The broadening of spectral lines due to dipole-dipole interactions with ground-state atoms of the same element is calculated with the impact and classical path approximations. Summation over all contributing intermediate states accounts for the multiplicity of the levels involved and results in a width proportional to the geometrical mean of absorption and emission oscillator strengths. Higher multipole interactions are shown to be almost always negligible, and for Lyman-a the iterated solution of the time-dependent Schrodinger equation describing the collisions is extended to fourth order. Estimated errors of its calculated width and of the widths of other lines are about 5% in the validity regime of the impact approximation, apart from any uncertainties in oscillator strengths and statistical weight factors.

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The recent measurements by Cassini Ion Neutral Mass Spectrometer (INMS) showed the presence of numerous carbocations and shed light on their composition in Titan's upper atmosphere. The present research identifies an important class of ion-molecule reactions proceeding via carbocation collision complexes, and its implications in the chemistry of Titan's thermosphere and ionosphere. An analysis (based on the kinetics and dynamics of the elementary chemical processes identified) of the Cassini measurements reveals the mechanism of formation of the three-membered Huckel aromatic rings-Cyclopropenyl cation and its cyclic methyl derivatives. For carbocations, a nonclassical three-carboncenter two-electron-bond structure is no longer a controversial topic in chemistry literature. Emphasis has been placed on a future coordinated effort of state-of-the-art laboratory experiments, quantumchemical calculations, and astronomical ALMA and JWST observations including planetary in situ measurements at millimeter and submillimeter wavelengths to elucidate the structure, energetics and dynamics of the compositions of carbocations detected by Cassini cationic mass spectrometry. The cabocation chemistry in Titan's upper atmosphere has a possible bearing on the organic chemistry and aromaticity in the atmosphere of primitive earth.

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