Unusual mechanism for H₃⁺ formation from ethane as obtained by femtosecond laser pulse ionization and quantum chemical calculations

The formation of H₃⁺ from saturated hydrocarbon molecules represents a prototype of a complex chemical process, involving the breaking and the making of chemical bonds. We present a combined theoretical and experimental investigation providing for the first time an understanding of the mechanism of H₃⁺ formation at the molecular level. The experimental approach involves femtosecond laser pulse ionization of ethane leading to H₃⁺ ions with kinetic energies on the order of 4 to 6.5 eV. The theoretical approach involves high-level quantum chemical calculation of the complete reaction path. The calculations confirm that the process takes place on the potential energy surface of the ethane dication. A surprising result of the theoretical investigation is, that the transition state of the process can be formally regarded as a H₂ molecule attached to a C₂H₄² entity but IRC calculations show that it belongs to the reaction channel yielding C₂H₃ ⁺ H₃⁺. Experimentally measured kinetic energies of the correlated H₃⁺ and C₂H₃ ⁺ ions confirm the reaction path suggested by theory.