Development of an Infrared Action Spectroscopy setup to Study Analogs of Cosmic Carbonaceous Nanograins in Extreme Conditions

Polycyclic aromatic hydrocarbons (PAHs) are an abundant component of interstellar matter. PAHs emit characteristic emission features in photodissociation regions (PDRs) and their study is the focus of observational programs with the James Webb Space Telescope (JWST, see [1,2] for results of the PDRs4All program). In molecular clouds and protoplanetary disks, PAHs could be in the form of clusters [3,4].These clusters are good candidates for very small carbonaceous grains [3] on which small molecules such as water could stick [5]. Our objective is to perform the IR spectroscopy of PAH cation clusters (PAHn+) and mixed PAH-water cation clusters ([PAHm(H₂O)n]+) under extreme conditions, i.e., similar to the interstellar medium. The obtained data will be compared with JWST observations and quantum chemistry calculations, in order to find evidence for PAH clusters and their interaction with water molecules in astrophysical environments. The presented poster will detail the main components of our experimental setup. It consists of (i) a molecular source, that can produce homogeneous and heterogenous ionized clusters [6], (ii) a cryogenic ion trap, in which ions will be trapped and thermalized at low temperature (<10 K), and (iii) a time-of-flight mass spectrometer to analyze the species and their mass evolution. The trapping conditions are also designed to perform tagging between the ionized clusters and rare gas atoms or small molecules (e.g., He, Ne, Ar, H₂O, CO, etc.). During trapping, the ionic species will be irradiated by a tunable IR laser, ranging from 2.5 µm to 4.5 µm, in order to perform their action spectroscopy within the C-H and O-H stretching regions. In its final configuration, the developed setup will be combined with PIRENEA 2 [7] to study the properties of the produced species in a cold and collision-free environment (~10 K, ~10-11 mbar), which can mimic interstellar conditions. Keywords: IR action spectroscopy, PAH clusters source, cryogenic ion trap, photodissociation regions. References [1]E. Peeters et al., 'PDRs4All III: JWST's NIR spectroscopic view of the Orion Bar', ArXiv E-Prints, p. arXiv:2310.08720, Oct. 2023, doi: 10.48550/arXiv.2310.08720. [2]R. Chown et al., 'PDRs4All IV. An embarrassment of riches: Aromatic infrared bands in the Orion Bar', ArXiv E-Prints, p. arXiv:2308.16733, Aug. 2023, doi: 10.48550/arXiv.2308.16733. [3]Pilleri, P., Montillaud, J., Berné, O., and Joblin, C., 'Evaporating very small grains as tracers of the UV radiation field in photo-dissociation regions', A&A, vol. 542, p. A69, 2012, doi: 10.1051/0004-6361/201015915. [4]K. Lange, C. Dominik, and A. G. G. M. Tielens, 'Turbulent processing of PAHs in protoplanetary discs. Coagulation and freeze-out leading to depletion of gas-phase PAHs', A&A, vol. 674, p. A200, Jun. 2023, doi: 10.1051/0004-6361/202245108. [5]S. Zamith, A. Kassem, J.-M. L'Hermite, and C. Joblin, 'Water Attachment onto Size-Selected Cationic Pyrene Clusters', J. Phys. Chem. A, vol. 126, no. 23, pp. 3696-3707, Jun. 2022, doi: 10.1021/acs.jpca.2c02195. [6]I. Braud, S. Zamith, and J.-M. L'Hermite, 'A gas aggregation source for the production of heterogeneous molecular clusters', Rev. Sci. Instrum., vol. 88, no. 4, p. 043102, Apr. 2017, doi: 10.1063/1.4979639. [7]A. Marciniak et al., 'Studying the Interaction Between VUV Photons and PAHs in Relevant Astrophysical Conditions', in European Conference on Laboratory Astrophysics ECLA2020, V. Mennella and C. Joblin, Eds., Cham: Springer International Publishing, 2023, pp. 237-243.