Experimental study of the interference dips observed on the collision-induced absorption fundamental band of H2: their relevance to planetary atmosphere characterization

Introduction: 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 opacity sources in the near-infrared spectral range between 1 and 5 μm. This is a spectral range widely investigated for Jupiter not only by ground-based instruments, but also from space, presently through the eyes of JIRAM on board JUNO, JWST, and in future also of MAJIS on board JUICE.

Jupiter, in fact, represents an archetype for the giants’ gaseous planets, and the understanding of its magnetosphere, composition, and opacity of its dense and complex atmosphere are all important elements for the most comprehensive view about how the Jupiter system(s) works.

In this work, we performed an experimental study of the H₂ CIA in the [3600, 5500] cm^-1 spectral range at high resolution, to investigate not only the overall opacity due to the CIA, but also to study the narrow features called interference dips, not taken into account by the existing models.

Experimental setup: The experimental setup employed here is called PASSxS (Planetary Atmosphere Simulation System x Spectroscopy) [1]. It consists of a simulation chamber that contains a Multi-Pass cell coupled with an IR Fourier spectrometer (FTIR) 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. The FTIR has a maximum spectral resolution of 0.002 cm^-1.

A picture of the setup can be visualized in Figure 1.

Figure 1: Experimental setup, consisting of a Fourier Spectrometer coupled with a simulation chamber (in grey behind the FTIR)

Results and discussion: Binary absorption coefficients due to both the H₂-H₂ and H₂-He collisions in the [3600, 5500] cm^-1  spectral range for temperatures going from 120 to 500 K has been recently published in [2]. Superimposed on the CIA absorption, some narrow features have been observed at all the temperatures. These interference dips correspond with a smaller absorption at specific frequencies with respect to the overall CIA band contour.

They have been previously observed in other experimental works [3-7] at temperatures up to 300 K. To study the behavior of those features with density and temperature, we performed measurements of the H₂ CIA fundamental band at a resolution of 0.05 cm^-1, temperatures from 305 to 499 K, and different pressures.

Figure 2 shows the measured absorption coefficients for three pressures at 399 K.

Figure 2: Experimental absorption coefficients measured at 399 K for three different pressures

The interference dips are well visible on the left side of the main peak of the band.

Furthermore, they are also present around 4161 cm^-1, 4500 cm^-1, 4700 cm^-1, and 4900 cm^-1, but the latter three are superimposed on several sharp absorption lines due to the H₂ quadrupolar transitions, located approximately in the centre of the dips.  

The phenomenon generating those dips has been previously investigated by Van Kranendonk [8]. They are caused by the interference of induced dipole moments in consecutive collisions and are not reproduced by the existing CIA model simulations.

Van Kranendonk calculated a symmetric theoretical profile to describe their shape as a function of the intracollisional halfwidth δ and the frequency of the dip’s peak ν_c.

He also predicted a linear behavior of the intracollisional halfwidth with density.

However, Kelley and Bragg [5] observed an asymmetry of the main peak of the dips. Consequently, they used a modified version of Van Kranendonk’s profile by adding a phase α to fit the asymmetric line profiles as shown Equation 1.

Equation 1: Asymmetric profile [5]

We used their profile to fit the Q(1) dip near 4155 cm^-1 for all the pressures considered at the investigated temperatures and retrieve the δ parameter.

Figure 3 shows the fit performed over the Q(1) dip measured at 12.7 bar and 399 K.

Figure 3: Q(1) interference dip (black solid line) measured at 399 K and 12.7 bar. The light blue dotted line represents the fit made with the asymmetric profile [4].

The intracollisional halfwidth has been then plotted against the density, finding a linear behavior for all three temperatures considered, 305 K, 399 K, and 499 K, as can be seen in Figure 4, as expected by Van Kranendonk's theory.

Figure 4: Behavior of the intracollisional halfwidth (δ) with density for the three temperatures considered

CIA of H₂ plays an important role in investigating Jupiter’s atmosphere, and accurate laboratory measurements along with models are of primary importance to study the chemistry and physical properties of a gas giant atmosphere.

Laboratory data can also potentially provide additional elements, such as the dependence of the interference dips on density, that can extend the retrieval of atmospheric parameters otherwise difficult to access.

References:

[1] M. Snels et al. (2021), AMT 14, 7187–7197,

https://doi.org/10.5194/amt-14-7187-2021.

[2] Vitali F. et al. (2025), JQSRT, Vol. 330, doi: https://doi.org/10.1016/j.jqsrt.2024.109255

[3] J. D. Poll et al (1975), Can. J. Phys., 53, 954

[4] A. R. McKellar et al. (1975), Can. J. Phys., 53, 2060

[5] J. D. Kelley et al. (1984), Phys. Rev. A, 29, 1168

[6] J. P. Bouanich et al. (1990), JQSRT, 44, 4

[7] J. Westberg et al. (2025), Optics Express, 33, 5

[8] Van Kranendonk J. (1968), Canadian Journal of Physics, Vol. 46 N.10,

doi: https://doi.org/10.1139/p68-150

Acknowledgments:  This work has been developed under the ASI-INAF agreement n. 2023-6-HH.0. The upgrade (in progress) of this experimental setup is partially funded by the EMM (Earth Moon Mars) project of PNRR (task 1500-13).