A theoretical study of SnF ~(2+), SnCl ~(2+), and SnO ~(2+) and their experimental search

摘要

We present a detailed theoretical study of the stability of the gas-phase diatomic dications SnF ~2, SnCl ~2, and SnO ~2 using ab initio computer calculations. The ground states of SnF ~2, SnCl ~2, and SnO ~2 are thermodynamically stable, respectively, with dissociation energies of 0.45, 0.30, and 0.42 eV. Whereas SnF ~2 dissociates into Sn ~2 F, the long range behaviour of the potential energy curves of SnCl ~2 and SnO ~2 is repulsive and wide barrier heights due to avoided crossing act as a kind of effective dissociation energy. Their equilibrium internuclear distances are 4.855, 5.201, and 4.852 a _0, respectively. The double ionisation energies (T _e) to form SnF ~2, SnCl ~2, and SnO ~2 from their respective neutral parents are 25.87, 23.71, and 25.97 eV. We combine our theoretical work with the experimental results of a search for these doubly positively charged diatomic molecules in the gas phase. SnO ~2 and SnF ~2 have been observed for prolonged oxygen (~(16)O ~-) ion beam sputtering of a tin metal foil and of tin (II) fluoride (SnF ~2) powder, respectively, for ion flight times of about 10 ~(-5) s through a magnetic-sector mass spectrometer. In addition, SnCl ~2 has been detected for ~(16)O ~- ion surface bombardment of stannous (tin (II)) chloride (SnCl _2) powder. To our knowledge, SnF ~2 is a novel gas-phase molecule, whereas SnCl ~2 had been detected previously by electron-impact ionization mass spectrometry, and SnO ~2 had been observed before by spark source mass spectrometry as well as by atom probe mass spectrometry. We are not aware of any previous theoretical studies of these molecular systems.