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Context. Clouds seem unavoidable in cool and dense environments, and hence,are necessary to explain observations of exoplanet atmospheres, most recentlyof WASP 96b with JWST. Understanding the formation of cloud condensation nucleiin non-terrestrial environments is therefore crucial to develop accurate modelsto interpret present and future observations. Aims. The goal of the paper is tosupport observations with infrared spectra for (TiO2)N clusters in order tostudy cloud formation in exoplanet atmospheres. Methods. Vibrationalfrequencies are derived from quantum-chemical calculations for 123(TiO2)-clusters and their isomers, and line-broadening mechanisms areevaluated. Cluster spectra are calculated for several atmospheric levels fortwo example exoplanet atmospheres (WASP 121b-like and WASP 96b-like) toidentify possible spectral fingerprints for cloud formation. Results.Rotational motion of and transitions in the clusters cause significant linebroadening, so that individual vibrational lines are broadened beyond thespectral resolution of the medium resolution mode of the JWST mid-infraredinstrument MIRI at R = 3000. However, each individual cluster isomer exhibits a"fingerprint" IR spectrum. In particular, larger (TiO2)-clusters havedistinctly different spectra from smaller clusters. Morning and eveningterminator for the same planet can exhibit different total absorbances due todifferent cluster sizes being more abundant. Conclusions. The largest(TiO2)-clusters are not necessarily the most abundant (TiO2)-clusters in thehigh-altitude regions of ultra-hot Jupiters, and the different cluster isomerswill contribute to the local absorbance. Planets with a considerable day-nightasymmetry will be most suitable to search for (TiO2)-cluster isomers in orderto improve cloud formation modelling.

Infrared spectra of TiO2 clusters for hot Jupiter atmospheres