Collisional Excitation in Space: from the Interstellar Medium to Planetary Atmospheres

Our perception and understanding of many astrochemical environments, such as the interstellar medium (ISM), have been transformed over the last approximately 50 years. In the last decades, the synergism between astronomical observations and laboratory studies (increasingly based on the interplay of experiment and theory) has led to the discovery of an ever growing number of molecules, with the observed spectra becoming more and more detailed. However, in space, physical conditions pose severe constraints to the chemical processes it hosts, which greatly differ from those occurring in terrestrial environments. For instance, in the ISM, the molecular energy level populations are rarely at the local thermodynamic equilibrium (LTE) since the density is usually so low (~10^2-10^6 cm-3) that collisions compete with radiative processes. Under such conditions, deriving molecular abundances from spectral lines requires the knowledge of the collisional rate coefficients of the molecule under consideration for the most abundant perturbing species. The nature of the latter depends on the investigated astronomical environment. For instance, in the interstellar clouds the most important perturber is molecular hydrogen (H2), with He also considered as approximation of H2; otherwise, if we aim to constrain molecular abundances in cometary comas, collisional excitation is generally dominated by H2O (and/or electrons) and, for comets at large heliocentric distances, by CO; in the planetary atmosphere, moreover, the molecular composition could further change: in the thermosphere of Titan, for example, N2 and CH4 are the most abundant collisional partners.Given the extreme conditions of those environments, which are difficult to be reproduced in the laboratory, the knowledge of the collisional rate coefficients heavily relies on theoretical calculations. However, broadening measurements are frequently employed to provide experimental validation of the computational procedure.During my PhD, I have mainly focused my research on the theoretical and experimental determination of the relevant collisional parameters for molecules of astrochemical interest. The aim of this research is to gain more information regarding their population distribution through rotational levels in order to accurately determine their abundance and excitation temperature via radiative transfer models.