Infrared spectra of TiO2 clusters for hot Jupiter atmospheres

Context. Clouds appear to be an unavoidable phenomenon in cool and dense environments. Hence, their inclusion is a necessary part of explaining observations of exoplanet atmospheres, most recently those of WASP 96b with the James Webb Space Telescope (JWST). Understanding the formation of cloud condensation nuclei in non-terrestrial environments is therefore crucial in developing accurate models to interpret current and future observations. Aims. The goal of the paper is to support observations with infrared spectra for (TiO2)(N) clusters to study cloud formation in exoplanet atmospheres. Methods. We derived vibrational frequencies from quantum-chemical calculations for 123 (TiO2)-clusters and their isomers and we evaluated their line-broadening mechanisms. Cluster spectra were calculated for several atmospheric levels for two example exoplanet atmospheres (WASP 121b-like and WASP 96b-like) to identify possible spectral fingerprints for cloud formation. Results. The rotational motion of clusters and the rotational transitions within them cause significant line broadening, so that individual vibrational lines are broadened beyond the spectral resolution of the medium-resolution mode of the JWST mid-infrared instrument (MIRI) at R = 3000. However, each individual cluster isomer exhibits a 'fingerprint' IR spectrum. In particular, larger (TiO2) clusters have distinctly different spectra from smaller clusters. The morning and evening terminator for the same planet can exhibit different total absorbances, due to the greater abundance of different cluster sizes. Conclusions. The largest (TiO2) clusters are not necessarily the most abundant (TiO2) clusters in the high-altitude regions of ultra-hot Jupiters and the different cluster isomers do contribute to the local absorbance. Planets with a considerable day-night asymmetry will be most suitable in the search for (TiO2) cluster isomers with the goal of improving cloud formation modelling.