023-6-HH.0, and supported by the EMM (Earth Moon Mars) project of PNRR (WP 1500-13)

References

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