Charge Transfer, Complexes Formation and Furan Fragmentation Induced by Collisions with Low-Energy Helium Cations

The present work focuses on unraveling the collisional processes leading to the fragmentation of the gas-phase furan molecules under the He⁺ and He²⁺ cations impact in the energy range 5-2000 eV. The presence of different mechanisms was identified by the analysis of the optical fragmentation spectra measured using the collision-induced emission spectroscopy (CIES) in conjunction with the ab initio calculations. The measurements of the fragmentation spectra of furan were performed at the different kinetic energies of both cations. In consequence, several excited products were identified by their luminescence. Among them, the emission of helium atoms excited to the 1s4d ¹D₂, ³D_1,2,3 states was recorded. The structure of the furan molecule lacks an He atom. Therefore, observation of its emission lines is spectroscopic evidence of an impact reaction occurring via relocation of the electronic charge between interacting entities. Moreover, the recorded spectra revealed significant variations of relative band intensities of the products along with the change of the projectile charge and its velocity. In particular, at lower velocities of He⁺, the relative cross-sections of dissociation products have prominent resonance-like maxima. In order to elucidate the experimental results, the calculations have been performed by using a high level of quantum chemistry methods. The calculations showed that in both impact systems two collisional processes preceded fragmentation. The first one is an electron transfer from furan molecules to cations that leads to the neutralization and further excitation of the cations. The second mechanism starts from the formation of the He-C₄H₄O^+/2+ temporary clusters before decomposition, and it is responsible for the appearance of the narrow resonances in the relative cross-section curves.